[12Z NAM forecast map for early Monday (green=rain, purple=ice, blue=snow); map courtesy "tropicaltidbits.com"]
Overview The combination of increasing warmer air in the upper atmosphere, moisture advancing from the southwest, and the residual effects of the currently entrenched bitter cold Arctic air mass is setting the stage for a significant icing event late tonight and Monday morning; especially, to the north and west of the big cities from DC-to-Philly-to-NYC. The increase in clouds today is associated with an area of moisture in the southern part of the country that is heading in our direction. It appears that it’ll stay cold enough at low levels of the atmosphere late tonight and Monday morning for an extended period of icing in suburban locations of the big cities along I-95 and slippery spots are likely for the morning commute.
Details DC metro area Precipitation begins most likely in the form of freezing rain between 10pm and 1am; however, there can be sleet mixed in at times especially at the onset. Snow is unlikely here, and if it does occur, it will be for a very brief period of time only. The freezing rain will continue into the early morning hours in the District and immediate suburban locations and into the mid-to-late morning hours in the farther out northern and western suburbs (e.g. Frederick, Loudoun and Carroll counties) where there can be a buildup of ice on untreated surfaces. Slippery spots are possible on roadways for the morning commute throughout the region. All areas in the metro region should see plain rain during the afternoon.
Philly and NYC metro regions Precipitation likely begins as snow between 3 and 6am and it could produce small accumulations of snow before changing to sleet and/or freezing rain. The sleet and/or freezing rain will continue for an extended period of time and a significant build of ice is likely on untreated surfaces; especially, in the northern and western suburbs. Slippery spots will have an impact on the morning commute. The icing continues into the early afternoon hours in the northern and western suburbs before finally changing to plain rain later in the day. The precipitation may change back to ice and/or snow late tomorrow or early tomorrow night before ending as colder air filters back into the region.
Looking ahead The upcoming week generally looks quite cold and there are two other potential precipitation events. The first such event will take place Tuesday night/early Wednesday as moisture from the south could push into the area causing some snowfall. Late in the week, there is likely to be a storm near the southeast US coastline. Current indications are that this storm will move off-shore, but the overall pattern suggests it is still worth monitoring.
A temporary break in the cold pattern is likely to begin later next weekend, but January is still likely to end up as cold or even colder on a nationwide basis than last year in a winter that is likely to go on and on along with these occasional warmups.