Archive for maximums
Evening Thoughts- Breaking down the storm threat
Posted by: | Comments9:24 PM
I am very tempted this evening to issue text alerts about the potential for a significant winter storm for Sunday afternoon through Monday evening. The ingredients are there, and one of the most reliable model guidance for the 5 to 7 day period suggest the game is on.
This evening the first stage of developing an environment for potential accumulating snow as the Arctic cold front exits into the Atlantic. Temperatures that peaked in the lower to mid 50′s this afternoon are now crashing through the 40′s. The temperatures are 850 MB have also fallen below freezing, suggesting strong cold air advection is on the way. Conditions will be significantly colder tomorrow than what was experienced today.
Model guidance leading up to this weekend continue to have significant issues with handling the interactions of the two disturbances interacting over the Gulf Coast and the development and interaction of the Upper Low within the Polar jet stream. For tonight’s discussion I want to focus on the GFS and the ECMWF and keep other guidance of various solutions out of the conversation for clarity sake. These two models strongly make the case for the primary solutions of both camps.
ECMWF-Saturday Morning-Penn State
Despite the significant differences in the way the GFS and ECMWF produce a surface solution for the period of Saturday morning through Monday morning, there are actually plenty of strong similarities that strongly point to a potential storm for much of the northern Mid Atlantic.
For example, both guidance produce a building ridge over the West coast, with a ridge axis around Montana. The ridge in a building process enhances the jet stream dynamics downstream, which leads to strong support of impressive vorticity maximums. Both models suggest that the NAO will be trending weaker, which is a major warning sign for a developing Nor’ Easter as well. Both the GFS and the ECMWF also suggest that the Sub Tropical disturbance and Polar disturbance will have a strong interaction and some phasing will occur. Both models also suggest strong moisture advection from the Gulf of Mexico and the Atlantic with special enhancement from the Sub Tropical jet stream. Finally, both models strongly support a sustained cold air mass at the coastal plain down to the
GFS-Saturday Morning-Penn State E-wall
boundary level with the hint of a CAD and strong upper level support of high pressure or at least the influence of high pressure to the north of the storm track.
The key difference between the models is how the upper low over eastern Quebec will interact with the developing upper level features over the Tennessee Valley on Saturday morning. Note the difference between the two models over New England and the Eastern Great Lakes. The GFS produces an intense disturbance over the St. Lawrence Valley that lingers back through much of eastern Ontario. The trough and upper low takes on a strongly positive tilt which forces the strong developing trough to the south to take on a positive tilt as well. The result is that the storm track is suppressed and forced to move east into the Atlantic. The ECMWF on the other hand keeps the influence of this upper low over the Canadian Maritimes and over Quebec. The feature is much more representative of a negative NAO structure. The trough axis over the Tennessee and Mississippi Valleys is neutral, and as a result the storm is able to move further north than the GFS would suggest.
The reason why I would lean on the ECMWF is due to the recent errors in the model guidance overall and also the signs of what I think is the GFS solutions clear over phasing issues that plagued the early GFS version. So much for upgrades. The idea of the upper low dropping south far enough to produce strong PVA over Syracuse and Buffalo sounds off to me given the other upper level features. One must ask, what would cause the upper low to retrograde that far? What forces this development. I have yet to find a mechanism to displace what is essentially a 50/50 low on the ECMWF. I think the GFS is trying once again to produce a strong phase of these upper level features and thus altering the track of the surface low this weekend.
My thoughts for this storm continue to bring me to these key ingredients, plenty of cold air throughout the atmosphere, an ample source of moisture, and two strong disturbances driving towards the Mid Atlantic coast. These signs plus the others that I have listed over the past few days continue to strongly point me towards a potential winter storm for much of the Philadelphia and New York City metropolitan area. The exact track and the degree of expansion of the precipitation field is simply not known right now. However, the fact the Sub Tropical jet stream will have a major role argues for at least the potential for an expansive precipitation shield. Given the tight thermal gradient at all levels of the atmosphere in the vertical and horizontal, the development of CSI and other convective banding features will likely have to be discussed in the future as well. Further, I would keep an eye on the developments of the other upper low dropping south towards western Ontario, which may have the influence of slowing the exit of the storm than currently forecasted.
