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11:10 AM | ***First significant winter weather event of the season is on the way***

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Weather forecasting and analysis, space and historic events, climate information

11:10 AM | ***First significant winter weather event of the season is on the way***

Paul Dorian

 12Z NAM surface forecast maps from Thursday 7AM (hour 24) to Friday 1PM (hour 54) [blue=snow, pink, purple=ice, green, yellow=rain]; courtesy NOAA/EMC, tropicaltidbits.com

12Z NAM surface forecast maps from Thursday 7AM (hour 24) to Friday 1PM (hour 54) [blue=snow, pink, purple=ice, green, yellow=rain]; courtesy NOAA/EMC, tropicaltidbits.com

Overview

The first significant winter weather event of the season is on the way for much of the Mid-Atlantic region and Northeast US. Low pressure will intensify along the Mid-Atlantic coastline on Thursday and impact the region from early tomorrow into early Friday. Accumulations of snow and ice are quite likely from this first of the season winter storm – even in the metro regions of DC, Philly and NYC. Be prepared for slippery road conditions both tomorrow and tomorrow night as surface temperatures will not get too far above freezing during much of this upcoming event.

 12Z NAM forecast map at the surface level for tomorrow evening with the look of a “dry slot” in terms of precipitation as well as mixed precipitation associated with the upper-level low over the Ohio Valley and western Mid-Atlantic; courtesy NOAA/EMC, tropicaltidbits.com

12Z NAM forecast map at the surface level for tomorrow evening with the look of a “dry slot” in terms of precipitation as well as mixed precipitation associated with the upper-level low over the Ohio Valley and western Mid-Atlantic; courtesy NOAA/EMC, tropicaltidbits.com

Details

A classic setup for accumulating snow in the I-95 corridor usually includes strong high pressure located to the north of here over New England or southeastern Canada along with intensifying low pressure near or just off the Mid-Atlantic coastline and that general pattern appears to be unfolding for Thursday/Thursday night. A cold air mass has pushed into the Mid-Atlantic region in the overnight hours and it will be anchored on Thursday by strong high pressure to the north. As a result, moisture that arrives from the south early on Thursday has an excellent chance of being frozen – even in the big cities – and accumulations are likely before any potential changeover to plain rain. Preliminary estimates for accumulation amounts are a coating to an inch or two in the metro regions of DC, Philly and NYC with 2-4 inches possible in the northern and western suburbs. Precipitation likely arrives in DC around daybreak (i.e., 6 or 7 am), by mid-morning in Philly (i.e., 9-10 am), and mid-day to early afternoon (noon – 2pm) in the NYC metro region.

Temperatures will have a hard time climbing through the 30’s on Thursday thanks to the strong high pressure system to the north which will hold its ground for quite awhile. Frozen precipitation is likely to changeover to plain rain during the afternoon hours in the immediate DC metro region, but that transition will come later in the day to the north and west of DC and it’ll be a real struggle in all areas north of the PA/MD border. In fact, temperatures could hold pretty close to freezing for much of the day in the Philly and NYC northern and western suburbs with slippery road conditions quite likely to last all day.

 12Z NAM forecast map at the 500-mb level for Friday morning with a powerful upper-level low centered over New Jersey; courtesy NOAA/EMC, tropicaltidbits.com

12Z NAM forecast map at the 500-mb level for Friday morning with a powerful upper-level low centered over New Jersey; courtesy NOAA/EMC, tropicaltidbits.com

Potential “dry slot” and impact from a dynamic upper-level low

There is the chance that precipitation cuts off for awhile late tomorrow or early tomorrow night in parts of the I-95 corridor as a “dry slot” tries to form within the overall structure of the intensifying coastal storm. If so, this dry period will likely not be the end of the story as far as precipitation is concerned. In fact, there could be some interesting weather later tomorrow night into early Friday as the powerful upper-low spins overhead and this time period could feature gusty winds, maybe a rumble of thunder, and perhaps a burst of heavier sleet and snow as dynamic cooling takes place in the atmosphere. The storm will then pull away to our northeast during the morning hours on Friday and skies could turn partly sunny in the afternoon and it’ll turn a bit milder.

 12Z Euro model forecast map of total snowfall from upcoming event (using Kuchera method); courtesy ECMWF

12Z Euro model forecast map of total snowfall from upcoming event (using Kuchera method); courtesy ECMWF


A couple of final cautionary notes about this pending storm: (1) there is the chance that freezing rain lasts for several hours in some spots and if gusty winds do develop as expected, the potential ice buildup could become a problem for some tree limbs and branches, and (2) localized flooding may become a serious problem in some spots as plain rain could fall heavily at times - perhaps on top of ice and snow patches and fallen leaves.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com 

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