Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 211855
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
255 PM EDT Fri Apr 21 2017
A cold front push southeast across the region tonight. The
front will stall to the south Saturday and low pressure will
develop along the boundary...impacting our area for the weekend
through early next week.
.NEAR TERM /THROUGH TONIGHT/...
Latest surface analysis indicated a cold front is currently
stretched from near State College PA southwest across far
western MD and central WV into eastern KY. A stalled warm front
extends southward from near State College towards Baltimore and
Annapolis, then east across the Eastern Shore. Aloft, a strong
shortwave is located well to the north over the Great Lakes with
another over the western Plains. Weak ridging is present over
the southeastern US.
While its a pretty potent front and there is sufficient moisture
and instability across our region, the relative lack of mid and
upper level forcing seems to be limiting convection at the
moment. This may change over the next few hours now that the
cold front is entering the CWA since we will at least have some
surface forcing entering the picture, though the upper levels
still seem relatively weak forcing-wise. Have maintained chance
to likely pops into this evening with the front expected to
reach DC metro this evening, and clear the region by midnight.
Given the instability and good shear present, there is potential
for organized storms, but think that the lack of upper level
forcing means the marginal risk from SPC should cover the risk.
Will need to watch storm interaction with the stubborn warm
front located over Maryland, as that could be a focal point for
severe, but otherwise best instability is over Virginia. With
the front sliding down, precip chance should end from NW to SE
overnight, with temps sliding into the 40s NW and 50s elsewhere,
and humidity dropping.
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
The front stalls in the Carolinas Saturday as a slow moving wave
of low pressure moves eastward along it. High pressure nosing in
from the north may produce a dry period Saturday especially in
Maryland, but chance of rain extends through the day, with
greater risk as one heads further south and west. By Saturday
night, rain looks likely as the low approaches and some decent
FGEN develops over our region. This FGEN and the low itself may
shift southward on Sunday as a shortwave passes to our north,
displacing it, but the low is then expected to start pushing
back north Sunday night, ending any gap in the rain by the end
of the night. Given continuing uncertainty among guidance as far
as timing and amounts of rainfall, have not added any flood
mention to HWO yet, but certainly if some areas get the rain
modeled, we could have one. The question is whether several
inches of rain can indeed fall over any one area over the next
With cool Canadian high pressure wedging southward on the east
side of the mountains and the air mass becoming saturated with
the nearby front and approaching low, the weekend will be quite
cloudy and cool, with temperatures below normal. Sunday looks
like the coolest day, with highs likely to get stuck in the
50s. Temperatures Saturday will be rainfall-dependent, but may
also be stuck in the 50s. Lows at night will probably nudge into
.LONG TERM /MONDAY THROUGH FRIDAY/...
Low pressure along the southeast coast will remain cutoff from
the jet stream and slowly drift northeastward early next week,
keeping the area in a cool and dreary pattern. The wind and
subsequently the temperatures do an about-face around midweek
with significantly warmer temperatures (possibly near 90) by
next weekend. One interruption in the warming trend will be a
front boundary that may clip the region Thursday. Other than
that, temperatures will start the week at levels more typical
for early March, and end the week at levels more typical for
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Slowly improving conditions at BWI/MTN this afternoon as the
stubborn warm front slowly pushes east and low clouds/fog break.
Main concern through this evening is going to be scattered
showers and thunderstorms which may briefly reduce cigs and vis
while also bringing the potential for some strong wind gusts and
hail. This threat should end towards 0Z as the cold front slides
south and winds shift northerly. Should have a VFR period
overnight into early Saturday, but then rain may spread back
into the area as we head through the day with lowering cigs and
vis returning. Looking like a significant period of IFR is
possible Saturday night into Sunday night with northeast winds.
Sub-VFR likely early next week with low clouds/northeasterly
onshore flow around 10 knots.
Main concern this afternoon/evening is potential for a gusty
thunderstorm to cross the waters, necessitating a special marine
warning. Storm risk should end later this evening behind cold
front as winds shift northerly. SCA potential remains Saturday
as high pressure nudges south, but odds look pretty low still so
have not issued yet especially given relative stability. Better
risk of SCA Sunday as low pressure to the southwest starts to
intensify, increasing the gradient.
Northeasterly flow between high pressure to the north and low
pressure to the south may result in Small Craft Advisory level
gusts across the waters early next week.
A persistent onshore flow will develop behind a cold front
Saturday through early next week. Elevated water levels are
expected during this time...and minor flooding cannot be ruled
out near times of high tide...especially Sunday into Tuesday.