12:20 PM (Friday) | ***Cold, very dry, dense Arctic air paves the way for a wintry mess on Saturday with snow, sleet, freezing rain and some accumulations…watch for slick spots***
Paul Dorian
Different colors on this surface forecast map produced by the 12Z NAM (12-km version) suggest a wintry mess for the Mid-Atlantic region on Saturday at 3 PM to include snow (blue), sleet (purple), freezing rain (pink) and “plain” rain (green); map courtesy NOAA, tropicaltidbits.com
Overview
Dew point temperatures have plunged in the last 24 hours as a cold and very dry Arctic air mass takes hold in the Mid-Atlantic region and this will set the stage for a wintry mess on Saturday to include snow, sleet, freezing rain and some accumulations. This Arctic air mass is being anchored by strong high pressure building into the southeastern part of Canada and it will be very reluctant to give up its ground on Saturday as precipitation moves in from the Ohio Valley. As a result, temperatures will struggle to climb above freezing on Saturday; especially, in the northern and western suburbs along the DC-to-Philly-to-NYC corridor and this will likely result in an extended period of frozen precipitation.
Low-level temperatures will start the day at levels well below the freezing mark in the DC-to-Philly-to-NYC corridor (7 AM forecast map by the 12Z NAM (3-km version)); map courtesy NOAA, tropicaltidbits.com
Details
A strong cold front pushed through the region early yesterday and it ushered in an Arctic air mass anchored by strong high pressure to the north. As is characteristic of Arctic air, this air mass is cold, very dry and quite dense and this will lead to problems on Saturday. First, the extreme dryness of the air will result in “evaporative cooling” as the initial batch of precipitation arrives. As a result, temperatures will actually drop a bit in certain levels of the atmosphere enhancing chances for an extended period of frozen precipitation. Interestingly, the dry air will actually make difficult at first for the precipitation to reach the ground as it’ll evaporate until the lower atmosphere becomes sufficiently moist (aka “virga”). In fact, if you look at real-time radar maps at the onset of this event on Saturday they may indicate it is snowing when nothing is actually reaching the ground. Second, since the Arctic air mass is so dense it will be slow to retreat as milder air tries to push in from our southwest. The milder air will be forced to lift up and over the low-level cold air and this process should last for awhile as the Arctic air will be reluctant to give up its ground. As the milder air makes inroads in the upper part of the atmosphere, the snow will change to sleet and then to freezing rain with low-level temperatures hanging at or below freezing for much of the day; especially, in the northern and western suburbs along the I-95 corridor.
00Z Euro forecast map of dew point temperatures as of 7 AM Saturday indicating extremely dry air across PA, NJ and NY with single digits and even negative numbers; map courtesy Pivotal Weather, ECMWF
Precipitation is likely to arrive in the DC metro region during the early-to-mid morning hours in the form of snow or perhaps a combination of snow and sleet with likely accumulations of a coating to an inch or so. The snow is likely to start in the Philly area during the mid-morning to mid-day hours and accumulations at the front end can end up in the 1-3 inch range with the higher amounts in that range across the northern suburbs (e.g., Bucks and Montgomery Counties). In NYC, the snow will likely begin between late morning and early afternoon and there can be 1-3 inches of accumulation up there at the beginning of this event with the higher amounts to the north and west of the immediate metro region.
Low-level temperatures will continue to hover near the freezing mark in the DC-to-Philly-to-NYC corridor as late as 7 PM tomorrow night according to this forecast map by the 12Z NAM (3-km version); map courtesy NOAA, tropicaltidbits.com
In all areas, there may be a bit of a break in the precipitation after the initial burst of snow (or burst of snow and sleet in DC) and then sleet is likely to fall when the precipitation picks up again, maybe even for an extended period of time north of the PA/MD border. As the milder air pushing in from the southwest continues to erode away the low-level Arctic air, the sleet is likely to change to freezing rain and then ultimately, to “plain” rain in some areas with above freezing temperatures; primarily in the immediate urban areas and points to the south and east. The precipitation will wind down tomorrow night and then Sunday will turn out to be another cold and windy day as a reinforcing shot of cold air arrives and there can be some snow shower activity. Temperatures could drop into the upper teens for lows on Sunday night and again on Monday night as some of the coldest air so far this season begins the new work week in the Mid-Atlantic region.
The storm that will impact the Mid-Atlantic region and NE US on Saturday/Saturday night is now taking shape over the middle of the country; Long-wave IR image courtesy NOAA, College of DuPage
One final note, watch for slick spots on the roadways tomorrow - even on treated surfaces - when, for example, there is sleet falling as the ice pellets will likely not melt on contact with the asphalt given below freezing surface temperatures.
Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com
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