I don’t expect the model guidance to build a strong concenous either way until possibly tomorrow night or the Thursday morning model runs.
Beyond this weekend, additional threats appear to be possible with at least one clipper leading to a swatch of light to moderate snowfall over the northern Mid Atlantic for the middle of next week and a larger storm possible towards the end.
What? Did you think I was kidding about the end of this month being cold and stormy?
-- Weather When Posted --
- Temperature: 44°F;
- Humidity: 65%;
- Heat Index: 44°F;
- Wind Chill: 39°F;
- Pressure: 30.01 in.;
Thoughts On Model Guidance This Morning
Posted by: | Comments12:01 PM
I know a lot of you are about to jump off a bridge after looking at the 12Z model guidance. Well, sit down and just take a deep breath for a moment.
What did the models really say here? Did the models say that there was no negative tilt in the trough axis? No. Did the models produce no jet maximum on the western side of the trough? No. Has anything fundamental about the overall pattern changed? No.
In fact the only aspect that changed in the models is the position of mid level forcing. Note that there is VERY strong lifting just off the coast on all guidance. So is there an east trend and is this storm not going to happen?
Let me once and for all put an end to this model trend nonsense. There is no trend in model guidance. Period. You can’t have a trend when model guidance over the past 4 days have produced every type of storm track possible for this storm. Of course, the supposed God like ECMWF has to be right in days 4 to 7, except for the fact that in that time period model guidance has gone from a blizzard for Ohio to a light snow over the East coast. Not exactly a reliable track record for this pattern. I’m going to say this again. MODELS DO NOT CONTROL THE WEATHER. Period. There is a lot of research that proves model trends don’t exist and should not be relied upon for forecasting. Here is one link that has been circulating at Eastern US Weather for a few days that everyone should read.
As for the forecast, I wouldn’t change anything in the alert status or thinking of the overall pattern right now. Once again, the question is not IF the low will track towards the bench mark and produce precipitation. The question is the timing of when the trough goes negative and the intensification of the upper level dynamics like the development of the 700 MB low and 850 MB low, which leads to strong lifting along the coast, and therefore the heavy snowfall. There is very much still a strong potential for heavy snow to fall over the forecast area based on the way the pattern is currently evolving. The difference between a 3 to 6 inch snowfall across the forecast area and a foot of snow across the forecast area will be determined on when the 500 MB features intensify and at what rate. As such, I don’t think models nor humans can forecast such an intensification rate. The best way to illustrate this storm is basically how I have it laid out. Yes, there’s potential and yes snow will fall on Tuesday. How much is still very much unknown.
-- Weather When Posted --
- Temperature: 48°F;
- Humidity: 34%;
- Heat Index: 48°F;
- Wind Chill: 43°F;
- Pressure: 29.91 in.;
Afternoon Thoughts
Posted by: | Comments4:40 PM
I’m going to be heading out for the evening for a family event, but I want to leave some thoughts before I head out this evening.
Keep an eye on that moisture over south central PA. If that precipitation makes the jump over the mountains, that means that the mid level forcing is stronger than forecasted. If that band of precipitation jumps the mountains, then even Philadelphia could see some flakes from the first wave of precipitation tonight into tomorrow morning.
As for the secondary low. The issue isn’t the track of the storm, but the development and timing of the secondary low. The actual track is just fine for snowfall for this region. The issue at hand is that the mid level dynamics appear to take too long to produce much of any precipitation and the PVA lags behind too long to be much help. In yesterday’s 12Z guidance, the model had all features come together perfectly. It is clear, at least to the guidance, that is not going to happen.
What would I need to see change? Well, we are at NOW-CASTING stage now. For one, the vorticity maximum would have to interact with the coastal low much faster, which would enhance the mid level forcing at 700 MB and 850 MB. Is there still a shot that happens? Sure, but I doubt it. Never the less, I”ll keep an eye on the situation as clippers tend to do what ever they want despite what model guidance thinks. If you see an explosion of precipitation over the DELMARVA tomorrow evening, then you know something is up.
