Archive for screams
Long Range Thoughts
Posted by: | Comments8:15 AM
There is something brewing in the Tropical Pacific and it is called an MJO or Madden-Julian Oscillation. To get a great overview on the technical side of what’s going on, I want to introduce you to a meteorologist with the initials HM who has written a great post on Eastern US Weather.
To break this down to a basic level, the location of forcing in the Tropical Pacific is starting to change as the MJO, which is basically a global lifting mechanism shifts to the east towards the date line. Why is this important?
Note that the main concern I had about issuing a winter forecast in September is that I wanted to wait to see how the atmosphere will react to a relatively weak El Nino event. Up to this point, while there have been cases of impressive negative NAO signatures, the PDO or Pacific Decadal Oscillation has been up to this point rather negative. A negative PDO with an El Nino basically brings a lot of frustration to many snow lovers along the East coast as the Pacific jet screams into western Canada, moderating the Polar air masses and keeping the pattern progressive.
However, with the MJO taking shape, changes have begun! The cold pool over the Gulf of Alaska has
SST anomalies- 9/24
started to slowly shrink over the past several days, which means that we are starting to see a trend towards the negative PDO breaking down. This is an important observation because the relationship between the SST and the upper level development of the pattern is one of feeding back and forth on each other. In a way, it’s a bit of a chicken or the egg
SST Anomalies 9/28
debate. The point though is that a change towards warm SST anomalies allows the upper level pattern to develop and maintain itself.
The model guidance is now starting to pick up on the changes as the convection over the tropical Pacific shifts to the east. The most important development can be seen in the 8-10 day means on the Penn State E-wall where we can see a strong agreement between the GFS and ECMWF of a split pattern developing. In the winter, this pattern strongly supports the potential for strong snow storms and note I have a plural there.
The first key is an upper low develops over the Aluetians, which forces a ridge to build into western Canada. The
ECMWF/GFS 8-10 Day Mean
El Nino influence causes the pattern to split with an upper low over the Southwest and a ridge over Canada. What would this pattern produce? Well, for one a deep trough will develop in the East, as seen on the model guidance which will have a strong influence from northern Canada and the North Pole, so there is the cold air source. Meanwhile, the upper low over the Southwest will send disturbance after disturbance into the southern Plains and through the Mississippi Valley, drawing up moisture from the Gulf of Mexico. Then the cold air and the moisture clashes over the Mid Atlantic. I think you know where this leads.
So yes, this morning there are a lot of interesting developments unfolding. Developments that would make many happy in the winter (or frustrated depending on how you like winter) and would make my discussion for the winter forecast MUCH easier to develop.
More in the coming weeks!
-- Weather When Posted --
- Temperature: 51°F;
- Humidity: 76%;
- Heat Index: 51°F;
- Wind Chill: 48°F;
- Pressure: 29.62 in.;
Snowy Tuesday Or Just Snow Showers?
Posted by: | Comments10:12 AM
The medium range forecast obviously is focused on Tuesday as to whether the forecast area gets a heavy snowfall or nothing at all. It’s amazing just how much the model guidance has changed in the past few days where the forecast from ALL guidance went from a driving rainstorm for the forecast area and a blizzard for western New York to possibly a complete miss for the East coast.
Obviously my trust in the models is rather low, but still can’t ignore the trends of the further east solutions for each run. Let’s discuss this situation to get a better grasp of the situation.
We have a better set up for a snowfall for the forecast area than forecasted a few days ago. For one, the development of the 50/50 low has trended to a better position with a weak negative NAO, which indicates the POTENTIAL for a significant winter storm for the forecast area. The one issue at hand though is that the disturbances over Ontario and Quebec on Monday night prevent any solid convergence to develop over eastern Canada, thus a lack of a strong high. What is in place is a stalling cold front along the coast and cold air advancing to the east.
Boundary Layer Issues: I never like a situation where the forecast area is waiting on cold air to move in, especially with coastal lows. The idea on some model guidance is that as the low deepens, the cold air will be pulled towards the coast. Well, that works great at 850 MB, where there is less friction to slow down an advancing cold air mass. However, that isn’t the case at the surface where the Appellation Mountains can slow down the cold air mass to a crawl due to increasing friction factors. That is what is screaming at me right now. Sure, 850 MB temperatures fall like a rock, but if the surface cold front is starting to stall and slow down, that means the coldest air core of the air mass is as well. So while temperatures may be falling along the coast on Monday night and Tuesday morning, the temperatures may still range in the mid to upper 30′s, which means rain, not snow despite a colder 850 MB level. I can’t make that determination right now, even if I knew the exact storm track, which I don’t. So right off the bat, there are serious boundary layer issues that must be resolved before any support of a significant snowfall can be warranted.
