7:45 AM

I have a lot of information to go over this morning so get your coffee or tea and get ready for a lengthy read.

This morning, the water vapor satellite picture shows a developing and energetic Sub Tropical Disturbance about to interact with a strong Polar disturbance racing south along the eastern Rockies.  Eventually these two disturbances will phase to produce a rather impressive blizzard over the Plains.

On a side note, I used to live out there and this storm would be crippling for many in that part of the country.  So if you have travel plans out there, I wish you the best of luck, you’ll need it!

Anyway, the feature that caught my attention is that the developing storm to the west, but the strong upper low to the east.  This upper low ushered in a reenforcing shot of cold Arctic air into the northern Mid Atlantic.  The position of this upper low coupled with the storm to the West is producing a strengthening ridge over the eastern Great Lakes that will slide east towards St. Lawrence River Valley.  The jet structure developing due to the interaction of all the features over North America would take about an hour to break down, but the overall theme has been described and compared to the January, 28 1994 set up for several days now.  What the water vapor satellite data tells me is that my suspicions were right about this set up and thus I am prepared to go into more details for this storm.

First, I want to illustrate the air mass we have in place over the northern Mid Atlantic.  There are two factors that everyone should key one.  The first is the obvious which is the cold surface temperatures in the 10′s and 20′s with highs today in the 30′s for most locations.  However, the most important factor is the dew points.  The dew points this morning are ranging from the single digits to upper 10′s depending on location.  This tells use that the air mass in place is not only cold but very dry.  This is going to have an impact on the start time of precipitation AND the potential for initial evaporational cooling for some locations.

Given the upper level structure and the development of the boundary layer thus far, my forecast idea is that the primary low pressure system will remain over the Great Lakes through at least Saturday morning.  The coastal low will have an influence in keeping cold air over the interior, however I do not believe the coastal low takes over as the primary as the UKMET had suggested a few days ago.  As such, the warm air will eventually win out, the question has always been when not if.

So here is the forecast!  Strong high pressure over Ontario will continue to ridge down through the coastal plain.  This will produce dry and cold conditions today through tomorrow morning.  The high pressure system will slowly slide east towards northern Maine while an occluded front approaches from the west and a warm front from the south.  Given the dry air, I’m delaying the start of precipitation from southwest to northeast until the early afternoon.  The forecast idea behind this map is based on the idea that this Arctic air is not going to exit the surface (especially the valleys) very quickly and with support from SREF, NAM WRF, and MM5 guidance.  I believe the coastal locations will not have a significant ice threat at all.  Precipitation may start as snow/sleet briefly on Friday afternoon but will eventually change over to rain.  This rain will be heavy at times through Saturday afternoon with over an inch of rain possible.  Just to the north and west, the cold air will linger a bit longer to allow for a brief transition to freezing rain.  A trace of ice is possible, however the warm easterly and southeasterly winds will help to erode the cold air mass fast enough to limit any impact.  The more significant ice problems can be found over northeastern Pennsylvania through the Hudson Valley and into Connecticut.  It is these locations where I believe the cold air will hold on the longest and will cause the most problems.  I also went back to previous ice threats of the past three years and noted a trend where the low level cold air tended to hold on longer than forecasted.  In this case, a strong high pressure system although leaving will continue to have an influence on the CAD that sets up.  I believe in this situation that it would be wise to error on the side of a slowly departing cold air mass at the surface rather than a fast warm up.

Eventually, all locations with the possible exception of the Mohawk and central Hudson Valley of New York, will change over to rain.  I’m staying below guidance on the high temperatures for this event.  I am banking on the melting effect of this snow pack to have an under forecasted cooling factor on the air temperatures given the basic laws of thermal dynamics.  As such, I’m keeping high temperatures on Saturday in the upper 40′s at warmest over southern New Jersey and Long Island.