Have a great night!
-- Weather When Posted --
- Temperature: 17°F;
- Humidity: 40%;
- Heat Index: 17°F;
- Wind Chill: 17°F;
- Pressure: 30.38 in.;
Arctic Cold Followed By Major Winter Storm On Sunday
Posted by: | Comments7:45 AM
I wanted to make a post about the ideas I have on the potential storm on Sunday, however I saw some data on the model guidance coming in at 00Z and thought better to do some research first before posting. So off I went diving into the KU book for some additional guidance and words of wisdom. I think many in the forecast area are going to like what I found and am thinking now.
First, let’s deal with an equally dangerous issue that will be unfolding over the forecast area on Friday and Saturday, that would be the Arctic cold air. The 850 MB temperaturs will support bruttally cold temperatures at the surface with a range of 14 below to 20 below zero on both days, that’s in celcius at that level. At the surface, temperatures will struggle to break 20 on Friday even along the coast and will “moderate” slightly on Saturday with upper 20′s along the southern New Jersey coast. Not exactly a warming comfort. Combined those temperatures with the stiff northwesterly wind, and you can bet the Philadelphia Eagles and their fans are glad to be in Arizona rather than at home for this weekend!
As I have been saying for the past few days, if you do not need to be outdoors in this weather, don’t be. This type of cold air can cause frost bite and can kill. If you have to go outside, layer as much as you can and cover up as much as you can. This is not the type of cold air to try to be a “tough guy” in.
Now, on to the topic that everyone is likely excited about. The research I did on this period is based on the placement and intensification of the ridge out West and why this ridge is so important. In almost all the text book examples of winter storms, whether a postive or negative NAO state or what ever is going on in the Atlantic, a key indicator of a winter storm a few days out was the placement of the ridge over western North America. The ridge axis leading up to Sunday is right around western Idaho, which basically is a big flashing warning sign for me that a major east coast winter storm is on the way. Why? Because the ridge position and intensification strongly points to intensification and digging of the trough over the East coast and a trough axis that would strongly support a storm track from the Southeast coast to around the 40N/70W benchmark.
This piece of data seen on all model guidance on Saturday is why I upgraded to Alert Stage 2 for Sunday this morning, by the way.
Now, there’s a lot that I need to adress here first as far as model guidance. The Non-American model guidance continues to strongly advertise the potentail for Sunday. The ECMWF, beginning to enter a forecasting period of high accuracy level, strongly suggest a major hit for the forecast area with snowfall amounts of over a foot over the forecast area, yes even you Philadelphia. The UKMET takes a similar position as well. The Canadian trails behind, but struggles with the vorticity maximum diving into the trough on Sunday morning. The GFS is catching on, but is showing an error seen in the model for the entire winter with tracking the initial clipper way too far north, thus producing a redeveloping low over the New England coastal waters. However, the 500 MB level is beginning to resemble the ECMWF, and I expect a trend in that direction over the next few days. The NAM (I know, at 84 hours) does show a leaning towards the ECMWF with at least a similar position of features at the surface and upper levels.
Now, there is still a lot of uncertainty here. For one, these storms don’t like to invite everyone to the “party” and someone is going to miss out. The question that needs to be answered is at what location and time does the new coastal low develop and intensify. Expect the model guidance, all model guidance, to waver in the placement of the best QPF. The fact that the ECMWF has shifted to a very snowy solution last night is promising, but temper that excitement for a moment. If the low intensifies only a few hours later, than Philadelphia and New York City may see next to nothing due to a significant dry slow between the closing 500 and 700 MB lows and the deepening surface low. Further, the Arctic air mass will be moderating at this point and the position of the high pressure system is not supportive of CAD. I do think some locations like Long Island and the immediate New Jersey coast may have boundary layer issues. What may be an interesting interaction is the strong CAA behind the weakening clipper (old primary) and the bombing out of the coastal low. The timing and interaction of the coastal front along with the fresh introduction of Arctic air may have very interesting (and promising) developments with mesoscale forcing over the entire forecast area, but will have to wait at least until 24 to 48 hours before this event before discussing this in detail.
