Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181847

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

A storm system will move offshore tonight. High pressure then
briefly builds over the area Sunday and Monday, before a 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.


As expected, showers have developed to our west this afternoon
in association with an approaching shortwave and cold front.
There have even been a couple thunderstorms near the surface low
center, which will move across our southern counties this
afternoon. As a result, showers look pretty likely with a chance
of thunderstorms. The storms may be vigorous enough to produce
small hail, perhaps close to severe-sized hail if a storm gets
particularly strong. The window of occurrence looks pretty
short -- perhaps 3 to 7 pm, but the convection has had a history
of small hail over West Virginia, and we are a little bit
warmer / more unstable. Thunder potential should generally be
south of the US 48/I-66/US 50 combined corridor.

The shortwave moves across the area, and as mentioned in the
morning discussion, energy transfers to an offshore low, leaving
an inverted trough behind, followed by the upper low itself.
This combination will keep precipitation chances in the eastern
half of the forecast area virtually all night. Forecast
soundings show precipitation changing over to snow at the tail
end of the precip (say after 08z tonight)...but temperatures are
at or above freezing and today has been reasonably warm with a
lot of sun. Temperatures do get close to freezing in extreme
northeast Maryland -- places like Frederick, Westminster, and
northern Baltimore and Harford Counties. I`ve added some light
accumulations to these areas (1/2 inch or less, generally), but
there`s some potential for around an inch of fresh snow there.
Further south in the cities, some flakes may fly late tonight
or early Sunday, but it currently looks too warm to accumulate.

Precip quickly ends Sunday morning, even in the upslope region,
leaving a partly to mostly cloudy day, and temperatures around
10 degrees cooler than today with a brisk north/northwest wind.


High pressure builds in for Sunday night and early Monday. A
westerly wind on Monday afternoon will help boost temperatures
into the mid/upper 50s or even lower 60s again. The 12z model
suite has delayed the onset of the next round of rain Monday
with another approaching cold front, and for most areas, Monday
should be dry during the day now. This next shortwave is rather
weak, with not much moisture return, so we are only carrying
chance PoPs for now.


A cold front will bring a chance for rain showers to the region
Tuesday and Tuesday night.

An area of high pressure will build southward from the Great Lakes
Wednesday through Thursday night. Drier air and colder temperatures
will ensue into the region.

This high pressure will move to the coast to allow for a return
flow to usher in milder air Friday and a chance for rain showers
Friday night and Saturday with an advancing warm front. The warm
front should move north of the region later Saturday, while even
milder air moves in from the south.


Although today turned out quite sunny, cigs/vsby will decrease
as the showers/rain move back in. Most of what is out there now
is VFR but after sunset, cigs will lower to MVFR and maybe even
IFR (IFR was only included in the MRB TAF though). Vsby is a big
question mark. Probably largely MVFR but a period of IFR is
likely, especially if rain changes to snow for a while,
accompanied by gusty winds.

Vsby improves once the rain/snow move out, and ceilings should
follow suit by midday Sunday. VFR after that through Monday.
Showers possible Monday night with some ceiling/visibility
restrictions possible.

MVFR conditions Tuesday and Tuesday night with a few rain
showers. VFR conditions Wednesday and Wednesday night. Winds
northwest 10 knots Tuesday and Tuesday night. Winds northwest 10
knots Wednesday becoming north 5 to 10 knots Wednesday night.


Winds have been light on the waters on this Saturday. Showers
move into the waters by early evening, and a thunderstorm is
possible south of DC/Annapolis with some small hail. By late
evening, northerly channeling develops down the bay with
sustained winds 15-20 knots and gusts 25-30 knots. Cannot rule
out a gale force gust but for now believe those will be further
down the bay into Virginia. As high pressure builds in Sunday
night, winds will begin to decrease especially in the northern
bay and tidal Potomac. Extended the SCA for the lower Potomac
and lower half of the Bay through Sunday night; additional areas
may need to be added to this if winds do not decrease as quickly
as currently forecast.

Winds light on Monday and Monday night with high pressure.
Showers possible Monday night.

Small craft advisories possible Tuesday, then again Wednesday. Winds
on Tuesday will be northwest 10 to 15 knots gusts 20 knots. Winds
Wednesday northwest becoming north 10 to 15 knots gusts 20


MARINE...Small Craft Advisory from 11 PM this evening to 7 PM EDT
     Sunday for ANZ530-531-535-536-538-539-542.
     Small Craft Advisory from 11 PM this evening to 6 AM EDT
     Monday for ANZ532>534-537-540-541-543.


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