Upper Level Dynamics: The biggest question I have in the whole set up of this storm is the position of the ridge axis over the Western United States. I know, you thought I was going to say the trough axis. No, it’s the ridge axis I am keeping an eye on. Currently model guidance has trended towards a less amplified ridge from the previous runs, which means the amplification of the trough and the timing of when the short wave produces a negative tilt in the trough happens later and later. THIS IS WHY THE STORM TRACK HAS SHIFTED EAST. Sorry for yelling, but I keep on hearing comments like if this model trends this way or that, then we’ll have snow. Models do not produce snow cause if they did, I would have made one already. Anyway, the reason why I issued the Stage One Alert is because the jet streaks over the Pacific and the teleconnections of the overall pattern would suggest a more amplified ridge over the West. Model guidance has not and likely will not include the impact of these stronger jet streaks over the central Pacific until roughly the 12Z Sunday or 00Z Monday runs of the model guidance. A perfect example is the poor job the 12 NAM does in handling the jet streaks southwest of Alaska and southwest of Hawaii at initialization, which plays into the development of the upper level pattern on Monday night. That fact alone would suggest a track closer to the coast and a possible bench mark track.
However, the other issue at hand is the position of the 50/50 low through Tuesday morning, which gradually moves further north to around 70/50, which would allow a coastal low to track further east. However, I’m not sure whether the implications of the ridge axis over the West coast will have on the position of the upper low over Labrador considering the whole development of the trough axis over the eastern United States.
So clearly there are questions that need to be answered that won’t be answered until at least Sunday night.
So where do I stand?
Well, I am pretty confident that precipitation will fall over the forecast area Monday night and Tuesday. Ironically, the precipitation will not be directly related to the actual low pressure system over the Gulf of Mexico. As the cold front stalls, moisture from the Gulf of Mexico will be moving from southwest to northeast. The strengthening thermal gradient from the coast to the interior at the boundary layer and the tightening gradient at 850 MB will support strong frontogenesis from 950 MB to 700 MB from Monday night through Tuesday. Given the uncertainty of the location of the cold front before stalling and the speed of the cold air mass at the surface, I am not prepared to label a rain/snow line nor accumulation amount. However, I can say that these set ups can produce surprising precipitation amounts above forecasted guidance do to mesoscale lifting parameters which can not be forecasted just yet. What I can tell you is that some locations over the forecast area will get a moderate snowfall out of this frontogenesis. My best guess is that will occur from the northern suburbs of Philadelphia, north of Trenton, through the southern Hudson Valley, and into Connecticut. I would place the best potential there given the seasonal position of the coastal front this time of year and the nature of the trough axis on the average model guidance.
As for the storm itself. I think the majority of the precipitation of this storm is going to be too far east for the interior to see a moderate or heavy snowfall. The interior is basically northwestern Connecticut, the central Hudson Valley including Poughkeepsie and Albany, back through northeastern Pennsylvania. Points on south have the potential for a moderate to possibly heavy precipitation, but accumulation amounts would be tempered by the timing of transitioning from rain to snow.
My local forecast will keep the rain/snow forecast for now and I’ll wait on more data before going into more details. I think that’s the smartest way to go. So to wrap this part up, yes there is potential, but this forecast is no slam dunk as there are a lot of questions that need to be answered. There is support for this storm to be closer to the coast than model guidance suggest, but that doesn’t mean snow neccassarily because of boundary layer issues. That’s where I stand now.
After this storm, the pattern becomes quiet again. The trough over the East will produce below normal conditions through next Friday with slow moderation by Friday afternoon into the weekend. A few clippers may produce some scattered snow showers, but no significant precipitation is expected.
-- Weather When Posted --
- Temperature: 23°F;
- Humidity: 49%;
- Heat Index: 23°F;
- Wind Chill: 10°F;
- Pressure: 30.01 in.;
Defined Thoughts And Increasing Confidence
Posted by: | Comments10:52 AM
I’m going to cover all the thoughts for the next seven (7) days in this one post. The reason is because the short and medium range periods are really interconnected so there’s no point in splitting them up. So get that coffee cup and prepare to read a lot. I’m going to add more description in this post in exactly what I think will play out here. The good news is that despite the jumping around of the guidance, a basic idea is unfolding the question is now the details.
Today, the cold air is on the move through the forecast area. Most locations have already seen the highs for today and temperatures will only fall from this point on through the 40′s, 30′s, and 20′s. Before moving forward, we need to understand the type of cold air that is building into the forecast area. MOS Guidance has been trying to play catch up with the type of cold air moving in. This isn’t just cold air, it is dry, cold air. The type of cold air that does not leave quickly. More importantly, the cold air source over Southeast Canada is very impressive with temperatures in the 20 below range and dew points in the upper 20′s below range.