The point here is that the potential is high for a major winter storm and I think a winter storm is likely. Who gets what and how much is unknown, but you have to like the model trends leading up to this situation.
As for after this storm, I do expect a relaxation of the cold pattern for a brief period, however the negative EPO will continue to flex it’s muscles, and drive another cold, Arctic air mass into the eastern two thirds of the nation. There is potential for a very stormy and cold pattern setting up for late January and early February, but let’s deal with one storm at a time for now.
-- Weather When Posted --
- Temperature: 17°F;
- Humidity: 44%;
- Heat Index: 17°F;
- Wind Chill: 3°F;
- Pressure: 30.06 in.;
Active Winter Weather To Continue
Posted by: | Comments9:58 AM
The pattern will continue to amplify towards the end of the week and into the weekend. I want to touch on some points about the overall pattern first before discussion the forecast.
There is some discussion that the pattern appears to be like a return to mid December with a raging postive NAO. This is not true and I will tell you why. Look at where the strong upper low over Canada is located on model guidance up through early next week. The upper low through much of December was located over central and northern Canada, basically just east of the Yukon. As a result, the storm track basically was right over the forecast area, which produced storms that were snow to rain to snow and then very cold for several days. This time, the strong upper low sets up over southeastern Canada, which is a huge difference! The storm track in this set up is through the Ohio Valley and off the Mid Atlantic coast. The cold air stays in place while the -EPO pattern drives cold air into the central and eastern United States through January. Meanwhile, the influence of the strong -NAO block that is weakening will support a neutral to slightly negative look. No, the strong NAO block will be gone. That’s true, but if you love winter storms over the I-95, then you don’t want a strong NAO block, you want a weak and frequently transitioning NAO. That’s exactly what I see developing through mid January.
As for the forecast, on Thursday a weak disturbance will move through the forecast area with a few scattered snow showers, but nothing significant. The disturbance will do more in enhancing the Lake Effect Snow over Ontario than anything else.
Strong high pressure build build into the forecast area on Friday, supporting temperatures below normal and setting up a cold air mass for the next low pressure system moving over the Ohio Valley.
Now the atmospheric set up for Saturday and beyond through the next week is very interesting. We have a building ridge over the West coast and into Alaska with a strengthening negative EPO signature. The medium range model guidance of the GFS, ECMWF, Canadian, and Ensemble guidance all have various vorticity maximums or energy diving from northwestern Canada through the Plains and turning over the Gulf Coast, Mississippi Valley, Ohio Valley, and towards the Mid Atlantic Coast. The model guidance is having a difficult time handling the individual disturbances that move through the “long wave’ pattern here. For example, the GFS produces a clipper that drives through the Ohio Valley, redevelops along the coast, and produces a moderate snow storm for the forecast area. The ECMWF just sends a cold front across with a significant isentropic lifting event (possibly ice storm) over the Southeast and southern Mid Atlantic. The Canadian tries to combine the two threats into a clipper followed by a heavy frozen precipitation event moving towards the forecast area on Sunday.
The point here is that the potential is very high for a winter storm event Saturday through Sunday with more potential thereafter. How each disturbance moves within the pattern is clearly unknown by the guidance. However, teleconnections and previous experience in this type of pattern strongly points to a storm along the coast with the potential for significant frozen precipitation.
With the uncerntainty, I remain vague in the forecast for Saturday and Sunday. I think the best idea is to stay put and continue to advertise the threat. While others jump around with the guidance going storm, no storm, storm. No thanks. So yes, the threat is there. This period, late week through the middle of January may be the best potential for snow accumulation for the entire forecast area.
-- Weather When Posted --
- Temperature: 44°F;
- Humidity: 52%;
- Heat Index: 44°F;
- Wind Chill: 40°F;
- Pressure: 29.97 in.;
Major Winter Storm On The Way For Friday
Posted by: | Comments7:56 AM
This period of the forecast is growing increasingly interesting. While I continue to stress that the forecasting of snow/mix/rain lines is still around 24 (preliminary) to 66 hours away, the basic ideas in the model guidance are coming together for a significant storm to impact the forecast area. Let’s look at the general idea here.
