Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 270749

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

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.


Clear skies and light winds have led to radiation cooling...with
temperatures across the area nearing the dewpoint. Shallow low-level
moisture (as sampled by the 00z IAD RAOB and forecast by model
soundings) is trapped beneath low-level inversion. This has led
to the gradually development of shallow fog (and some low
clouds) east of the Blue Ridge over the past few hours. Fog is
expected to spread in coverage and may become locally dense
later this morning...and may necessitate a dense fog advisory if
the areal extent becomes large enough.

Any fog should quickly burn off shortly after sunrise as decent
mixing commences and erodes low-level inversion. 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.


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 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 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 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.


The backdoor cold front will still 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


VFR expected to develop at most sites over the next several hours as
fog continues to expand/develop across the area. Fog could become
locally dense...with LIFR possible near sunrise. Fog should rapidly
dissipate a few hours after sunrise as strong mixing develops. VFR
then expected to remain through the remainder of the day. 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 very
low at this point. 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

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.

Southerly flow will increase today...with low-end small craft wind
gusts near 20 kts requiring a SCA (begins at noon for all waters).
Winds will begin to diminish overnight...with SCA winds becoming less
likely. 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 influence 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.


Tidal anomalies remain elevated across the area. Coastal Flood
Advisory continues for Straits it is currently exceeding
minor flood stage. Added a Coastal Flood Advisory for this tide
cycle at Annapolis as anomalies have been increasing and latest
forecast places tidal levels a tenth or two above minor flood stage.
SNAP-EX ensemble guidance also confirms this trend. Minor flooding
is also possible this morning at DC...though confidence is still to
low to act on it now...will continue to monitor.

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).


MD...Coastal Flood Advisory until 10 AM EDT this morning for MDZ014.
     Coastal Flood Advisory until 6 AM EDT early this morning for
MARINE...Small Craft Advisory from noon today to 5 AM EDT Friday for
     Small Craft Advisory from noon today to midnight EDT tonight
     for ANZ530-535-536-538-542.


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