Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170826

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
326 AM EST Fri Feb 17 2017

High pressure builds overhead through late morning. A weak system
passes nearby Saturday night into Sunday morning. A backdoor cold
front will be near the region on Monday followed by the return of
high pressure.


Chilly to start today with nearly the entire CWA AOB 30F near
daybreak due to strong radiational cooling of a dry airmass. High
pressure has built, winds are relatively light and
wind chills aren`t too far below temperatures. This morning is the
last through this forecast period with lows below freezing.

Transition day today as high pressure crosses the area and slides
just to the east with southwesterly return flow developing by
afternoon. Onset of warm air advection has trended a bit earlier
from last night...which will mean slightly warmer forecast highs
across central VA (where U50s/L60s are likely). Elsewhere little
change to forecast highs. Near 50 F for DC/Balt...cooler (L40s)
across northeast Maryland where warm advection arrives later in the


Warmup continues Saturday...with highs nearly 20 F above normal.
M/U60s in most places...except right along the Chesapeake and Tidal
Potomac where U50s/L60s will be more common. While we start the day
sunny, clouds increase during the afternoon as a weakening
disturbance approaches. This disturbance will bring a chance for
some light precipitation to the area Saturday night into early
Sunday morning (mainly south of Washington DC). Any precipitation
expected to clear the area near daybreak Sunday leaving the rest of
the day dry with increasing sunshine. Highs again well above normal
(in the M/U60s).


A sharp ridge of high pressure over the area Monday will lead to
warm, tranquil conditions. While the same ridge will be the
influencing feature Tuesday, the result may be a little bit
different. Current guidance suggests that the ridge axis will be
atop the area, resulting in onshore flow. Hence, the forecast
will have more clouds and cooler temperatures. The timing and
strength of this will need to be watched. Potentially, this
could result in a marine airmass advecting inland which would
result in a much different forecast than previously advertised.
Points east of the Blue Ridge would be most vulnerable.

By Tuesday night, northern stream shortwave energy will be
flattening the flow aloft (both mid and upper levels...850 mb
and above). Heights looking a bit flatter than yesterday, which
is allowing both GFS and especially the ECMWF to bring a surface
front and associated precipitation east of the Appalachians
Tuesday night into Wednesday. Any precipitation would be light
(a few hundredths inch of rain). And, thermal profiles warm
enough for this to be definitively rain.

The subsequent front would be left behind across the area on
Thursday. It remains to be seen how strong it would actually
be. Consequently, forecasts will have mid range clouds and low
end (20%) PoPs.


VFR conditions expected to prevail through Saturday as high pressure
remains in control of the weather. Weak disturbance moves through
the area Saturday night into Sunday morning...with a chance for
light showers. Light rainfall rates should keep flight restrictions
rather marginal/brief...with perhaps a brief period of sub-VFR
(best chance at KCHO). VFR returns after daybreak Sunday.

VFR conditions should continue into Monday. Tuesday could be a
little questionable, as there is a hint of onshore flow. What is
yet to be determined is whether that would bring flight
restrictions inland due to low clouds/fog.


Winds have fallen off rapidly overnight as high pressure continues
to build overhead. Thus, expired the SCA headline a little early.
Relatively light winds expected through Sunday...with the typical
increases/decreases during the day/night due to diurnal mixing

Winds should primarily be 10 kt or less Monday and Tuesday. In
terms of direction... northwest on Monday, but becoming onshore
Tuesday as a ridge axis passes overhead.




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