On Thursday the 500 MB pattern is well into an amplification phase over the Atlantic. The Pacific pattern is not very favorable for a coastal storm to impact all of the Mid Atlantic, however enough building heights begin to develop over the West to make things interesting for the forecast area. Now, let me be clear, if not for the negative NAO that has formed and the strong 50/50 low by this time period, not only would this storm be a lake cutter, but the forecast area would have seen very warm temperatures ahead of this clipper. This is why the negative NAO is so important! Anyway, the model guidance is coming around to the idea that the position and strengthen of the 50/50 low will support the disturbance diving towards the Ohio Valley to dive further towards the Mid Atlantic by Friday morning. Meanwhile, over the forecast area, one high departs to the east while another stronger high pressure system develops over Ontario. A weak CAD will form along the coast reaching down as far as Philadelphia, but that’s about it.
Now, the period from Friday morning through Saturday morning will exhibit some very volatile weather conditions. The general idea here is that the energy/PVA can not move further east from the position over the Great Lakes due to the block, but must be forced to dive south and east towards the DELMARVA Peninsula and the Virginia coast.
As of this morning, almost all guidance has redevelopment of the clipper low somewhere between the Virginia coast and the New Jersey coast on Friday afternoon followed by intense deepening the following 6 hours.
The way I see this storm is based on the overall dynamics the models are illustrating, that a coastal low will form just south of the DELMARVA Penninsula and track to or just north of the benchmark of 40N/70W. The reason for this track is based on the position of the developing jet maximum at 200 MB, 250 MB, and 300 MB on almost all guidance. Further, the strongest PVA drives towards the Mid Atlantic and right through the forecast area on Friday evening on all guidance.
So by Friday evening, we have cold air in place. Note this is not brutally cold air, but basically marginal cold air yet cold enough for this storm track. The storm track and development suggest that this coastal low will be compact and the heaviest precipitation just north and west of the actual storm track. The rapid deepening of the low combined with the rapidly deepening 850 MB low in the 6 hour period strongly suggest that cold air will be pulled quickly towards the falling pressures off the coast. The crashing heights also suggest this.
This storm is not for much of the Mid Atlantic, that much is true. I don’t see storm having much of any impact south of Philadelphia and I do think there will be mixing issues for Philadelphia through southern New Jersey. However, I do think the the northern suburbs of Philadelphia, central New Jersey, and the New York City metro have better keep an eye on this storm for Friday evening.
Bust potential analysis: Obviously the storm track along with timing of when the coastal low develops will be key for this forecast, which is why any specific accumulation forecast is way too early. Also, there is a significant chance that potions of the forecast area will be dry slotted as energy transfers from the Great Lakes to the coastal plain. There should also be boundary layer issue for the immediate coast, which may produce a situation where precipitation starts as snow changes to rain and back over to snow. So there is a lot to iron out, but the general theme is that for the forecast area, this storm bares watching. For locations to the south, this is not a big deal at all, while New England may get the worse of this storm overall.
The pattern there after does not give anyone a break. The negative NAO and Greenland block will be firmly established over North America producing below normal temperatures over the forecast area. Meanwhile, the Pacific pattern improves with strong indications of a returning negative EPO AND positive PNA pattern developing. So don’t let your guard down as January looks to be a very active month.
-- Weather When Posted --
- Temperature: 35°F;
- Humidity: 64%;
- Heat Index: 35°F;
- Wind Chill: 30°F;
- Pressure: 30.06 in.;
12Z Thoughts And The Friday Evening Winter Event
Posted by: | Comments4:09 PM
I’m sure everyone has seen the latest 12Z guidance. The GFS is having trouble with the upper level features and the ECMWF is beginning to strut it’s stuff in the type 3 to 7 day span of dominance of accuracy.