Another observation we should look at is the interaction of the moisture along the cold front and behind the cold front. Note that the cold front is well to the south, cutting through North Carolina. Yet, there is basically a “virga” storm over central/southern New Jersey, the Philadelphia metro, and points south. The strong initial dry air advection below 850 MB is preventing precipitation, but enhancing the fall of temperatures. The key point here though is the developments seen on the satellite images, strengthening mid level forcing over the southern half of the forecast area. This forcing is being produced by a tightening thermal gradient at 850 MB that will build down to the surface as the cold air moves into the forecast area. Is this development by itself meaningful? No, not by itself. But when you combine these observations with the pattern going forward, I’m getting some insights on how this pattern will unfold. So let’s go forward and look at this coming week!
Sunday Evening/Monday: Moisture is pulling over central Rockies and beginning to move east once again. For much of tonight through Sunday afternoon should remain dry as the area of moisture over the Southeast exits and some dry air works in ahead of the next area of moisture. The movement of moisture and the cold air in place is what is going to drive this pattern, and with disturbances exiting from the opening upper low over the Southwest, models are having a difficult time handling this pattern. Knowing this, that is exactly why I am not jumping around from model run to model run. Sorry, little bit of a rant.
Anyway, the moisture will move moving towards the southern half of the forecast area by Sunday evening. The strong thermal gradient in place over the forecast area at the surface and up through the mid layers of the atmosphere (850, 700 MB) will produce vertical and horizontal lifting of the atmosphere. In a more basic term, the atmosphere is favorable for rising air in this time period. The environment is set, the question is if a match will be struck.
Introducing match: Various model guidance is starting to pick up on a mid level disturbance at 700 and 500 MB that drives through the Mid Atlantic on Sunday night and Monday morning. Note, model guidance did not have any indication of this possibility 48 hours ago, but there it is now. Why? Because that is the type of pattern we are dealing with. This isn’t about one major storm, but a series of disturbances that will bring periods of light snow. An inch there. Two inches here. Before you know it, you have a snow pack developing and a more organized low on the way to complicate matters, but I’m getting ahead of myself.
The disturbance will race through Maryland and Delaware on Sunday night and Monday morning. I expect a band of light snow to develop to the north and east of this racing disturbance, which would be set up over central New Jersey through the Philadelphia metro and on south. Note the map here:
Blue: A period of light snow on early Monday morning. A dusting is expected overall, but some locations may reach an isolated 2 inches in a heavier band. The snow is exiting the forecast area by late Monday morning or early afternoon.
Light Blue: A few flurries, but nothing significant to impact travel.
Grey: Cold, a few clouds. A disappointed sigh.
I wasn’t expecting much in this time period at all, but the point here is that the models could not and have not handled this potential well at all. Note a few days ago, most model guidance had RH at 700 MB below 50%, now we have indications of snow. That’s why this is meteorology and not modelology.
Monday Night Through Tuesday Morning: The weak disturbance exits and clearing skies return. The thermal gradient at the vertical and horizontal remains, but the dry air advection behind this exiting disturbance will allow some clearing to take place. Temperatures will be cold, in fact I am staying below MOS guidance here in this period. I wouldn’t rule out a flurry from a passing mid level disturbance at 700 or 800 MB, but otherwise a dry period ahead of a more significant system.
Tuesday Night Through Wednesday Morning: I’m going with an average of model guidance for this period with some tips from the HPC, which has an excellent discussion this morning on this period. A significant amount of moisture is seen exiting from the west as the upper low opens and begins to move east as a progressive trough. An initial impulse or disturbance will move along the stationary front over the Mississippi Valley and Mid Atlantic on Tuesday night. I think model guidance is starting to pick up on the threat, but there is a lot of “noise” out there to make an accurate forecast. Taking a blend of guidance reduces that “noise” and taking a clue from previous similar patterns gives me an idea what to expect. The majority of the significant snow that falls on Tuesday night and Wednesday morning will be focused from central New Jersey through the Philadelphia metro and into Baltimore. This is based on what I think the low level and mid level thermal gradient will set up and the amount of moisture that will be present in the atmosphere. This disturbance will have influence from the Pacific but also interactions from the Gulf of Mexico. Be careful with the forecasted QPF amounts here. There is a growing potential for someone to get under a heavy band of snow due to the strengthening WAA. This disturbance has the potential to produce over 4 inches of snow from central New Jersey on southwards through the Philadelphia metro on Tuesday night. As for locations further north, I think there is a significant cut off in the precipitation area, and there is a potential for New York City to miss out here. In these types of storms there is a have and have nots that is pretty well defined. Monmouth County (for example only) could get 5 inches of snow while JFK gets just a few flurries. Yes, I think the gradient will be that tight. As far as precipitation type, with high pressure right to the north of this low pressure track seen again on the average of model guidance and using GEFS data, I expect all snow for this period throughout the forecast area. Again, to stress, this storm looks to be for locations SOUTH of New York City.