A new strong Arctic high pressure system will take hold with relatively dry conditions and cold temperatures.  The high pressure system will be centered over the Ohio Valley through much of the week, producing a sustained northwesterly flow from the Great Lakes into the Mid Atlantic coast.  As a result, there is potential for a few stray lake effect snow showers off of Lake Ontario and Lake Erie to drift through some locations with a brief snow shower or two through the week.

As the negative NAO pattern becomes established and a ridge builds over the West, another potential winter storm will be developing over the Gulf Coast on Wednesday night.  There is strong support on the GFS Ensembles and the ECWMF that an impressive Gulf Low will race towards the North Carolina coast and towards the 40N/70W bench mark.  Obviously, the potential for changes in the storm track are there, however the set up of the 500 MB pattern on the majority of the guidance strongly supports the development and track of this storm.  We can also expect this storm to be moisture loaded as the low pressure system exits the Gulf of Mexico and pushes north into a sustained, fresh Arctic air mass.  I think this storm has potential to be another major snow producer for much of the northern Mid Atlantic and will have to be monitored carefully.

Once this storm exits, a pattern will unfold that I believe will produce a period of snowfall that will be talked about for some time.  I know this is a bold statement, but the data I am looking at points me to one direction.  Remember back in my winter forecast I stated that I felt the pattern will lock in by the end of December and that pattern will be the prevailing theme through much of the winter.  The pattern that locks in and has strong support on many of the teleconnections I’ve been using up to this point is a sustained negative AO pattern which supports a sustained negative NAO pattern.  Further, the continued convection around the date line points me to a sea saw effect between a negative EPO to positive PNA patterns over the eastern Pacific.  I also want to go on record on saying that I feel the coldest conditions will be located over the northern Plains to the Great Lakes, however the battle ground between the warm air masses and the Arctic air mass will be over the Mid Atlantic coast.  This position of the air masses, which we’ve seen hinted at all December, will support the potential for intensifying coastal storms in the coming weeks.  Further, I believe the strong El Nino will help to enhance this pattern with an energetic Sub Tropical jet stream.  Get ready because I can bet that with these disturbances that models will not catch on to the magnitude of potential winter storm until with in two to three days if not later.  In short, I see no support what so ever of any potential sustained warm up through early February.

7:04 PM

At around 6:30 PM, right around when I was going out to fuel up on coffee from D&D, light snow started to fall over Freehold, New Jersey.  The snow is very light and wet, which means that initially the snow will have some difficulty in sticking to surfaces.  Current temperatures in slightly above freezing, however evaporational cooling has begun as temperatures have fallen from 35 to 33 degrees in the past 30 minutes.

10:52 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.

7:48 AM

Obviously the news of the Arctic Blast that will impact the forecast area on Thursday through Sunday has been talked about in several areas.

The main threat I want to convey about this air mass is not to take this situation likely.  I have a strong feeling that the MOS guidance is still too warm for many locastions, especially over the interior where temperatures can really crash.  I have stayed below MOS guidance within reason, but I am still not confident that I am cold enough, especially for Friday. 

The main theme I want to convey is if you can stay indoors on Thursday night through Sunday, do so as much as possible.  This is not the type of cold air mass to play around with and try to “tough it out” with some hot coffee and some gloves.  Any exposed skin will have a potential to be inflicted with frost bite in a matter of minutes, believe me, frost bite is NOT fun.  It can be very painful for days, weeks, and even months in some cases. 

As for the forecast, temperatures are going to struggle on Friday and Saturday to break 20 along the coast and the single digits over the interior while lows will crash well below zero over the interior and around 10 degrees along coastal locations.  Any moisture in the atmosphere will lead to light precipitation as quick evaporation of the moisture can be expected.  So don’t be surprised if a snow shower passes your way unexpectingly on Friday and Saturday. 

Again, if you can stay in doors, do so.  If you know someone that must be outdoors, for what ever reason, do what ever you can to give them the proper protection in these elements.