Do I believe the ECMWF, well to an extent yes. I don’t want to put the cart ahead of the horse here, but there is a simple reason as to why the ECMWF produces an environment favorable for a bomb of a winter storm from Philadelphia through Boston, the position of the 50/50 low from Thursday through Saturday.
The important factors of the 50/50 low is the following. First, the ridge that is building over Greenland will continue to build, but I don’t believe the ridge will crush and flatten the 50/50 low as seen on the GFS. The reason why is because the 50/50 low at 500 MB will be intensifying due to a clipper than will strengthen off the coast of New England on Wednesday night/Thursday morning.
A word on the clipper. I like the idea of this clipper tracking through central Pennsylvania through central New Jersey and south of Long Island based on the position of the 500 MB upper level low in this time period. I think any track to the north of this position will force the low to shear out, and the best area of weakness on the model guidance suggest a track similar seen on the ECMWF/Canadian/NAM/WRF/UKMET family. I think there is a reasonable threat for some light to moderate snow to fall on New Year’s Eve for northern Pennsylvania, northern New Jersey, the Hudson Valley, and Connecticut with light snow over the immediate New York City metro. However, what’s important here is the impact of this clipper at the 500 MB level. This clipper will allow for the 50/50 500 MB low complex to intensify, which forces the storm track south.
As a result, the next clipper and 500 MB disturbance is forced through the Ohio Valley and to redevelop along the Mid Atlantic coast. With the strong PVA being driven towards a coastal plain with a tight thermal gradient and a strengthening jet maximum, the coastal low explodes with an intensification rate of over 2MB/hr on Friday night through Saturday morning with the low tracking from eastern Virginia to just east of Cape Cod, MA. Should the ECMWF be correct, then this would produce very heavy precipitation along the coastal plain specifically from southern New Jersey/Philadelphia through southern New England. Given the rate of intensification, the cold air would be drawn to the coast as the dense air tries to fill the rising air of the deepening low pressure system. With high pressure to the north around 1032 MB, a cold air source will be available to draw from and a CAD will be established ahead of the storm.
Based on the ECMWF, the suggest is there for an all snow event. However given that I don’t know the exact track, strength of the coastal front, position of the 850 MB low on Friday night, nor the degree of cold air available at the surface, I think it is wise to assume at this point that mixing issues will have to be handled. Still, based on the physics and dynamics of the situation, I would certainly lean towards more frozen precipitation than not for the forecast area for this event.
-- Weather When Posted --
- Temperature: 66°F;
- Humidity: 68%;
- Heat Index: 66°F;
- Wind Chill: 65°F;
- Pressure: 29.85 in.;
Winter Returns, But Questions Remain
Posted by: | Comments10:22 AM
The up coming forecast for the middle and end of this week is going to be very difficult. The overall idea here on all model guidance is that the negative NAO block will be established while a series of clippers move through the forecast area. One clipper intensifies, and produces a strong Miller B storm along the Mid Atlantic or New England coast. That’s the basic overview, which all models agree with. The question is in the details like when and where does coastal cyclogenesis begin? How strong is the disturbance that moves through the Great Lakes on Thursday/Friday? What position will the 50/50 low be in? I’m going to try to answer some of those questions today with the pretense that the forecast is likely to change. Let’s start with Wednesday.
By Wednesday, the 50/50 low will be well on the way of developing. A weak clipper with some reenforcing cold air will be moving through the forecast area. The low should track from the Great Lakes through the southern Hudson Valley with a band of light to moderate snow along and to the north of the low track. A few inches of accumulating snow will be possible with this clipper, especially over extreme northeastern Pennsylvania, the Hudson Valley, and Connecticut. A few snow showers can also be expected over the entire forecast area as the cold front passes. At the upper levels, the Greenland block will continue to build while the 50/50 low intensifies.
As we move to Thursday, some interesting and important processes begin. Strong convergence and confluence intensifies over southeastern Canada, specifically over Ontario and western Quebec. An area of high pressure will be over the forecast area, producing clear skies. However, what is important is that as the surface high over the forecast area departs, a stronger high pressure begins to build over Ontario. This is extremely important for the up coming storm. Why? Well, if the stronger high was over the forecast area, then southwesterly winds would develop over the coastal plain, thus producing strong WAA ahead of the approaching low presure system. Instead, the strong high, which will be strengthening through the entire forecast period, will be over Ontario. This means, the overall flow of air is from the northwest and north, not the south. This also means that the support of a developing CAD will be strong along the coastal plain.