Wednesday Night Through Saturday: This period is like the end of a great classical ensemble. The trumpets are blowing, the drums are thundering, and everything is coming to a climax. To this point, the disturbances that have come through the forecast area are pieces broken off from the main upper low/disturbance over the Southwest coast. The driver of the southern branch is coming east here and will have a big story to tell.
First, I want to warn about the forecast of precipitation types here. I would advise hold off on any “rain” ideas just yet until we see the type of snowpack on the ground on Wednesday morning and the type of cold air advection that develops behind the departing low. Also, beware of the weak yet still present convergence zone over eastern Quebec on Wednesday night, which is supporting the CAD seen on the 06Z GFS and ECMWF. The cold air is not going to give up and leave. Yes, the warm air will eventually win out along the immediate coastal plain on Wednesday night and Thursday morning, but not before there is a significant period of mixed precipitation. The low level boundary layer is going to be difficult to determine and what’s even more troubling is the set up of the storm track. This storm track with a pronounced CAD in the mid range remember is from central Virginia through Delaware and southeast of the New Jersey coast. This storm track argues for a north to northeasterly wind, not east or southeast. We’ll have to watch how this plays out as far as wind direction and the boundary layer, but don’t rule out frozen precipitation type for a good deal of Wednesday night through Thursday.
Overall set up here is still not one strong low, but two developed, organized yet weak low pressure systems. The model guidance on the ECMWF suggest at most a 1004 MB low. Not exactly historic. That’s a huge indication on the type of pattern we are dealing with. The position of the main PVA is going to be key, but I am willing to bet the storm track is going to remain the same, but a slightly colder solution will play out. I expect two weak low pressure system to move south and east of the forecast area, with cold air remaining over the forecast area, especially at the low levels. I also think there is potential for more precipitation than what guidance is suggesting at the beginning of this next phase on Wednesday night. The initial strong push of warm air ahead of the develop low screams of a strong isentropic lifting over the Philadelphia metro and up through New York City on Wednesday night into Thursday morning.
So that’s where I pretty much stand for this up coming week. To recap:
1. Most of the action will be south of New York City Sunday through much of Wednesday.
2. There is a growing potential for a moderate snowfall over the Philadelphia metro on Tuesday night through Wednesday morning.
3. A more organized, but still overall weak series of low pressure systems will impact the forecast area Wednesday night through early Saturday morning. A variety of precipitation is expected with the potential for significant snow/ice for the majority of the forecast area. The cold air will lose some ground, but will not give in to the warm air advection as easily as model guidance suggest.
4. Models will continue to waffle with a great deal of uncertainty. Count on it!
5. The best way to forecast this pattern is take each disturbance individually. NOW-CASTING and reevaluation of the forecast will be needed. Someone is going to get a good deal of snowfall out of this pattern, the question is who and how much.
-- Weather When Posted --
- Temperature: 35°F;
- Humidity: 40%;
- Heat Index: 35°F;
- Wind Chill: 24°F;
- Pressure: 30.01 in.;
The Only Consistency In The Models Is Inconsistency
Posted by: | Comments8:32 PM
I’ve been getting a lot of emails today about the period from Sunday through Thursday. Actually, I’ve been getting a lot of emails over the past 4 days about this period. I want to just send an apology if you didn’t get a response, consider this post your response.
My confidence for the period of Sunday through Thursday is low. Very low in fact. Don’t confuse my low key attitude for this period as being unimpressed, the attitude I have is basically increasing uncertainty. Looks like I’m not the only one either as the fine meteorologist at the HPC seem to be equally unsure about the way this pattern will evolve and the just how well the models are handling the changing pattern. Again, I think the best approach is to do three things. One, learn from history. As you know, I’ve been studying the pattern (not the precipitation amounts) of the PDII storm and there are a lot of similarities. I want to stress that just because a pattern is similar does not mean the same results will transpire. Two, look at all the data and observations that is currently available. Three, confirm the aspects of the upcoming pattern that all the guidance basically agrees with.
Before I cover point one and two, let’s jump to three. All guidance agrees with the idea that a tight thermal gradient will set up through out the Mid Atlantic from the surface to 850 MB. In fact, the thermal gradient on many model guidance looks like a multi-layered colored cake, with all the creamy fillings. Another agreement in the model guidance is strong convergence and confluence over Southeast Canada and the Great Lakes, which will support strong high pressure to the north of a storm track. I am stating this similarity at the 500 MB level as models tend to have different depictions of the surface high from model to model and run to run. A third agreement is the idea of a split pattern with an active southern branch of the Polar jet stream sending a stream of disturbances along this stalled cold front. The disagreement between model runs and the models themselves is the intensity of each disturbance and naturally the location. So clearly, all the models agree that there will be weather. That’s about it.
