Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 271348
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
948 AM EDT Thu Apr 27 2017

.SYNOPSIS...
A weak cold front crosses the area from the west tonight. A backdoor
front will slide into the area Saturday evening into Sunday before
lifting north as a warm front early Monday. A stronger cold front
will move through the region from the west Monday night.

&&

.NEAR TERM /THIS AFTERNOON/...

Atmosphere is in a rather stagnant pattern. Without a wind to
mix out low level moisture has manifested itself as low cloud
and fog. On visible satellite this can be seen to be eroding
along the edges. This will continue during the morning. By
afternoon the sun should reappear.

Remainder of today will feature warm weather as ridging slides
across the area and moderately strong southerly flow develops.
Southerly wind gusts up to 25 mph by afternoon with highs in the
L/M80s and dewpoints rising into the L/M60s.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/...

Cold front will be approaching the area from the west this evening
and will weaken as it crosses the area. Thunderstorms likely west of
the Blue Ridge...as time of arrival near or shortly after sunset
will allow some instability to remain. While forecast soundings
suggest instability will be rather limited across this area (due to
relatively poor mid-level lapse rates)...strong shear could allow
for some organization into convective clusters...which could briefly
become marginally severe with isolated wind damage. SPC has outlined
this area in a marginal risk because of this.

Any storms will be moving into a progressively hostile environment
as they progress east of the Blue Ridge...since instability rapidly
wanes across this area. Thus, expect any storms to rapidly become
elevated and weaken as they approach the metros...with a rather
precipitous decline in lightning activity.

Warm weather continues Friday...with afternoon highs similar (if not
a couple of degrees cooler) to today. Though, cold front will have
shunted best moisture to the south...so dewpoints will be slightly
lower.  Warm nose near 800 mb will keep the majority of the area
moderate-to-strongly capped which will suppress convection across
much of the area. One exception could be across across our far
southern counties closer to where better moisture resides...with an
isolated shower/thunderstorm possible. Currently believe any
activity would be just to our south...so left forecast dry for now.

Saturday appears to be the warmest day of the period...with highs
nearing 90F and dewpoints peaking near 70F. This will create
moderate instability by early afternoon (MLCAPE 1000-2000 J/kg).
Backdoor front may slide into the area during the afternoon...which
may focus thunderstorm development. Still some uncertainty with
regards to the southward spatiotemporal evolution of front...and
thus whether we would be able to take advantage of the primed
environment. Best chances for thunderstorm activity would reside
along/near front with strong thunderstorms possible given strong
instability and shear (EBS > 40 kts).  Any thunderstorms would
become increasingly elevated overnight with a gradual weakening
trend. SPC has introduced a Day 3 Marginal Risk across the northern
half of the forecast area. Spatial location and category look very
good considering aforementioned discussion of parameters and
uncertainty in frontal position.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
The backdoor cold front will be lurking across the forecast
area at the start of the extended forecast period (Sunday).
Present indications are that northeast/north-central Maryland
will be most affected. Exact frontal location/movement will be
key, as temperatures north/east of the boundary will be in the
70s (at best), while it`ll be approaching 90 south/west of the
front. Confidence improving a little vs. yesterday, but still
not all that high. Will seek to strike a balance in the
forecast. Will need the warmer air mass to generate enough
instability for thunderstorms.

The front will retreat north Sunday night, but without much fuel for
storms. Mid levels will actually be on the dry side. Surface
dewpoints, on the other hand, will be high for late April. Most
locations will have lows in the 60s.

The forecast area will be in the warm sector before the arrival of a
fairly well-defined cold front Monday night. The PM hours have the
best chance of seeing thunderstorms. It remains to be seen how
strong those storms will be, as temperatures won`t fully recover
from the effects wedge, and thus instability values/lapse rates will
be on the lower side.

Behind the front, surface high pressure will reside under zonal
flow. That would support scattered clouds and perhaps a few late-day
sprinkles/showers.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...

With the exception of DCA ceilings/vsby have been
oscillating/doing a slow improvement but are still either IFR or
MVFR. This afternoon VFR is expected to return.

A few showers/thunderstorms are possible this evening into the
overnight. The best locations for thunderstorms would be at
KMRB...with less of a chance as activity spreads east of the
Blue Ridge.

Could be some fog around again tonight...though confidence is
low. VFR expected Friday.

Shower and thunderstorm chances then return Saturday as frontal
boundary slides into the area. Some strong storms are possible
depending on where the boundary sets up (best chance across
northern half of the area).

The backdoor cold front will be lurking across the terminals on
Sunday, especially in the morning. There is a potential for
flight restrictions with east winds to the north of the front.
Frontal position remains a challenge to forecast. BWI/MTN stands
a better chance at being impacted than CHO though.

The front will return north by Sunday night. There could be some
showers/storms Sunday PM, but confidence and areal coverage both
limited.

A much better defined cold front will arrive from the west Monday
PM. Scattered to numerous thunderstorms appear to be a good bet at
this time, with brief flight restrictions possible within storms.

&&

.MARINE...
SCA begins at noon for all waters. Winds will begin to diminish
overnight. Relatively light winds expected over the waters
Friday into Saturday. Shower and thunderstorms are possible
Saturday afternoon as a front slides into the area...some storms
could be strong.

The waters will be influenced by a backdoor cold front on
Sunday. Mixing likely will be poor, and the gradient won`t be
all that strong either. The front will retreat by Sunday night,
permitting stronger southerly flow on Monday. A better chance at
Small Craft caliber gusts will develop ahead of a cold front.
But, mixing may not be ideal at the water/air temperature
interface.

&&

.TIDES/COASTAL FLOODING...
Tidal anomalies remain elevated across the area. Coastal Flood
Advisory continues for Straits Point...as it is currently exceeding
minor flood stage.

Southerly flow today will keep tidal anamolies elevated.
Appears most sites will remain below minor flooding stage for
todays high tide cycle (it is the lower of the two) but will
then threaten minor flooding tonight (especially the sensitive
sites).

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 1 PM EDT this afternoon for
     DCZ001.
MD...None.
VA...Coastal Flood Advisory until 1 PM EDT this afternoon for
     VAZ054.
WV...None.
MARINE...Small Craft Advisory until 5 AM EDT Friday for ANZ531>534-537-
     539>541-543.
     Small Craft Advisory until midnight EDT tonight for ANZ530-535-
     536-538-542.

&&

$$
.UPDATE...WOODY!
.PREVIOUS...MSE/HTS


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