At this point, we enter a very difficult period in the forecast. I’m still staying away from specifics for this time period, but I do have a general idea. I’ll say this, I don’t agree with what the GFS is producing. All model guidance agrees that a 50/50 upper low structure will develop. The question lies in the position of the 50/50 low and the strength of the disturbance that dives through the northern Plains and towards the Mid Atlantic/New England coast. The GFS representation produces a closed and intense low over the Great Lakes, which basically ignores the 50/50 low. The GFS also wants to make the disturbance over the Plains stronger than the developing 50/50 low. The ECMWF meanwhile has the disturbance over the Great Lakes as an open wave with the 50/50 low much stronger, thus forcing the disturbance south. Redevelopment along the coast happens faster and overall a colder solution is the result.
I like the ECMWF representation better because of the physically reasoning. I can’t find much upper level support in jet dynamics for either model to support the intensification and development of the Great Lakes disturbance to the degree that the GFS has. The GFS depiction appears to be a typical error of over amplifying a short wave disturbance in this time period. More importantly, the idea of such a disturbance being so far north with a strengthing negative NAO block in this time period argues against the entire physical/synoptic set up. Now, is it possible that the storm acts to some degree like the GFS is showing? Yes, if the 50/50 low is displaced too far north than the storm track is likely. I am siding with the ECMWF because the model makes more sense synoptically and also more consistent. While the GFS has been flipping between the current 00Z depiction and the 18Z depiction, which is similar to the 00Z ECMWF by the way.
The overall idea for Friday through Saturday is this. I think the clipper will dive towards the Ohio Valley on Friday. The upper level short wave will remain open and will not be as intense as the 50/50 low to the north. The strong convergence and confluence over Ontario/western Quebec will support strong high pressure over Ontario that will ridge down the coastal plain, producing a CAD effect along the coast. The strong thermal gradient along the coast will support cyclogenesis somewhere between southern Virginia and the Delmarva coastal waters. The forcing of the shortwave to the southeast due to the placement of the 50/50 low will support strong PVA towards the coastal plain and strong divergence and difluence with a developing jet maximum along the coast at 300 MB to 200 MB.
A developing coastal low will quickly race to the northeast on Friday night and Saturday morning. This low pressure system will take over as the primary and rapidly intensify along the coast or over the coastal waters of the northern Mid Atlantic and then somewhere along the coast or over the coastal waters of southern New England. The track at this point is obviously unknown and will have a significant impact on precipitation type and amounts. There is a significant potential for some locations to be dry slotted depending on the track of the storm and upper level features. All of those details are obviously unknown as well as precipitation type. What I am confident in is that locations south of Philadelphia will likely miss out on this event. I am also confident in that much of New England will get a good amount of snow out of this storm. The question lies in the impacts for the forecast area, namely the northern Mid Atlantic.
So that’s where I am now on this storm. With the way conditions are setting up, I issued an Alert Stage 2 for the entire forecast area, which means the atmosphere is setting up for a likely winter storm event.
-- Weather When Posted --
- Temperature: 62°F;
- Humidity: 77%;
- Heat Index: 62°F;
- Wind Chill: 60°F;
- Pressure: 29.96 in.;
A Snowy Tricky Forecast For Friday
Posted by: | Comments5:58 PM
The watches are going up and the excitement is growing. Well, for some at least. We have another winter storm on the way for the forecast area on Friday. However, I have a slightly different take on this forecast and I still have a lot of uneasiness about this storm in general, especially over the New York City metro through central New Jersey and the northern side of the Philadelphia metro.
I want to lay this on the line right now, I think this storm is going to be slightly further south than what guidance is currently suggesting. No, not enough to have flakes fly over Atlantic City, but far enough south to bring a dangerous mixed bag further south than forecasted on the guidance on Friday evening. However, I have my reasons, which I will explain in detail right now.