So knowing that I would have better luck figuring out this pattern with an etch-n-sketch, than the model guidance. I think it is time to look at what we have in the real world. Currently there are three key features that tells me a lot about what will unfold. First, there is clearly an Arctic air mass on the move that is diving towards the Plains and heading for the East coast. There is not only strong model agreement on this idea, but you can actually see the Arctic air roll down the eastern slopes of the Canadian Rockies on the IR satellite images. It’s a pretty impressive site to see. So we know cold air is on the way. We also know that an upper low is spinning off the West coast. This upper low is diving south and will begin to slowly open up. This feature is very similar to what was seen in the pattern leading up to the PDII storm, which allowed the Southeast ridge to build but not dominate, thus forcing the thermal gradient a bit further north than expected. The third feature, which is associated with that diving upper low is the significant moisture that is just pooling over the Southwest and throughout California. It should be noted as well that California and the Southwest did get above normal rainfall ahead of the PDII storm event in the East. Another one of those pesky indications from the past, you know. So let’s summarize. Cold air? Check. Developing split pattern with upper low off the West coast to amplify but not agressively build the Southeast ridge? Check. A large moisture source building over the Southwest? Check.
Okay, so clearly we have the ingredients to make for a rather active and potentially snowy period for the Mid Atlantic. So let’s discuss what can go wrong here before I go into what can go right. The first warning I have to give out for all you snow lovers out there is that the high may end up to be too strong on Sunday night through Wednesday to allow any significant isentropic lifting to develop. In other words, sure there’s a great thermal gradient and a fertile ground for low level forcing, but the subsidence is drying out the air mass and killing any chance for snow. That’s basically what the model guidance (take your pick) is showing on Sunday night through Monday. The result of this type of situation playing out is the storm track remains too far south for anyone in the forecast area. Another problem I see in this pattern is the potential for phasing of disturbances and producing a low that is too strong. That would result in the Southeast ridge to build significantly in this period, thus bring rain to the entire forecast area. The ECWMF decided to take a turn with this solution after the GFS started to trend away from it. Personally, I think that is just cruel of the EURO’s to do, so I’m going to send a letter to the UN on everyone’s behalf here for an apology.
What can go right? Well, history tells us this pattern has a significant potential to bring a lot of snow to the forecast area via the impact of various waves of precipitation moving through the forecast area. The cold air remains in place as the high pressure system sits to the north of the forecast area, supporting a strong CAD. The moisture comes in waves with light to moderate snow overall and a few bands of heavy snow via mesoscale forcing. The low pressure systems are generally weak, which keeps the thermal gradient in the same position through the long period. That is the potential here and no model has ruled out the overall potential at all. In fact, the Canadian and ensemble guidance is suggesting this very threat.
Where do I stand?
Well, at this point, I am being very cautious with this forecast. I think it is a bad idea to go with the no snow idea right now. For the simple fact that there are “players” on the field that would at least argue that the potential is real and very much present over the CONUS this evening. I also don’t want to jump the gun here and scream big snows on the way. Why? Because a lot of that potential has to do with the development of individual disturbances at 500 MB AND 700 MB, which frankly can not be forecasted very well. Sure, we have better data in the Pacific in the 12Z guidance, but not to the extent where we can define such a forecast. I do think that there is potential for light snow over the Philadelphia metro and central/southern New Jersey on Sunday night into Monday. The average model guidance does suggest the potential for isentropic lifting developing via the set up of the thermal gradient and the developing mid level winds turning west and southwest on Sunday night into Monday morning. I would not be surprised if snow showers started to develop over the southern portions of the forecast area, and in fact some SREF guidance is beginning to pick up on this potential in the last 15Z run.
Going forward from Monday, a lot of the potential will have to basically be NOW-Casted. The developments of the previous day will have an influence on the next. The depth of snow pack, the position of the stationary front after a wave passes, and the amount of dry air in place after a wave of low pressure leaves will all have an important influence on the next wave. Further, the strength and position of each disturbance following Monday will also have a significant impact on the forecast. So if I sound vague here, it is because I think it is best to be so.
The theme here tonight is to take model guidance with a big grain of salt. Keep an eye on the water vapor and developments on the radar. Those that take a cautious approach to this forecast I think will do best, so that’s exactly what I am doing.
-- Weather When Posted --
- Temperature: 33°F;
- Humidity: 47%;
- Heat Index: 33°F;
- Wind Chill: 27°F;
- Pressure: 30.02 in.;
Forecast Ideas Coming Together, But Still A Lot Of Uncertainty
Posted by: | Comments7:23 AM
The basic ideas of the forecast for the period of Sunday through Wednesday is starting to come together slowly but surely. Let me first say the GFS 00Z and 06Z ideas have little support in developing a strong and defined trough in the West. As such, I basically through the model guidance out in my overall thinking. However, the fact the the GFS is over phasing disturbances in this period isn’t exactly surprising.
