Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 182001

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
301 PM EST Sat Feb 18 2017

A weakening area of low pressure will cross the area tonight. A
backdoor cold front will enter the area Monday into Tuesday.
Another disturbance early Wednesday. High pressure will be
located off the southeast coast of the United States during the
end of the week.


High pressure is located off the southeast US coast this
afternoon. S/SW flow has been established, helping to push
temperatures into the 60s and 70s. One exception is by the
colder waters of the Chesapeake Bay, where a light onshore flow
is holding temperatures in the 40s and 50s as of 2 PM.

Closed low pressure in the upper atmosphere is located near
Memphis and will open/weaken as it moves east tonight. The arc
of precipitation associated with the low is expected to fall
apart as it heads toward our area tonight, but models vary on
what degree. Overall, atmospheric moisture is lacking and
forcing resides more in the mid levels, so any precipitation
that does fall will be very light rain showers (less than a
tenth inch) or just sprinkles. The highest chance will be in the
Potomac Highlands.

Increased cloud cover is more certain for tonight. Low
temperatures will only drop into the 40s...perhaps staying above
50 in a few locations.


Low level trough axis extending between low pressure in Quebec
and the weakening wave to the south will have cross the area
early in the day. This will turn the wind direction to the W/NW
and allow some cold air advection aloft. However, the
downsloping flow with good mixing will allow temperatures
similar or a few degrees cooler than today. Any morning clouds
will thin though the day.

Upper level pattern amplifies Sunday night into Monday, with
a shortwave trough deepening the mean trough in New England.
Meanwhile surface high pressure will be building into Quebec. A
backdoor cold front will slip southwestward into the area. Winds
will turn northerly on Monday, although cooler air aloft only
slowly seeps southwestward. This will lead to a tricky
temperature forecast, with temperatures near 70 likely
persisting in central VA, while northeast MD may struggle into
the 50s. Overall airmass remains dry though, with mostly sunny
skies across the entire area.

Flow turns a bit more onshore by Monday night as the low level
ridge axis moves into New England. RH fields are still on the
drier side though, with any sort of overrunning moisture signal
for low clouds generally limited to the southern Potomac
Highlands. The lows may depend on cloud cover, as dew points
drop into the 20s and winds will be light.


The tail end of a cold front, driven by low pressure departing up
near Hudson Bay, will approach the area Tuesday, then cross the area
Tuesday night. Most of the forcing will be removed well to the north
of the area, so expect most showers along the front to be relatively
scattered and in a weakening state as they pass through.

The front will stall just to the south Wednesday before returning
north as a warm front during the second half of the week. The next
upper trough/low pressure should bring a cold front near the area
during the first half of next weekend, though the track of the low
and best forcing remain uncertain at this time.

Overall, temperatures will be milder than normal by February
standards for much of the week.


VFR should prevail for the TAF period. SSW flow will turn W/NW
late tonight as a trough axis passes. Chances for precipitation
remain low, and if anything does occur, it would be more on the
order of sprinkles. Cigs likely remain above FL050. This chance
will end with daybreak. Some gusts of 15-20 kt possible as
mixing increases on Sunday.

A backdoor cold front will drop south toward the area Monday,
with flow becoming northerly. Model RH still do not indicate of
clouds, with only a limited signal for low clouds. Even if some
stratus does form, it may be VFR.

Mainly VFR expected Tue-Wed. Brief sub-VFR possible in -
SHRA Tue night. SELY winds around 10 kts Tue become SWLY behind
cold FROPA Tue night-Wed.


S/SW flow continues into tonight, generally 15 kt or less. A
trough will cross the area tonight and turn winds to the NW by
Sunday. Wind fields indicate marginal SCA conditions will be
possible by Sunday afternoon, but temperatures will remain warm
over cold water, so mixing is in question. The best opportunity
would be on the upper Potomac and nearshore/inlets of the
northern part of the Chesapeake Bay.

The flow will turn north Monday and perhaps easterly by Monday
night, as a backdoor cold front slips toward the waters.
Gradient appears to remain weak enough that winds will stay less
than SCA levels regardless of frontal position.

With a front stalled near the waters, the gradient/winds
should remain light enough to preclude any Small Craft Advisories
during the middle part of the week.




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