11:55 AM | *Overall weather pattern looking more and more winter-like*

00Z EPS forecast map of 500 mb height anomalies on Sunday night with a strong upper-level low in the Northeast US; map courtesy tropicaltidbits.com

Overview
After a couple of warm days to end the week, a wintry blast will arrive in the Northeast US on Saturday night and it promises to be winter-like in the Mid-Atlantic region on Sunday and Monday with well below normal temperatures, strong winds, and perhaps even some snow shower activity in the I-95 corridor.  This weekend cold blast will set off the Great Lakes snow machine and there will be widespread accumulating snow as a result in areas just downstream of the water; especially, in higher elevation locations.  In fact, the Great Lakes snows may develop explosively this weekend given the expectation of cold, Arctic air flowing over the still relatively warm waters - thundersnows could be the result. The chill sticks around here early next week and then it should ease somewhat by mid-week, but we may then have to deal with a storm system by Thanksgiving Day.  Looking ahead, signs continue to point towards a more sustained colder weather pattern as we head through late November and into December.

00Z EPS forecast map of 850 mb temperature anomalies on Sunday night with a cold blast in the eastern US; map courtesy tropicaltidbits.com

Details
High pressure will slide off the coast by the end of the work week and temperatures could reach the mid or upper 60’s in the I-95 corridor on Friday afternoon and then it should stay quite mild on Saturday ahead of an approaching powerful cold front although clouds and wind may be on the increase during the PM hours.  This strong cold front will blast through the region on Saturday night – perhaps accompanied by some rain shower activity – and then colder air will pour in on increasingly strong NW winds with highs on Sunday confined to the 40's.  The combination of a significantly colder-than-normal air mass and still relatively warm water temperatures in the Great Lakes will crank up the Great Lakes snow machine and some areas just downstream of the water could get 6-12 inches of snow accumulation by early next week.  Sunday and Monday will turn out to be quite a bit colder-than-normal around here along with strong winds to boot and - given a strong upper-level system in the Northeast US - there will be plenty of instability in the atmosphere and the result could be some snow shower activity even in the immediate I-95 corridor.

00Z GEFS 500 mb height anomaly forecast map for Tuesday, November 29 with blocking pattern across Canada/Greenland (oranges) and an upper-level trough across much of the US (blues); map courtesy NOAA, tropicaltidbits.com

The chill sticks around as we begin the new work week and then it should ease somewhat by mid-week.  However, at this time a storm system may be gathering strength as it heads towards the eastern states from the Midwest and there can be an impact around here by Thanksgiving Day.  That potential storm could encounter enough cold air in the Upper Midwest and interior Northeast US for snow to fall at the onset in areas north of a line from about Chicago to New York.  

AO, NAO indices for the current and past (in black) and future (in red); courtesy NOAA

Longer term outlook
Looking ahead to beyond Thanksgiving Day, there continue to be signs of a more sustained cold pattern in the eastern US as a blocking pattern will likely unfold across the northern latitudes (Canada/Greenland). The prospects for frequent blocking events this winter was a key part to the colder-than-normal winter outlook for the Mid-Atlantic region.  High-latitude blocking is tracked by meteorologists through two indices known as the Arctic Oscillation (AO) and its closely-related cousin called the North Atlantic Oscillation (NAO).  When these indices are consistently negative during the winter season, the chances for sustained cold air outbreaks in the eastern US increases as blocking tends to form across the northern latitudes. In fact, both of these are forecasted to tank into negative territory later this month which favors the idea of blocking to form up north of here.

As November winds down, 500 mb heights are forecasted to become lower than normal (i.e., upper-level troughing, blues) across much of the US and, at the same time, high pressure (blocking) will set up shop to our north across Canada and Greenland (00ZGEFS forecast map above). The deep troughing across the US at this time is suggesting to me that the southern branch of the jet stream will become quite active from the Pacific Ocean to across the southern US which will increase chances for snow around here given the cold air supplied by the blocking pattern up to our north.  Indeed, the last month of the year is quite likely to turn out much different than last year when temperatures averaged well above normal (e.g., PHL +13.7 degrees) with numerous cold air outbreaks likely and some accumulating snow chances in the Mid-Atlantic region.

Stay tuned.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com