Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 220759
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
359 AM EDT Mon May 22 2017

.SYNOPSIS...
A warm front will cross the region this morning. A cold front
will move through the area later today. High pressure will
briefly return Monday night before low pressure develops over
eastern Carolina Tuesday and moves northeast off the DelMarVa
Tuesday night. Upper-level low pressure will linger over the
area Wednesday through Friday.
&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
It`s rather difficult to pick out precisely where the warm
front resides early this morning. Based on dew points and wind
flow, best estimation is somewhere across southside Virginia.
However, the isentropic lift and the overruning precipitation it
has induced has enveloped central Virginia, with lighter
returns across the Baltimore-Washington area. Just about
everyone east of I-81 will be seeing precipitation by sunrise.
Have likely or categorical PoPs for the morning, with the
highest along I-95... coincident with the best lift.

Trends this afternoon are more optimistic. There is a common
signal that echoes will become more scattered in nature.
Further, RAP, NAM, and even GFS suggesting that the cold front
will be entering the western portion of the forecast area during
the afternoon. Best moisture remains east of this boundary, so
won`t have PoPs higher than chance through frontal passage. Will
be ending precip chances along with the dry air infiltrating
behind the front.

Instability will be limited at best due to early precip timing.
However, in daytime heating behind the first round of precip,
few hundred joules of CAPE will be possible east of the Blue
Ridge, and have entertained a chance of thunder in this area.
&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
High pressure will briefly build across the region tonight.
However, the frontal zone won`t be all that far offshore, and
clearing unlikely for point east of the Blue Ridge.

Guidance has, for a couple of cycles now, been inducing
cyclogenesis across the southeastern United States late tonight
near a 500mb shortwave and the right rear quad of the upper
jet. The track of this low will bring rain back to central
Virginia and southern Maryland by late tonight. The focus will
remain across southern/eastern areas, with a good dose of
associated precipitation though Tuesday.

One difference this cycle though is the compact nature of the
low (tight precip gradient on the northwestern edge) and
progressive speed. That means that precip will be exiting by
Tuesday evening. Have backed off on Tuesday night precip
substantially. Am hesitant to drop PoPs entirely as current
guidance suggests, but am at a lower confidence "chance".

A closed upper low will be moving south into the Mid-west Wednesday
while a sfc low and associated cold front moves eastward across the
Ohio Valley. A brief period of subsidence is expected across the Mid-
Atlantic Wednesday leading to mostly dry conditions for most of the
day however instability will increase across the Allegheny Front
and creep northward into central VA Wed afternoon. This may
cause a few showers and thunderstorms to form however confidence is
low through the afternoon. Mostly dry conditions will come to an end
Wed evening as rain progresses from south to north across the
Mid-Atlantic region into Thu morning. Elevated instability is
expected and thunderstorms are possible Wed night.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
A cold front will be in vicinity of the Mid-Atlantic region
Thursday. Showers and a few thunderstorms should be ongoing Thu
morning as low pressure rides along the cold front. Depending on the
speed of the front showers and thunderstorms may continue through
the day Thursday. Shear profiles show weak shear and therefore
thunderstorms are expected to be sub-severe. The caveat is that
heavy rain is possible in thunderstorms. Will need to monitor the
threat if training storms occur.

An upper level trough will move over head Thu-Thu night. The cold
front should be east of the Mid-Atlantic region by Fri morning and
northwest flow is expected. Disturbances may move across the region
under the influence of the upper level trough Friday and a chance
for showers exists.

High pressure will build into the Mid-Atlantic region Friday-
Saturday. Dry weather is expected through Saturday. Low
pressure will move into the Ohio Valley Sat night and Sun and
showers and thunderstorms are possible during this time.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
MVFR flight conditions prevail across most of the terminals
except for MRB, which is further away from the forcing. MTN has
actually just dropped to IFR. Expect a downward trend through the
morning push, with aob IFR common...due more to ceilings than
vsbys.

After the initial round of showers pass east, ceilings should
gradually lift. The afternoon will be at least above IFR. By
evening, VFR should prevail. The precise timing of category
crossings still a little bit nebulous. Also, there could be a
few more showers in the afternoon, and maybe even a thunderstorm
DCA/BWI/IAD. Spatial and temporal uncertainties preclude a TAF
mention.

VFR for the rest of the TAF period. However, more rain, with
reduced flight categories will return for the morning push
Tuesday. Once again, IFR a good possibility for much of the day,
with improvement in the evening.

SHRA/TSRA possible Wed night-Thu night as a cold front moves across
the terminals. Sub-vfr conditions possible during this time.

&&

.MARINE...
Winds have dropped to 10-15 kt, and all Small Craft Advisories
have been dropped. Mixing should be rather poor today in
vicinity of showers. Southerly flow will become west/northwest
this evening with a cold frontal passage.

The pressure gradient will tighten Tuesday as low pressure
moves up the coast. Have raised a Small Craft Advisory for the
mid Bay and lower Potomac, where the gradient will be the best.
As the situation unfolds, the Advisory may need to be expanded
or extended.

A cold front will cross the waters Thu-Thu night. Showers and a few
thunderstorms are possible Wed night through Thu night. Winds become
W-NW Friday.

&&

.TIDES/COASTAL FLOODING...
Water levels remain about a foot or less above astronomical
normals. High tide has passed at Straits Point, and the tide
came in under minor threshold at Annapolis. Both of those
Advisories have been dropped.

At SW DC and Alexandria, guidance indicates that tides will
bring a surge along with this high tide. HEC-RAS forecast on
track so far, so have no reason to dispute the results. Have a
bit of uncertainty is it will indeed be as high as forecast, but
at least think the Advisory threshold will be crossed.
Therefore, have raised the Advisory for high tide this morning.

The next tide cycle will be the lower astronomically, and
anomalies may be easing slightly. A cold front tonight will
bring northwest winds. Therefore there are no indications that
any additional flooding is imminent. However, a coastal low
Tuesday provides some uncertainty. Guidance ensembles then
diverge substantially through midweek.



&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 9 AM EDT this morning for DCZ001.
MD...None.
VA...Coastal Flood Advisory until 9 AM EDT this morning for VAZ054.
WV...None.
MARINE...Small Craft Advisory from 2 PM to 6 PM EDT Tuesday for ANZ533-
     534-537-543.

&&

$$
SYNOPSIS...HTS
NEAR TERM...HTS
SHORT TERM...HTS/HSK
LONG TERM...HSK
AVIATION...HTS/HSK
MARINE...HTS/HSK
TIDES/COASTAL FLOODING...HTS



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