Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180009

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
809 PM EDT Sat Mar 17 2018

Low pressure will move across the Carolinas tonight, with high
pressure building over the mid-Atlantic on Sunday. Low pressure will
pass through the Tennessee Valley on Monday, with another
coastal low developing off the Mid-Atlantic coast Tuesday. A
third low pressure system will follow the coastal low and pass
our region to the south Wednesday. High pressure will return for
late next week.


The earlier rain and snow showers have exited the area, leaving
behind clearing skies from the north. However, the radar is
still green (as well as yellow and red) with showers and even
thunderstorms in association with an upper wave diving through.
Radar and hi-res model trends are shifting a bit further north
with this precip shield, so I`ve updated PoPs accordingly, with
chances roughly south of a line from Harrisonburg to Orange in
Virginia, and better chances the further south you go from that.
Will be watching the northern extent of precip closely, and may
have to increase PoPs even further later this evening.

There is a very, very tiny chance that if there is precip in
Pendleton or northern Highland counties, it could briefly be in
the form of light freezing rain around midnight. However, the
chance of any precip is only 20 percent and the chance of
freezing rain is even less than that.

There is quite a bit of model spread for low temperatures
tonight. I trended temperatures upward just slightly with the
update, but most places still do likely drop below freezing by
daybreak on Sunday.

Sunday then looks like the nicest day of the next week, with
plenty of sunshine, not as much wind, and highs that actually
get close to seasonal normals (mainly 50s) for a change. The
last time DCA had a March where we got this far along in March
and temperatures never rose to 60 degrees or higher was back in
1962. (Of course we topped 60 degrees 8 times each in January
and February this year, so March is the new winter, apparently.)


High pressure will build overhead through Sunday night. Dry and
seasonable conditions are expected. The high will shift to the
East Coast later Sunday night.

An area of low pressure is expected to track across the Tennessee
Valley Monday. As it reaches the Appalachian Front Monday night,
it will transfer energy to the East Coast in order to form a
coastal low. This newly-formed coastal low is expected to
intensify as it moves northeast and parallel to the mid-Atlantic
Coast Monday night and Tuesday.

Precipitation could begin as early as Monday afternoon in the
form of a mix of rain and snow. There should be rain since the
high pressure is along the coast and there will be an east to
southeast wind bringing modified March air onshore ahead of the
storm. There should also be snow since the high pressure that
will be at the coast will have already surged a swath of chilly
air along the eastern slopes of the mountains down into the
Carolinas about 12 hours before the precipitation arrives.

Gradually and steadily this wintry mix of precipitation will
overspread the region from southwest to northeast late Monday
through Monday night. Also, with cooling and saturation of the
atmosphere from top down as the coastal low develops at the
coast, precipitation should transition to all snow for a period
of time before either changing back over to a rain and snow mix
or remaining all snow and tapering off late Tuesday. That
period of time is the big question in terms of precipitation
types, precipitation duration, and precipitation amounts. Model
guidance is agreeing more and more each run that we are
anticipating a winter storm to some degree of magnitude. To what
degree and affect will it have on our region.

The latest forecast track takes a blend of model guidance which
continues to keep the best chance for snow north and west of the
Metropolitan areas. Confidence still remains low on the track,
plus it is low on location of developing coastal low, and how
much wintry precipitation is expected through Monday night that
will linger into Tuesday.


A low pressure system will be moving off of the NC/VA coast
early on Tuesday with a high pressure to our north centered over
Ontario. This system is expected to bring precipitation over
our area through the day on Tuesday as it intensifies offshore,
with precipitation transitioning from rain or rain and snow mix
to snow later on. The system will then move northeastward into
the west Atlantic Tuesday night into Wednesday with
precipitation lingering over our region into possibly late on
Wednesday. Latest guidance still differs on the solutions for
its track, which impacts the amounts and p-types that we could

A high pressure system will then build over our region Wednesday
night into Friday night before a frontal boundary could affect us
next weekend.


VFR with no aviation impacts through the valid TAF period
(Sunday night). Cannot completely rule out a shower or lowered
ceilings at CHO but the chance is about 25 percent as of the 00
UTC forecast.

Low pressure will track through the Tennessee Valley before
transferring its energy to a coastal low off the Mid-Atlantic
Coast Monday night. Rain will likely overspread the terminals
later Monday into Monday night. Rain could mix with or change to
snow Monday night as colder air works its way into the area.
SubVFR cigs/vsbys are likely later Monday through Monday night.

Sub-VFR conditions expected Tuesday into Wednesday with coastal
storm possibly moving near by during this time. Breezy
conditions on Tuesday and Wednesday, some gusts up to 20 kts
possible. VFR conditions expected on Thursday with high pressure
building in.


High pressure will build over the waters tonight into Sunday.
There`s still a chance of some SCA wind gusts late tonight with
northerly channeling. Will give this a further look in the
coming hours. Otherwise, winds should be light through Monday.

Low pressure will track through the Tennessee Valley before
transferring its energy to a coastal low off the Mid-Atlantic
Coast Monday night. A Small Craft Advisory will likely be needed
for the waters Monday night.

Wind gusts will be above small craft criteria Tuesday into
Wednesday as low pressure system moves near our region...
therefore a small craft advisory is likely these days. Winds
should decrease Thursday below criteria.




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