To start my forecast this morning, I decided to look at the actual water vapor satellite images for the northern hemisphere (at least on our side of the globe) to see exactly what type of moisture was building over the Pacific and West coast. I wanted to see the actual movement of this mid level moisture and the impacts at the lower levels in precipitation. Overall, model guidance had a difficult time picking up the actual precipitation over the West. Why is this important? Well, this gives me an idea on how well the data from the South Pacific and Pacific in general is being handled.
Let’s get started with the overall idea here. The pattern that is unfolding for this period strongly supports the foundation of deep Arctic air over the forecast period Sunday through Wednesday. If precipitation falls, it will be in the form of snow and/or sleet and I don’t expect any sort of rainfall out of this situation anymore. The ECMWF is keeping the majority of the precipitation suppressed to the south as strong high pressure appears to dominate the forecast area from the Great Lakes. Before you scream in frustration, and I know you guys in Philadelphia are right now. Let’s really look at the data here.
Note that there is a lot of moisture out over the West that is not being handled very well by the model guidance. Note that on the GFS, UKMET, Canadian, Ensemble guidance, and GEFS guidance all have a strong thermal gradient from the surface to 700 MB running from North Carolina through southern New York. We know where the best thermal gradient is, what about the moisture? The ECMWF is suppressing a good deal of the moisture to the south of the forecast area through much of this period. The GEFS is a bit further north. The UKMET appears to be a bit further north, but I have limited data to view the complete evolution on the guidance. The Canadian is also further north with precipitation threats. The difference in position of precipitation locations is how each model guidance handles the strength and track of each disturbance moving through the Mid Atlantic.
For now, in an attempt to give a nod to the ECMWF and Canadian guidance, I am going with a mostly dry and cold Sunday forecast, however I am keeping in snow showers for Sunday evening just to cover the potential here. I don’t like all that moisture over the West being completely suppressed to the south. I also think the best threat for precipitation on Sunday night will be south of New York City.
From there, I am going to put a threat of snow for the forecast area, specifically south of the Hudson Valley and Connecticut through the middle of next week. The forecast idea at the surface is that where the Arctic front will stall, weak areas of low pressure will capitalize on the deep moisture available from the South Pacific and Gulf of Mexico to produce a persistent band of light to moderate snow over the Mid Atlantic. Locations just to the north of this stalled out Arctic front are going to see a long period of cloud conditions and waves of precipitation. The most likely areas to see this unfold look to be from central New Jersey through the Philadelphia metro and points south. I think New York City has a shot to see the same, but the overall development of the pattern points to higher potential the further south you go. Don’t be surprised if the amount of precipitation in this period is greater than what guidance is currently suggesting.
As you can tell, I am very uneasy about this entire set up. The general synoptic idea is pretty well established, however the details and specific location of precipitation, disturbance, not to mention mesoscale lifting parameters are far from being settled.
I think the best route to take over this period is to keep an eye on radar and water vapor trends, update the forecast, and hang on. This forecast is likely to change.
-- Weather When Posted --
- Temperature: 17°F;
- Humidity: 61%;
- Heat Index: 17°F;
- Wind Chill: 7°F;
- Pressure: 30 in.;
Virga Building, Carefully Monitoring
Posted by: | Comments2:50 PM
If you are wondering why I am not shooting out rapid updates, it is because I am trying to be very careful in what I am seeing this afternoon.
Personally, the issue at hand is I don’t want to go off screaming snow storm developing and then it does. That would kill my credibility of my skills and this site. On the other hand, virga is developing all over the forecast area, when it shouldn’t. The satellite images are showing rising air throughout the coastal plain from the mouth of the Hudson Valley down to the Carolina’s. It is clear that a disturbance is starting to support cyclogenesis off the southern Mid Atlantic and Southeast coast. It is also clear that the primary low over the Great Lakes is weakening and thus so is the warm air advection over the region. This precipiation and virga is forming due to the interaction of PVA and the developing theramal gradient from the surface to 700 MB over the East coast.
As far as temperature trends, notice that temperatures have stopped rising over southern New Jersey. Virga over these locations are blunting the warm air advection. In fact, some locastions over east-central PA are observing temperatures falling AHEAD of the cold front.
So here is the question that is running in my head. If virga is developing along the coastal plain and there seems to be a developing thermal gradient setting up along the coast, then would this mean that precipitation is more likely along the coast. My instincts say yes, but I don’t want to jump the gun yet. If that precipitation over eastern North Carlina starts to back build and move into northeastern Virgina and the DELMARVA Peninsula, then we are looking a potentially HUGE bust in guidance. I’m not ready to make that announcement yet, but if the trends continue, I will have to.
