4:23 PM  05/17/10

For once, dry air actually helped out the northern Mid Atlantic for at least a mostly dry day throughout the New York City and Philadelphia metropolitan area.  In the winter, this same set up would basically create havoc among snow lovers in the region as the storm would never seem to actually get to southern New Jersey.  Trust me, that won’t be the case here.

Steady moderate to heavy rain has been stalled over south-central Pennsylvania on southeastward through Delaware as a coastal low starts to take shape off the Delaware/Maryland coast.

The coastal low will overwhelm the mid level dry air and allow the rain to mark north tonight, with rain starting throughout much of the region around of just after midnight.  The latest guidance strongly suggest the best low and mid level forcing, due to an increasing thermal gradient, will be along the New Jersey coast and up through central and eastern Long Island.  The new 12Z ECMWF is the most aggressive here with over 2 inches of rain right along the Monmouth, Ocean, and Atlantic; New Jersey coastlines.

The latest water vapor satellite image gives us a good idea of where the weather pattern is going.  First, the obvious is the amount of moisture being drawn northward by the upper level low, and thus the potential for heavy rainfall tomorrow morning.  However, what’s caught my eye is what is happening out over the West!  Not the strong sinking air over the Plains and the trough developing over the Rockies.  This upper level pattern is changing and that change is a deep trough in the West.  Such a change in the pattern strongly confirms the idea of a ridge developing within the Polar jet stream over the eastern two-thirds of the United States.  Secondly, note that the upper level low over the Canadian Maritimes is moving east, shifting to an east based negative NAO.  All of the upper level features are moving towards a much warmer and more volatile pattern for the northern Mid Atlantic, that should be locked in place by the last week of this month.

So for tonight, the rain will continue to slowly work northward and eventually will overwhelm the dry air that’s been in place over the northern Mid Atlantic all day.  Rain will become steady after midnight and will be heavy at times along the New Jersey coast, the New York City metropolitan area, and much of Long Island.  Rainfall amounts will generally range from 1 to 2 inches along the coast and under an inch the further west you go.  The rain will taper off to scattered showers late Tuesday night through Wednesday afternoon.

High pressure will take hold again Wednesday evening through Saturday with clear skies, light winds, and temperatures moderating at a rapid pace back into the 70′s and 80′s once again.  In fact, there is still some debate as to whether I should include upper 80′s in this forecast for the Delaware River Valley.  I decided to hold off on that until I get a better picture of the pattern around the next upper low approaching the region.

That next upper low is expected on the door steps of the northern Mid Atlantic by Sunday morning.  Now, the pattern for the end of next weekend into early next week is very interesting.  There is growing support in the model guidance and in the teleconnections discussed above of a very warm, humid, unstable air mass over all of the Philadelphia and New York city metropolitan area by this time period.  At the same time, a cold, dry air mass at 500 MB or in the upper levels will be in place.  This is a recipe for summer afternoon thunderstorms.  The type of thunderstorms that can produce very heavy rainfall, frequent lightning, strong wind gusts, and even a rare tornado.  So this pattern will need to be watched carefully.  With high pressure off the coast and that upper low to the east, next weekend might be a very busy time for me.

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7:43 AM

I would love to say this morning that I know exactly what this storm is going to do and that I have complete confidence in the way the models are handling the moisture advection at the mid levels, the expansion of the precipitation shield, and the strength of the Sub Tropical disturbance.  If I could say that, then this forecast would be very easy.  Of course, I can’t say that and this morning, while issuing the first snowfall map, I am also going to address my concerns with this storm.

To the left is the snowfall forecast I am currently going with based on all the guidance I have available and some tweaking via observations of the radar and satellite images.  My original forecast for a moderate snowfall event for northeastern Pennsylvania through the New York City metropolitan area remains with generally 3 to 6 inches for New York City and 2 to 4 inches to the north over Connecticut and the Hudson Valley.  The heavier precipitation and best lifting will be focused to the south as this storm is expected to really nail Washington D.C. and Baltimore with potentially over 20 inches of snow.  The Philadelphia metropolitan area will come in to 6 to 12 inches of snowfall with higher amounts over southern and southeastern New Jersey of 10 to 15 inches of snow.  Cape May, New Jersey is looking like the prime area to be for regional snow lovers as up to 20 inches of snow will be possible.  Go figure!  Who would have thought Cape May, New Jersey would end up with more snow this season than Allentown, Pennsylvania?

