Area Forecast Discussion
Issued by NWS Sterling, VA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

FXUS61 KLWX 171907

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

Low pressure will pass our region to the south this afternoon
and evening. High pressure will return Sunday and Sunday night.
A second, yet stronger, low pressure system will track across
the Tennessee Valley Monday. It will transfer energy to the East
Coast to allow for a developing coastal low to spawn that will
ride up the 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.


Weak energy is bringing some widely scattered rain and snow
showers across Maryland, eastern West Virginia and northeast
Virginia. Little to no snow accumulation expected across these
areas. For the most part, trace amounts is about all we
anticipate with this activity. This will be mostly across
central Maryland and the higher elevations of eastern West

Surface observations reveal a weak boundary across the Central
Shenandoah Valley and just south of the Virginia Piedmont.
Temperatures on the north side of the boundary are in the middle
30s to near 40. Temperatures on the south side of the boundary
are in the lower to middle 50s. It is along this boundary that a
weak low pressure system will ride from northwest to southeast
and bring a couple of rain showers to parts of the central
Appalachians, central Shenandoah Valley and lower Virginia
Piedmont late this afternoon into this evening. Rain amounts
will average a few hundredths of an inch and not everyone will
encounter one of these showers. The remainder of the night
tonight, drier air will filter in from the northwest as high
pressure builds into the region.


High pressure will build overhead Sunday 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.


For another hour or two, a brief period of light rain or snow
cannot be ruled out at the MRB, BWI, and MTN terminals. Brief
MVFR cannot be ruled out with this activity, but for the most
part VFR conditions will evolve elsewhere. High pressure will
keep VFR conditions in play through Sunday night.

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.


Small craft advisory remains in effect through 6pm for the upper
Chesapeake Bay. High pressure will build over the waters tonight into
Sunday. The pressure surge may cause a period of SCA wind gusts
late tonight into early Sunday across middle portions of the Bay
but confidence was too low for an SCA at this time.

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.


MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ530-


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