In the meantime, the virga developing over the forecast area will continue to cool temperatures this afternoon and eventually snow and rain depending on locastion will fall. At this point, the precipitation is going to be light, so don’t expect a quick and significant accumulation. However, like I said, the trends are troubling for those that thought the forecast area would see nothing but partly cloudy skies and generally dry conditions.
-- Weather When Posted --
- Temperature: 35°F;
- Humidity: 43%;
- Heat Index: 35°F;
- Wind Chill: 35°F;
- Pressure: 29.82 in.;
Thoughts For Sunday Night
Posted by: | Comments9:10 PM
As I was alluding to this morning, model guidance is wavering on how to handle the intensifying trough and the large strong disturbance that will be diving towards the Mid Atlantic on Sunday and exploding somewhere off the coast on Sunday night into Monday.
As a forecaster, I know that trying to nail down details like low pressure track, precipitation amounts, and precipitation types is a futile attempt at scoring some sort of, I was the first title. No thanks! I’m not playing that game.
What I will tell you is this, the atmospheric set up for this period continues to be very favorable for an East coast storm on Sunday and Monday. Note that all guidance produces a strong and amplifying ridge over the West and the ridge axis remains over roughly western Idaho, which is a key signature for Miller B low pressure development. Why? Because the trough over the East is able to (1) dig into the Southeast, (2) enhance the jet dyanmics entering and exiting the trough, and (3) keeps the trough axis in a position to support a storm track close to the 40/70 bench mark. Combined these factors with an Arctic air mass in place, and potential is screaming for a winter storm.
Now, naturally, the model guidance is displaying all sorts of solutions out of the 500 MB pattern that is forecasted. The 12Z ECMWF backed off on the monster winter storm (for now) at the surface with a much less robust event for the forecast area, however the upper level pattern which supported the 12/14/09 00Z ECMWF solution remained. The UKMET was a bit more organized with a solution with a forecast similar to the older 00Z ECMWF as well. The GFS and Canadian seem lost, but is trending to catching on as the pressure field off the East coast shows a lot of “bagginess” and several weak areas of low pressure meandering off the coast.
So what the model guidance is trying to do is figure out what to do with the strong disturbance on Sunday (the clipper) and how to handle the strong PVA driving towards the coast. Similar set ups have produced major winter storms, in fact almost all of the KU storms show similar characteristics. However, I can also make a case that this storm can go out to sea if one of those disturbances kicks the whole storm east rather than enhance the overall storm.
So right now, I am standing pat with the forecast with the potential for a likely winter storm. I think by Thursday night or Friday morning, we’ll get a much clearer picture on what to expect on Sunday night.
-- Weather When Posted --
- Temperature: 21°F;
- Humidity: 41%;
- Heat Index: 21°F;
- Wind Chill: 17°F;
- Pressure: 30.15 in.;
Arctic Express On The Way Along With More Snow Potential
Posted by: | Comments7:49 AM
The theme going forward will likely crush the forecast for anyone going for a warm January. The combination of a strong negative EPO and the postion of the upper low over eastern Canada will support a very cold air mass to dive into the eastern two-thirds of the nation.
There can be no doubt that very cold Arctic air is on the way for the forecast area. The model guidance for the medium range is in amazing agreement on this. What’s also clear is that the forecast area will likely be under the influence of several passing and fast moving Alberta Clippers through the forecast area Monday through Wednesday, which will bring the forecast area a potential for light snow or snow showers each day. Where and how much is not forecastable at this time, but there is high potential for sure as the dry, cold air squeezes out all the moisture in the atmosphere.
Towards the end of the week, there are a lot of hints of some southern branch energy diving towards the Gulf Coast and develop a Gulf Coast low with plenty of moisture over the Southeast. This is screaming of a significant snow event for the Southeast. However, the set up over the atmosphere and the strengthening amplification of the pattern strongly suggest that this low will at least track towards the Mid Atlantic coast, producing snow chances, possibly heavy, for the forecast area. Especially for the immediate New York City and Philadelphia metros.
-- Weather When Posted --
- Temperature: 35°F;
- Humidity: 64%;
- Heat Index: 35°F;
- Wind Chill: 27°F;
- Pressure: 29.44 in.;
Thoughts On The Greenland Block Developing
Posted by: | Comments7:12 AM
I don’t think there needs to be any more debate on whether the NAO will go negative or whether a block will form over Greenland. The fact is that the block has already formed over the eastern North Atlantic (think northwestern Europe) and is starting to retrograde towards Greenland.
