Sunday mid-day | ****Winter storm to impact southern Mid-Atlantic region…snow arrives in DC near midnight…before daybreak in Philly…little impact in NYC…a look ahead to more cold and storm threats****

A look at the major winter storm system on Sunday morning with radar echoes colored by precipitation type (blue for snow, orange for sleet, red for freezing rain, green for plain rain) and cloud cover (using IR satellite imagery). Map courtesy NOAA

Overview

Low pressure will intensify today as it treks eastward through the Tennessee Valley and reaches the coastal waters of the southern Mid-Atlantic by Monday evening. This system will continue to produce significant snowfall along the way with a foot in many areas and there will be a swath of disruptive ice immediately to the south of the heavy snow zone. In addition, there will be an enhanced risk of severe weather later today and early tonight in the storm’s warm sector centered over the Lower Mississippi Valley region.

Snow should arrive in the DC metro region around midnight and into the Philly metro just before daybreak. The heaviest snow in both places will likely be on the front end meaning late tonight and during Monday morning. In fact, there can be a lull in the precipitation on Monday afternoon and then some areas will receive “backend” snow on Monday evening; especially, in the region from DC to the Delmarva to southern NJ. The advance of the snow shield to the north will be inhibited by very dry air stationed over the Northeast US. As such, the storm’s effects will be more limited in places like Philadelphia compared to Washington, D.C. and virtually nonexistent up across New York City. Looking ahead, all indications point to plenty of cold weather left in January – potentially some extreme cold – and multiple storm threats including the possibility of one by the weekend.

A high-resolution forecast map of radar echoes for 8AM, Monday, with snow indicated by blue, sleet by orange, freezing rain by purple, and plain rain in green. Map courtesy NOAA, Weather Bell Analytics

Late tonight/Monday

Low pressure will reach the Tennessee Valley region by this evening and produce significant snow and ice along its path as it moves from west-to-east likely then to reach the coastal waters of the southern Mid-Atlantic by late Monday. Snow will spread to the northeast and reach the DC metro region near or shortly after the midnight hours and then the Philly area during the hour or two before daybreak. The snow is likely to come down quite heavily during the front end of the storm from late tonight into Monday morning in the DC-to-Philly corridor. In fact, there can be a lull in the precipitation in some areas during the Monday hours; especially, north of the PA/MD border.

“Mesoscale” or small-scale heavy snow banding is likely to take place in the region from DC-to-Delmarva-to-southern NJ during this upcoming storm system in the area of strongest “frontogenesis”…something we’ll closely monitor on Monday. Map courtesy NOAA, tropicaltidbits.com

The advance of the snow shield to the north will be inhibited by very dry air stationed over the Northeast US. As such, the storm’s effects will be more limited in places like Philadelphia compared to Washington, D.C. and virtually nonexistent up across New York City.

(Left) As a cold conveyor belt flows westward in the cold air mass north of a mature mid-latitude low's warm front, it splits into two tributaries. One tributary curves clockwise away from the low's center and eventually ends up flowing eastward. The second, more important, tributary turns counterclockwise around the low, coiling cyclonically around its center. (Right) The cold conveyor belt associated with a mature mid-latitude cyclone carries moisture back into the cold air northwest of the low. The combination of ample moisture, cold air, and upward motion can create heavy snow northwest of the low's center.

Credit: David Babb, Penn State University/College of Earth and Mineral Sciences

Monday evening “cold, conveyor belt” snow

By Monday evening, and with the surface low pressure out over the open waters of the western Atlantic, cool and moist ocean air - just to the north of a warm front - will be transported westward and “lifted” as it moves into the cold air sector to the north and west of the low…traveling along the so-called “cold conveyor belt”. This process helps to create the “comma-shaped” appearance to the cloud pattern in mature mid-latitude winter storms (and sometimes to the radar echoes as well). This “cold conveyor belt” snow is also referred as “wrap-around” or “backlash” snow and it can produce additional accumulations on the backside of the storm. This type of backend snow is likely to develop late Monday – perhaps after a lull in the precipitation - in the region from DC to the Delmarva to southern NJ and potentially as far north as the Philly metro area.  

Snowfall estimates:

DC metro region:

5-10 inches with isolated higher amounts of up to 12 inches or so. The heaviest snow is likely to be on the front end of the storm system from late tonight into Monday morning when it can come down at 1-3 inches per hour. There can be a lull in the afternoon before backend snow develops late tomorrow/early tomorrow night. Sleet can mix in as well; especially, across southern sections of the metro region.

Philly metro region:

3-6 inches with the heaviest snowfall likely to occur during Monday morning. There can be a lull in the precipitation during the afternoon before the possible return late in the day or early tomorrow night of “wraparound” snowfall on the back side of the storm system. Sleet can mix in as well; primarily, to the south of the metro region.

New York City region:

Nothing more than a coating to an inch.

An active southern branch of the jet stream assures the overall active weather pattern will continue well into the month of January. A very strong upper-level jet streak later this week in the eastern part of the nation may play a role in storm development by the weekend. Map courtesy NOAA, tropicaltidbits.com

A look ahead…more cold and more storm threats

This cold and active weather pattern for the central and eastern US is likely to continue well into the month of January and perhaps even into February. Teleconnection index trends such as with the NAO which remains in negative territory suggest colder-than-normal weather will continue for awhile in the eastern half of the nation. The index known as the East Pacific Oscillation or EPO drops into negative territory at mid-month – which is a cold signal – so it is possibly suggesting the coldest weather in this pattern may take place from mid-month to the latter part of January…stay tuned.

The East Pacific Oscillation index drops into “negative” territory at mid-month and this suggests the coldest part of this pattern may indeed be from mid-month to late month. Data plot courtesy Meteorologist Joe Bastardi of Weather Bell Analytics, NOAA

On the storm scene, an active southern branch of the jet stream will likely help to produce multiple storm threats in coming days. In fact, there is a decent chance that strong low pressure forms over the Gulf of Mexico late this week and it could threaten the Mid-Atlantic region and Northeast US with accumulating snow this weekend…stay tuned.

January is going to be a wild month.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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