All guidance agrees that this storm will impact the northern Mid Atlantic from late Friday evening through Saturday evening with a heavy, wet snowfall.  Winds will be rather strong, especially along the coast, with sustained winds of 15 to 30 mph with gusts exceeding 40 mph.  Along with coastal flooding concerns, these strong winds will likely lead to significant reduction of visibility through out the region on Saturday morning, and travel is not advised.

Now, on to my concerns, and believe me I have plenty.  But since I don’t think it would be wise to write 3 pages for this post, I will focus on my main concerns.

First and foremost, lets look at the recent trends on the water vapor and radar images this morning.  Note that the water vapor satellite shows an impressive plume of moisture racing out of the Gulf of Mexico and from the south Pacific.  I compared observed areas of actual precipitation versus the model guidance from 1 AM to 7 AM, and not one model has the development of the 700 MB moisture field nor actual precipitation field forecasted correctly.  This storm is on a trend to have a wider area of influence than guidance is suggesting.  Second, the strength of the Sub Tropical disturbance continues to verify stronger than the previous guidance suggests.  This again suggests that the models are playing catch up with the intensity of this storm at 500 MB.  Note that in previous runs the 500 MB trough over the Mid Atlantic was neutral on many guidance, and now the guidance is starting to suggest a more negatively tilted trough and a closed low developing.  Watch for this trend in the next 24 hours!

So what does these observations mean to the forecast?  Well, my concern is that this storm is going to end up slightly further north and exit slower than currently forecasted.  This concern is based on the strength of the moisture advection being observed and the strength of the actual Sub Tropical disturbance versus modeled analysis.  Now, this further north idea is not one that brings heavy snowfall into northeastern Pennsylvania, the Hudson Valley, and Connecticut.  I’m generally comfortable with my forecast ideas in these locations.  However from the New York City metropolitan area on south through southern New Jersey, I do have my doubts on limiting the expansion of heavy snowfall into these regions.

Consider this solution, which is very much a possibility.  The Sub Tropical and Polar disturbances phase on Friday night, the trough goes into a slightly negative position and closes off over the Delaware rather than off the Delaware coast.  This would push the storm track slightly further north, roughly 15o to 100 miles southeast of Cape May, New Jersey.  The position of such the surface low in combination with the developing 850 and 700 MB low would lead to the band of heavy snowfall over Baltimore to creep north up to central New Jersey down through southeastern Pennsylvania through southern New Jersey on late Saturday morning through Saturday evening.  Model guidance does now portray the low slowing in exiting the Mid Atlantic until Saturday evening, just further south due to the late development of the upper low at 500 MB.  The difference in this solution and the current forecasted snowfall is roughly 3 to 6 hours of intensification rate at 500 MB, which at this time period in the forecasting process is very small.

So while I do expect the majority of the heaviest snowfall to be focused over the southern and central Mid Atlantic, those over the Philadelphia metropolitan area on east through the immediate coast should not let their guard down with this storm.  A slight push north combined with a slowing down of the exit of the surface low on Saturday afternoon would lead to significant higher snowfall than expected for locations shaded in red.

After this storm exits, high pressure will take hold by Sunday afternoon through Tuesday with dry conditions and near to slightly below normal temperatures.  Highs will generally range from the upper 20′s to mid 30′s throughout the region.

Another potential snowfall will be possible for Tuesday night into Wednesday as another Sub Tropical disturbance races towards the Mid Atlantic coast.  The currently forecasted 500 MB pattern does not suggest that the Sub Tropical disturbance will be able to intensify like this current storm, but certainly a moderate snowfall is possible for the region.

