Rain to messy snow early Friday morning; greatest accumulation chances northwest of the Beltway

* Winter weather advisory for D.C.’s far north and west suburbs 6 a.m. to 2 p.m. Friday *

A sloppy mix of rain and snow will zip into the D.C. region Friday morning — the official first day of spring. Capital Weather Gang's Jason Samenow has your forecast. (Video: The Washington Post)

Friday is the official beginning of spring and the start of the Cherry Blossom Festival, but winter is in no hurry to leave.  A sloppy mix of rain and snow zips into the D.C. region late Thursday night and exits Friday afternoon.  The snowiest period focuses in the 5 to 10 a.m. time frame Friday — coinciding with the morning commute — when some grassy accumulation is likely and even some on the roads during heavier bursts, especially north and west of the city.

Key points

• We’ve increased the most likely snow accumulation in the immediate metro area to a coating to 2 inches, from a coating to 1 inch, as we see the potential for several hours of steady snow.  We’re also increasing amounts to 1 to 4 inches (from 1 to 3 inches) in our colder areas to the north and west (northern Montgomery and Loudoun County.)
• The big question is how much of snow the sticks given temperatures near to slightly above freezing (30 to 35) as it’s snowing. It will stick more readily on grassy areas and in our colder suburbs. However, as some of the heaviest snow will occur early in the day, before the March high sun angle can easily melt it, snow may cause challenging road conditions – with conditions deteriorating as you go north and west from downtown Friday morning during the commute.
• This will be an elevation-dependent snowfall.  That is, the heaviest snow totals will occur above 500 feet (in the hills of upper Montgomery, Fauquier, Frederick and Loudoun counties) and especially toward to the mountains to our north and west above 1,000 feet, where 3 to 4 inches or more are possible.
• We expect some school delays and closings, especially in counties west of Interstate 95. We’ll post our SchoolCast and FedCast mid-afternoon.

Timeline

• Precipitation begins in the midnight to 3 a.m. time period, maybe a little earlier in our southwest areas (toward Warrenton and Gainesville).  It may begin as rain or a rain-snow mix near and east of I-95, with more snow in colder areas north and west of the Beltway.  Temperatures will initially be in the 35-to-40-degree range.
• Where precipitation starts as rain or a rain-snow mix, it should transition to snow before dawn, except from around Fredericksburg east into St. Mary’s County, which may stay more of a mix.  Temperatures fall to 31 to 36 degrees, coldest to the north and west.
• Snow is likely, possibly moderate to heavy for a time, between about 5 to 10 a.m. (possibly starting as soon as 3 a.m. or so in our colder areas north and west), with the exception of our far southern areas which may have more of a rain-snow mix or alternating snow and rain.  Temperatures fall to 30 to 35 degrees, coldest north and west.
• After 10 a.m., snow may begin to mix with and change to rain from around the District and gradually work north by around the noon hour.  Temperatures rise 32 to 37 degrees.
• In the afternoon, mostly rain is favored.  However, some of our colder areas in Frederick County and northern Maryland may hold onto snow into the late afternoon.  Temperatures rise to 35 to 40 degrees.
• Rain ends between about 4 and 8 p.m. Friday from southwest to northeast.

Uncertainty

As we wrote yesterday, snow amounts will be very sensitive to the exact temperatures.  If it’s a little warmer than we expect or on the warm side of model forecasts, little or no snow may accumulate in the immediate metro area and this becomes mostly a non-event.  However, if it’s a little colder than we expect or on the cold side of model forecasts, this could turn into a disruptive event, with hazardous conditions more widespread over the area.

We will have additional storm coverage and analysis this afternoon.