ICY Christmas night expected over interior, best snow pattern in years on the way

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.

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