Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 040228

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
928 PM EST Sat Dec 3 2016

High pressure will gradually move into the region through Sunday.
An upper-level disturbance will pass through the area Sunday night
and high pressure will briefly return for Monday. Coastal low
pressure will impact the area Tuesday before high pressure briefly
returns for Wednesday. A potent cold front will move through late
next week.


Surface high pressure is located over the Great Lakes and Ohio
Valley region this evening, with an upper level ridge axis in
roughly the same area. However, the gradient remains with low
pressure over maritime Canada, and winds have been slow to
diminish across northern parts of the area. Likewise,
stratocumulus has been persisting in this area longer than some
models indicated. Meanwhile, fairly dense cirrus has been
overspreading the area from a storm system along the Gulf coast.
While the winds will diminish with time as the high pushes east
overnight, the same can`t be said about the cloud cover. A
thicker mid-level deck will be approaching by daybreak. However,
hourly forecast temperature traces are verifying well, leading to
little change in the min temperature forecast of upper 20s to mid
30s. Some variance could occur due to local differences in
clouds/winds though.


Two systems will move across the region during the short term.
Both of them have the potential to bring precipitation other than
rain across parts of the area, primarily north and west of I-95
but possibly reaching the metro. Antecedent conditions suggest the
Tuesday event is a bit more threatening.

On Sunday, high pressure will slide east across the region as a
shortwave trough approaches from the west. Warm advection will
bring plenty of clouds through the day, with temps likely to
struggle to rise - highs should be mostly in the 40s. Warm
advection precip is likely to start breaking out late in the day
in SW VA and S WV, moving across our region on Sunday night. Low
level temps look warm on most guidance, but the mid level dry air
is more suggestive of snow or sleet possibly reaching the surface,
especially at the onset, thanks to evaporative cooling. Given warm
ground and a afternoon in the 40s with no chance for radiational
cooling, think impact to travel should be minimal with
accumulations hard to come by even in those locales that have a
period of all frozen precip, which is most likely to be NW of I-95
and closer to I-81. Higher terrain would be strongly favored for
any accums given the rapid cooling just above the surface, though
even in those spots it looks like accums would be perhaps an inch
or so. Lows Sunday night will be in the 30s, but look mostly above

This first system slides east on Monday and after some morning
clouds and perhaps a little lingering drizzle or light rain near
the bay, it should try to clear out during the afternoon with
temps trying to get back into the 50s. This is a short respite
however. In the evening, there is a window for radiational cooling
which could bring temps down to freezing or below before clouds
thicken again. The next system, which is currently sitting in
northern Mexico, will approach the region later Monday night with
warm advection/isentropic lift precip breaking out late at night
and early in the morning from SW to NE. With sub-freezing temps
possible, have highlighted this time period for potential impacts,
particularly near and NW of I-81. Still lots of uncertainty, but
guidance has trended a bit colder today, so the chance exists for
accumulating snow and/or ice primarily NW of I-95, with some mix
possibly reaching I-95 itself. Lows Monday night will be near
freezing especially NW, with highs struggling to reach the 40s on
Tuesday. Given this is a wedge situation, it would not be shocking
if many spots along I-81 got stuck in the 30s on Tuesday.


High pressure will build in on the west side of the coastal low
Tuesday night through Wednesday night. Drier air will filter in
from the northwest and temperatures will remain cool. Any
precipitation will be rain showers near the Chesapeake Bay and
snow showers at higher elevations of the Potomac Highlands.

The next storm system will approach from the southwest Thursday
and Thursday night. A chance of rain showers exist across the
region each period. We can`t rule out some wintry precipitation as
well, depending on the track of the storm system and whether or
not it repeats the previous storm`s process of transitioning its
energy to the East Coast. There is moderate to high uncertainty on
the exact outcome at this time.

A chilly dome of high pressure is expected to build in from the
west Friday through Saturday. This high could bring the chilliest
air so far this Fall. Some upslope snow showers could also occur
in the Potomac Highlands.


VFR thru Sunday afternoon with mainly mid and high clouds,
although stratocumulus has been slower to clear across northern
terminals. Those clouds should gradually clear as high pressure
builds. Likewise, winds will diminish late tonight into Sunday. Sub
VFR cig/vis (with decent indications of IFR cigs) expected Sunday
night as rain and perhaps a bit of sleet and snow moves through
the region.

Returning to VFR Monday then back to sub VFR by Tuesday as
another wave of precip, possibly including freezing rain, moves
across the area with the next system. Improving back to VFR
Wednesday and Thursday.


NW winds have been slow to diminish this evening, so have made
some adjustments to the SCA...namely extending until 3 AM for the
Potomac as winds aloft don`t diminish until that time. High
pressure will be overhead by daybreak Sunday, with further
lessening of the winds.

Sub SCA winds expected Sunday thru Monday night as a weak system
brings some rain and perhaps a little sleet Sunday night, with
Sunday and Monday daylight hours being dry. Another system moving
up the coast Tuesday may kick winds up to SCA levels, along with
some rain and perhaps a little sleet or even snow, then dropping
back to sub SCA later on Wednesday.


MARINE...Small Craft Advisory until 3 AM EST Sunday for ANZ531-532-535-
     Small Craft Advisory until 6 AM EST Sunday for ANZ533-534-537-


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