8:41 PM

I can tell the excitement is building for Saturday based on the emails that have flooded my inbox this afternoon.  Yes, the model guidance, and I mean all model guidance, is picking up on the potential for the Alberta Clipper on Saturday. 

Before we get into the model guidance, let’s get back to the basics for a second.  I’ve been advertising for a few days now, that this pattern holds a lot of potential for the forecast area.  Note the amplifying ridge over the West coast that in turn amplifies the trough over the eastern two-thirds of the nation.  This amplification allows for disturbances, like the disturbance for this weekend, to dig further south and become more energized than one would think.  Those, my reason why I wasn’t convinced that this clipper for Saturday was a non-event and I sure as heck didn’t buy the idea that the storm track was cutting through Lake Ontario.  Not with the amplification out west and the lingering effects of the weakening negative NAO to the north.  All indications pointed to this disturbance being “more than meets the eye”. 

So what have we seen in the past 12 hours from model guidance?  The ECMWF has taken the lead with driving this clipper through the SOUTHERN Ohio Valley and through the Delmarva Peninsula.  There has been a significant trend south, and the forecast period is in the ECMWF’s bulls eye of highest accuracy.  So that model alone would give me confidence in a much more southerly track.  However, I also have added support of the 12Z and 18Z GFS, the Canadian, and the UKMET.  The SREF guidance is trending in the direction of the ECMWF, while the NAM is lost in the wilderness and is an outlier.  So why the change?

Remember a few days ago, I was concerned tha the models were leaving too much southern stream energy behind.  That I didn’t like the idea of a bowling ball over Colorado under cutting a growing ridge and a strengthening northwesterly upper level jet.  That didn’t make much sense to me even though everyone seems to jump on the gravy train because the ECMWF said so.  Therefore, I didn’t change my forecast.  Now, the guidance is keeping some of the southern stream energy with the northern branch.  The disturbance on ALL guidance is much stronger than forecasted a few days ago and much further south.  Therefore we are seeing a change in the surface reflections on the model guidance.  So is the 18Z GFS right with the heavy snow then? 

Well, I continue to say, never let you guard down with a clipper.  Now, look the 18Z GFS is not exactly alone here as far as the low pressure placement, PVA strength, and strong thermal gradient at 850 MB to the surface.  The 12Z ECMWF isn’t too far away from this look and neither is the 12Z Canadian either.  However, I suspect the GFS is a bit too robust with the QPF on Saturday afternoon and evening.  Is heavy snow possible?  Yes, it is!  If this clipper rapidly deepens off the Delmarva coast, then much of the forecast area, (even you too Philly!) can see a period of heavy snow on Saturday afternoon.  Ironically, this would set up perfectly for the battle between the Giants and Eagles.  Anyway, should the clipper deepen off the New Jersey coast while the 850 MB low develops, then significant rising air would be present along the coast along with strong frontogenesis, PRESTO!  Heavy snow.  So, keep an eye on the guidance the next 48 hours.  The key here is how fast does the ridge amplify over the West and the resulting strength and location of the PVA (disturbance) on Friday night.  If this PVA is trucking through the central Mississippi Valley, then get your snow shovels ready.

Now, as for the period thereafter.  I want to be careful with what I say here.  I do believe we are heading for a very cold period of weather.  Possibly the coldest we’ve seen in this decade, which isn’t saying much considering how warm the winters have been.  I continue to think that the Arctic air is coming, but I also don’t buy into the idea that the forecast area or the Mid Atlantic will be completely dry.  This pattern at 500 MB has a lot of potential.  The guidance continues to send various disturbances into the trough through the middle of this month and these disturbance tend to head towards the Mississippi Valley and drive towards the coast.  Overall, these disturbances will likely be suppressed and produce only light precipitation.  However, as many of you know, you only need one such disturbance to make things interesting.  So I would get the heavy jacket ready for sure, and keep that shovel on stand by.  Looks like I am going to need more coffee!