Now what does this block mean for the overall pattern going forward? Well, let me just say this. The models may want to break down the ridge over Greenland too fast as these types of blocks do not give way to easily. That is the very nature of an upper level blocking pattern. The influence of this potentially strong negative NAO will have lasting impacts through the rest of the winter in my opinion.
For one, the ability to keep cold air over the coastal plain will be greatly increased through the rest of the winter once this block becomes established. High pressure will, in most cases, keep high pressure locked in over Ontario/Quebec or at least slow the progression of the high pressure down much more than the previous storms this winter.
Another impact will be the ability for low pressure systems to redevelop off the Mid Atlantic coast and in a much fast fashion. Remember in 2007/08 winter where low pressure systems would race into the Ohio Valley and redevelopment would begin along the coast plain. However, the primary low remained over the interior for too long, ending any chance for the cold air to remain in place for much of the forecast area. That will not be an issue under this type of blocking influence. Low pressure systems like to go to and develop in areas of least resistance. A blocking ridge over Greenland is the exact opposite of least resistance and as such, low pressure that track towards the Ohio Valley will be more likely to redevelop and transition towards the coast much faster through the rest of the season.
Expect the overall pattern to slow down. What I mean is that the pace of the disturbances that dive into the trough will diminish slightly as a “log jam” develops over the north Atlantic. The result of this “log jam” would likely intensify the block further for mid January, and that in itself will have lasting influences into February.
Finally, as we look in general into the rest of January and into February, I think while the pattern will likely relax at some point, as amplification has to decline at some point. However, the potential for a negative NAO to quickly intensify again will be much easier to develop for the rest of the winter and into March. Unlike up to this point where we had to get all the pieces in order and had to wait for the “puzzle” pieces to be built, for the rest of the winter season, the pieces are already in place. The block simply intensifies and the 50/50 low strengthens, and the cold pattern easily returns. So any idea of winter ending after January (you know, the same people that were screaming blow torch for January) will be wrong in my opinion. Winter does not end in January, February, nor March. That’s where I stand.
-- Weather When Posted --
- Temperature: 33°F;
- Humidity: 80%;
- Heat Index: 33°F;
- Wind Chill: 27°F;
- Pressure: 30.37 in.;
Thoughts For The Next Few Weeks
Posted by: | Comments1:10 PM
I can bet there will be some out there screaming that winter is over and January is a warm month after the end of this up coming week. A large ridge is going to build over the eastern third of the nation while an upper low digs into Alaska shutting down the northwesterly flow over much of North America that has brought cold air south into the United States.
However, this is a relaxation not an end. The SST and stratospheric conditions continue to support a perdominant negative EPO pattern. Patterns always go through weakening and restructuring stages, and this one will be no different. So here is how I see the weather pattern going into January for the Pacific.
The last week of December through the last weekend of December will be very warm for the eastern third of the nation as the ridge over the Southeast will force the storm track from the southern Plains to the Great Lakes. Sorry, no white Christmas this year for I-95ers. Meanwhile, an upper low will develop over Alaska and drive south towards the Pacific Northwest, further enhancing the ridge over the eastern third of the nation.
Heading towards New Year’s day though, the Pacific will amplify once again. A trough will develop over eastern Asia, which will once again force a ridge to build towards Alaska. The first week of January, expect a split pattern in the works. The pattern will develop very similar to the pattern that is currently about to break down. The southern branch will dig into the Pacific Northwest as the upper low that was over Alaska continues to dig south, but also begin to weaken. Meanwhile the northern branch of the jet stream will dive from western Canada and into the Plains. Expect a slow and steady “cold press” over the northern third of the nation. We may be looking at another round of snow and ice events over the Mid Atlantic once again, but with a colder air mass.
The second week of January I think will feature a strong negative EPO feature. The thinking here is that the upper low off the Northwest coast will pump up the ridge over Alaska. Meanwhile, a ridge along the interior West will begin to build north as well. If the negative EPO ridge and the PNA ridge connect, watch out for a very cold middle of January. I’m not convince that will happen as all the cards will have to come into play, but the potential is there, so I thought I should mention it. The point is that the negative EPO pattern supports a cold pattern for the forecast area, not a warm one.
As for “big storms”, a lot will depend on the potential for a negative NAO actually developing. Model guidance and the trends in the stratosphere once again supports the potential for a negative NAO to develop in roughly 8 to 10 days and possibly remained sustained. However, we’ve all seen this before. Based on the data alone, I would say that a negative NAO should develop by the end of this year and have a major role in the pattern for January. However, I’m in a wait and see position until it actually happens.
-- Weather When Posted --
- Temperature: 35°F;
- Humidity: 93%;
- Heat Index: 35°F;
- Wind Chill: 27°F;
- Pressure: 29.5 in.;
Freehold
77°
Philadelphia
81°
Albany
76°
Sussex
73°
Allentown
77°
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