7:30 AM

A cold front with an Arctic air mass behind the passage of the front is moving through the region this morning with scattered snow showers for much of the region.  The cold front is currently located in western Pennsylvania and will move east through the day.  Winds initially from the southwest will allow afternoon temperatures to rise into the 30′s and lower 40′s throughout the northern Mid Atlantic, changing the snow this morning into a rain/snow mix.

Tonight, the cold front will exit off the coast with winds veering to the northwest and temperatures crashing through the 30′s and into the 20′s.  The combination of strong northwesterly winds around 10 to 15 mph and the cold temperatures will create wind chills in the single digits and teens for many locations by tomorrow morning.  So get those jackets and sweaters ready, cause you will need them!  The rest of the day on Friday will have increasing clouds but very cold conditions with temperatures struggling in the 20′s and wind chills in the single digits and teens.

The water vapor satellite image this morning shows a lot of potential but also illustrates the one factor that will negate any potential for a significant snowfall for the Philadelphia and New York City metropolitan area.  As we can see to the left, a strong Sub Tropical disturbance is developing over the Southern Plains and driving a significant amount of moisture northward into the central Plains, Gulf Coast, and Mississippi Valley.  This disturbance is set to draw plenty of moisture from the Gulf of Mexico and is a display of what is now a very active Sub Tropical jet stream.  Unfortunately for snow lovers, there is one factor on the field that will keep much of that moisture to the south, and that would be the strong Polar Vortex diving into Ontario.  Note the area of dry air (in black) over the Mid West and south-central Great Lakes.  This is an area of convergence and confluence between the Polar and Sub Tropical jet streams.  On Friday through Saturday, that area of convergence will suppress any moisture from move north into much of the region, thus keeping the potential for snow near zero.

I don’t expect much change in this snow map over the next three days as there is pretty strong agreement on the model guidance and we can see the culprit that will suppress much of the moisture in the actual observations.  For much of the northern tier, only overcast skies are expected on Saturday through Sunday with no impact otherwise.  For the New York City and Philadelphia metropolitan areas, with the exception of a few snow showers or flurries, only overcast conditions are expected.  A stray snow show may produce a dusting of snow in this area given the very cold temperatures in place.  The accumulating snowfall will be focus over extreme southern New Jersey, basically in a line from Atlantic City to Salem.  The majority of the precipitation will remain south of New Jersey, however the northern extent of the precipitation shield will clip southern New Jersey with a light snowfall of 1 to 3 inches on Saturday evening.  The snowfall accumulations of up to 3 inches will be possible due to the thermal environment supporting a ratios of 15:1 with up to .2″ of precipitation possible.

Although I don’t expect any changes in the forecast, I will continue to monitor the storm for any changes, especially the position of the Polar Vortex, but any deviation northward in this storm looks to be minor at this point.

High pressure will dominant the weather pattern Sunday afternoon through Tuesday with dry conditions, scattered clouds and temperatures remaining in the 20′s and 30′s for highs on Sunday and Monday.  Moderation will begin to take hold on Tuesday afternoon as the high pressure system slides east and temperatures rising into the lower to mid 30′s over the interior and mid to upper 30′s along the coast.

A strong cold front and a Sub Tropical disturbance will move through the northern Mid Atlantic on Wednesday with scattered rain and snow showers coupled with falling temperatures in the afternoon.  At this time, the Sub Tropical disturbance is expected to remain weak.

7:49 AM

This morning we have an excellent example of why winter storms are so hard to come by in the northern Mid Atlantic and why all features must be in place just right.  In this case, the Sub Tropical disturbance certainly delivered with an energetic and impressive storm developing off the North Carolina coast.  A well defined precipitation shield has formed just off the southern New Jersey coast and the surface low pressure system is ranging from 995 to 998 MB over the past three hours, fluctuating through this morning.  Frustratingly for snow lovers in the Philadelphia and New York City metropolitan areas, this time the Polar jet stream was just in the wrong position with the trough axis over eastern New England and no blocking over the North Atlantic.  As a result, the upper level winds from the Great Lakes to the New Jersey coast was from northwest to southeast, suppressing the advance of moisture to the north and forcing the surface low pressure system due east.  Even still, the air mass in place was marginal at best so even if a phase had occurred with the Polar jet stream, the boundary layer would have ended up too warm to support frozen precipitation for many locations.

