Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210749
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
349 AM EDT Sun May 21 2017

.SYNOPSIS...
A quasi-stationary front will remain across the Carolinas today
before lifting north into the area as a warm front late today. A
cold front will move through the area from the west Monday. High
pressure will briefly return Monday night before low pressure
develops over the Carolinas Tuesday and moves northeast off the
DelMarVa into Wednesday. Upper-level low pressure will then linger
over the area into Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today`s weather will be quite similar to yesterday...as persistent
easterly onshore flow...resulting from high pressure to our NE off
the NY/NJ coast and low pressure over western Great Lakes...keeps
marine layer entrenched over the area.

As of 3 AM...quasi-stationary front has settled over far southern
North Carolina...it will remain there through much of today...with
our area remaining in the cool sector. Low clouds will remain
plentiful through the day hindering insolation and keeping
temperatures near 70 F. It is also expected to remain dry through
the day...except for far SW portions of the CWA where a late day
shower could approach.

Front will return northward as a warm front overnight. This will act
to increase rain chances from SW to NE as WAA lift develops in
vicinity of frontal boundary. Prefrontal showers chances continue
overnight.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
Shower chances continue Monday morning as cold front approaches from
the west. Front will struggle to move quickly through the area...as
it remains well displaced from parent low over northern Canada. By
afternoon...enough instability could reside ahead of the front for a
few thunderstorms to develop...though, airmass recovery from morning
rain showers casts doubt on how far west/north this would occur.
Best chance for thunderstorm development across central VA into S
MD. Weak shear/thermo space suggests disorganized general
thunderstorms would be the primary mode.

Respite from rain chances late Monday into Tuesday morning as high
pressure briefly builds into the area. Latest guidance develops low
pressure along frontal boundary to our south Tuesday and moves it up
to near the DelMarVa coast Tuesday night. This will act to bring
rain chances back to the area as we remain in the western quadrant
of the lows track. Tight gradient suggests western areas might not
see much rainfall...with best chances along/east of I-95.

Current QPF probs suggest 0.5/1/1.5 inches at the 25/50/75th
percentiles with the two systems through 8 AM Wednesday...with
highest totals along/east of I-95.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
The main synoptic feature dictating the forecast in the extended
periods remains the deep trough axis pivoting across the Great Lakes
Tuesday and tilting negatively across New England by Friday. There
is model agreement that there will be a cutoff low somewhere in the
base of the trough Thursday; where and how deep understandably still
to be determined. Regardless, the period will be wet/unsettled.

Precipitation will be dictated by location of vorticity maxima and
enhanced by diurnal heating. PoPs will be likely or close to it.
However, there will be dry periods. Too much moisture within the
column to hope for much sun, although the upper layer does dry out
by Thursday. Regarding thunder, ECMWF suggests there will be mean
layer instability both Wednesday and Thursday PM, while GFS is
focusing on only Thursday. Don`t believe these details can be
resolved at this timeframe, so will preserve chance of thunderstorms
both days. No good reason right now to deviate temps much from model
blend...which is climo.

The trough axis will move northeast of the area Friday, providing
the area with northwest flow on the back side of the deepening
surface cyclone. That setup suggests that precipitation chances will
be decreasing as drier air infiltrates the area. Cold advection will
initially keep temperatures down. The warm-up begin Saturday with
the return of warm advection.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
Cold front remains to the south today over the Carolinas...with
persistent easterly flow around high pressure offshore to our
northeast keeping low clouds across the area. VFR cloud decks should
prevail...with bases 3.5-4.5 kft...though brief periods of MVFR
ceilings are possible. Warm front lifts northward into the area late
today and a cold front will cross the area Monday. These features
will allow rain to develop with periods IFR (to perhaps locally
LIFR) developing tonight due to low ceilings. Improvement is
expected later Monday as cold front pushes east of the area.

Coastal low may create more aviation restrictions late Tuesday.

The forecast Wed-Thu will be unsettled, with numerous opportunities
for flight restrictions. However, showers (and perhaps
thunderstorms) will be hit-or-miss albeit numerous. The afternoon
and evening hours should be preferred for showers/storms, but it is
too soon to be detailed with forecast evolution.

&&

.MARINE...
Westerly winds gusting 10-15 kts will prevail through much of
today...before winds turn southerly later today as warm front begins
to lift back northward. Small Craft Advisory was issued to account
for low-end SCA criteria wind gusts starting late this afternoon
through tonight. Wind gusts may remain elevated Monday...though
confidence is currently low. Lighter winds expected late Monday into
Tuesday as high pressure briefly builds over the area. SCA wind
gusts could return late Tuesday depending on the track of the
coastal low...best chances across southern portion of the
Bay...which is closer to the current forecasted track.

The waters will be under the influence of an upper level storm
system crossing the area Wednesday and Thursday. Numerous
showers/perhaps thunderstorms will result. Gradient flow likely
won`t be that strong, although a coastal storm could develop
late Thursday. At this stage will keep winds under any Advisory
threshold.

&&

.TIDES/COASTAL FLOODING...
Easterly onshore has allowed tidal anomalies to rise rather quickly.
A Coastal Flood Advisory was issued for St. Marys county as
observational trends coupled with most recent guidance suggested
that Straits Point would exceed minor flood stage during this
mornings tide cycle. Annapolis may also approach minor tidal
flooding later this morning...though probability is too low at this
time...and will continue to monitor.

Anamolies increase further today...with minor tidal flooding likely
at Straits (Coastal Flood Advisory extends through tonights cycle),
Annapolis, and DC. Other sites may also approach minor flooding
tonight.

Evacuation from the estuary appears unlikely through at least Monday
(and perhaps longer)...which will keep tidal levels elevated with
continued minor flooding possible at the most sensitive sites near
times of high tide.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Coastal Flood Advisory from 8 AM this morning to 3 AM EDT
     Monday for MDZ017.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 5 PM this afternoon to 5 AM EDT
     Monday for ANZ530>533-538>542.
     Small Craft Advisory from 8 PM this evening to 8 AM EDT Monday
     for ANZ534-537-543.
     Small Craft Advisory from 8 PM this evening to 5 AM EDT Monday
     for ANZ535-536.

&&

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



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