Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181350

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
950 AM EDT Sat Mar 18 2017

Low pressure and associated cold front will move through the
area late this afternoon. High pressure then builds over the
area briefly, before another cold front crosses the area Monday
night. Stronger high pressure will take hold across the area for
most of the remainder of the upcoming work week.


An upper low is diving across the Great Lakes region this
morning, and will slowly traverse across the mid-Atlantic
through tonight. The associated surface reflection is rather
complex, with low pressure below the upper low, a second low
likely to develop near the triple point at the surface (over our
area), and then this evening the bulk of that energy will
translate off the coast and develop another surface low

For this afternoon, the initial shortwave will push across the
region. This turns on the upslope machine out west and also
serves to spark convective showers further east. South of the
triple point, the southern portion of our forecast area
(Charlottesville, Harrisonburg, Fredericksburg) will get into
the warm sector and see temperatures rise into the 60s. There
will be just enough instability that perhaps a rumble of thunder
or two will occur, and this has been added to the forecast for
just a few hours this afternoon.

As the shortwave pulls east, and the energy transfer occurs to
the offshore low, an inverted trough will be left behind and
that will serve as a focus for additional rain, especially in
the Frederick/DC/Pax River corridor and points east. Went with
likely PoPs for the morning update but if model agreement
persists with the 12z cycle, I may raise to categorical for the
afternoon update. Total rain could exceed 1/2 inch.

Some mixing with or changing to snow is likely late tonight at
the very end of the precipitation. However, surface
temperatures remain at or above freezing except right along the
Mason-Dixon line and at higher elevations. I kept the upslope
areas in the 1-2 inch range (similar to the previous forecast)
but I also added about 1/2 inch of snow in northern Carroll /
Baltimore / Harford Counties and around an inch on Catoctin
Mountain. Can`t rule out a dusting atop Skyline Drive either.
All totals look sub-advisory at this time.


Models now in agreement that precip ends by mid-morning Sunday,
with temperatures at least slightly cooler. Rather breezy though
so wind chills will be in the 30s to lower 40s all day. High
pressure shifts south Mon afternoon with scattered rain showers
expected with next frontal passage. Cold front clears the area
by 12Z Tue.


Computer model guidance is in good agreement about the extended
portion of the forecast. Aloft, troughing will persist through
the middle of the week, giving way to a ridge by Friday. At the
surface, a cold front will be settling south of the area Tuesday
morning. A wave of low pressure will ride along the front
Tuesday night, while a secondary front pushes south into the
area by Wednesday morning. Canadian high pressure will then
settle southeast, crossing the area Thursday and continuing
offshore Friday.

In terms of precipitation, the only notable chance comes with the
Tuesday night low pressure. While there is some spread in QPF
fields, the consensus at the moment has the best opportunity for the
southern quarter of the forecast area. Deterministic runs suggest
profiles will be warm enough for rain, but a few GEFS members crash
in enough cold air with the secondary front for a changeover to snow
before ending. At the moment, it doesn`t appear to be an impactful
system regardless of p-type.

The coldest period of the week will be Wednesday into Thursday
behind the second cold front, with highs in the 40s to lower 50s and
lows below freezing. The rest of the period will be fairly close to
seasonal normals.


Aviation forecast is pretty tricky. Rain showers could bring
some temporary vsby restrictions, but as precip mixes with or
turns to snow late tonight, both cigs/vsby could drop to MVFR or
lower for a few hours. Gusty N or NNW winds on Sunday.

A period of sub-VFR conditions is possible in rain and low clouds,
mainly at CHO, Tuesday into Tuesday night as low pressure moves
south of the area. Otherwise VFR prevails through Wednesday. NW
winds could gust to 20 kt both Tuesday and Wednesday.


Winds remain fairly light, but will strengthen this evening. SCA
begins at 11pm, which might be a touch early. Will assess that
further before the afternoon update. Model guidance and blends
show wind gusts that could get fairly close to gales on Sunday.
Definitely a solid 25-30 knots though. Although winds start to
slacken Sunday night, SCA gusts could persist into Monday for
portions of the waters.

After a cold front pushes south, NW flow will dominate Tuesday into
Wednesday, although it could become NE-N for a period as low
pressure passes to the south Tuesday night. SCA conditions will be
possible both days.


MARINE...Small Craft Advisory from 11 PM this evening to 6 PM EDT
     Sunday for ANZ530>543.


MARINE...JE/LFR/ADS is the U.S. government's official web portal to all federal, state and local government web resources and services.