Surface high pressure currently over Ontario will build south this afternoon and evening, promoting clearing skies for tonight.  A very nice weekend is setting up for the northern Mid Atlantic as high pressure slides just to the east of the region, supporting dry conditions, clear skies, and moderating temperatures to near and above normal levels.  In fact, Sunday afternoon will have many locations along the coast approaching 50 degrees.  A strong southwesterly flow will develop on Saturday night into Sunday as the high moves into the Atlantic and a strong cold front approaches the East coast.  Changes are certainly on the way, and this January thaw is about to come to an abrupt end.

Note the 500 MB pattern by Sunday night from the ECMWF guidance.  A negative EPO pattern is rapidly developing due to the changes in the stratosphere (rapid warming, which has been covered for several weeks in various premium discussions) and a crashing negative Arctic Oscillation supports a new Polar Vortex developing over central Canada.  The MJO wave, currently in stage 6, strongly supports a changing pattern, which is exactly what happens as a new trough axis is repositioning towards the Eastern third of the nation rather than the Rockies.  As a result, at the surface an impressive cold front will march through the eastern third of the nation.  The strong southwesterly winds at the surface and mid levels will pull a significant amount of moisture northward along the cold front supporting periods of heavy rainfall on Monday morning.  I’m concerned that the Monday morning rush hour will be significantly impacted with flash flooding and poor visibility, and I can’t rule out a few thunderstorms embedded in this rainfall to make conditions rather interesting to say the least.  In short, Monday morning is going to be rather wet and stormy.

The push of cold air at the surface is going to be slow, thus the real impact of the new Arctic/Polar air mass sliding south from Canada will first be felt over the Plains and Great Lakes before reaching the northern Mid Atlantic.  Temperatures on Tuesday will still push into the 40 early in the afternoon before rapidly declining in the evening.  As the trough is established with an axis over the Ohio Valley on Tuesday through Thursday, a series of weak disturbances or cold fronts will move through the region introducing a fresh cold air mass and a few scattered snow showers.  High temperatures Wednesday through Friday will be hard pressed to break through the lower 30′s along the coast and will likely remain in the 20′s just away from the coast.  So the cold air is back and well sustained at the 850 MB, 500 MB, and even stratospheric levels.

By the end of the week, a very interesting set up is starting to come to light.  The Polar vortex slides east over Ontario and Quebec, supporting a developing negative NAO pattern, thus leading to convergence over Ontario and a surface high pressure locking in over the St. Lawrence Valley.  Meanwhile, the Sub Tropical jet stream remains active and sends an area of low pressure into the Tennessee Valley by Friday night.  With cold air in place along with an established blocking pattern intensifying over the North Atlantic, there is a significant threat for an accumulating frozen precipitation event for Friday evening through Saturday.  I am using a combination of the Canadian, ECMWF, and Ensemble guidance that continues to point to this threat for three model runs thus far.

By the end of next weekend, in a span of 10 days, a dramatic shift in the pattern will have taken place.  With the MJO finally moving towards the more El Nino enhanced stages 6 and 7 in this time period, the Southeast ridge is gone and a trough is in place as a series of Sub Tropical disturbances cut through southern California and towards the Gulf Coast.  The negative EPO/negative AO pattern is established and the growing warming in the stratosphere will support the high latitude blocking pattern to become established.  The NAO pattern wanes from neutral to negative and back to neutral, supporting a higher potential for storms to move northward along the coast rather than be forced into the Atlantic like what was seen in early January.  Further, the heart of the Arctic air mass will be reestablished over eastern North America with the boundary setting up over the Mid Atlantic coast.  All of this will unfold if the majority of model guidance, teleconnections, and observed atmospheric data is correct.  So those expressing doubts that winter will return, will get their answer ten fold by the time February starts.

8:15 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!