500 MB and the model trends:
I want everyone to take a good look at the guidance the past 24 to 48 hours for this storm. Notice that the entire structure of this storm has been pushed south almost continuously. The model guidance in all models has moved from a solution of taking this storm through northern Ohio and into central New York to now taking the low through southern Pennsylvania and off the southern New Jersey coast. The 850 MB upper low structure and the 500 MB PVA has moved in all guidance from central New York on Friday evening to southern Pennsylvania and southern New Jersey. So why have we seen this change in the forecast? Let’s take a look at 500 MB for Friday morning via the 12Z model guidance of the GFS, NAM, and ECMWF.
Now, notice what I have circled in all three model guidance. First, there is a small short wave over northern Canada that will dive towards northern Ontario on Friday afternoon, which has been showing up stronger with each run. We aren’t talking about a strong disturbance, but one that would increase the convergence over eastern Quebec and force any disturbance to the south to move east rather than northeast. The second feature of note is the trough over the Northwest, which will weaken in this period, thus supporting a less amplified solution of the ridge over the Southeast. Finally, we have a 50/50 low or at least a disturbance putting on a good imitation of a 50/50 low that is keeping the strong convergence and confluence in place over southern Quebec and thus keeping the high pressure to the north of the forecast area.
My forecast suggests that these three features will force the 850 MB low and the surface low along a track from the Pennsylvania/Maryland boarder through southern New Jersey and to the south of Long Island on early Saturday morning. This track will force the 0° C line at 850 MB to run between Philadelphia and Trenton through southern Monmouth County and south of Long Island. The areas to the north of that line will have sounding at or below freezing from 850 MB to the surface through the entire event on Friday night. Also, the strong thermal gradient at 850 MB and the strong Omega values forecasted at 850 MB and 700 MB strongly suggest impressive frontogenesis at the mid levels and I would probably extend that down to 950 MB. The best dynamics via jet maximum dynamics at 250 MB and strong PVA over all of eastern Pennsylvania through the entire forecast area strongly suggest that the heaviest precipitation will fall between southern New Jersey and the central Hudson Valley through Connecticut. There is a case to be made that Albany, although all snow will see less accumulation potential than Poughkeepsie or White Plains due to the location of the best forcing, but that will be tweaked with later. So here is the forecast, which can and mostly likely will change as we approach Friday.
The interior areas for this forecast was easy to forecast for as forecast sounds has the majority of these areas remaining below freezing through the entire event. The strong lifting described above will support moderate to heavy snowfall over these locations. However, the large spread that is seen here is due to the uncertainty of meso-scale forcing.
The next two areas are much harder to forecast for. Any change north or south of the 850 MB low will have a significant impact on the forecast for this area. For example, a push to the south of the forecast would bring more snow to New York City metro through central New Jersey, while a push north by 10 miles may mean a quick change to rain with no accumulation at all. How would you like to make this forecast? The basic idea is that the 3-6″ area will be an area of the “battle ground” where that 0°C line will hover around. As a result, I would expect a variety of precipitation for this area from a steady snow at the start to a mix of sleet and rain. Depending on the mesoscale forcing, a location could see a change from sleet and rain to heavy snow and back to sleet and rain as the mesoscale disturbance moves through. Further to the south, the snow will quickly change over to rain with around an inch to an isolated 3 inches away from the coast before being washed away. Along the coast and much of southern New Jersey will have plain rain from this event.
The snow that falls will be of the heavy/wet variety with a high water content. Those seeing QPF totals of over 1.25″ may want to stay away from the idea of receiving over a foot from this storm. Unless a very impressive mesoscale disturbance does some magic, the snow/water ratio will be too low (1″/8″ to 1″/10″ ratio expected) from this storm.
-- Weather When Posted --
- Temperature: 39°F;
- Humidity: 86%;
- Heat Index: 39°F;
- Wind Chill: 33°F;
- Pressure: 30.09 in.;
Freehold
77°
New York
75°
Sussex
73°
Allentown
77°
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