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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
400 am EDT Tuesday Jun 30 2015

Synopsis...
low pressure will slide eastward over the eastern Great Lakes
today. A cold front will slide southeast into the middle Atlantic on
Wednesday...and then stall across the region on Thursday. The
stalled front will linger in the area through the weekend before
lifting northward again on Monday.

&&

Near term /through tonight/...
light showers have been crossing northwest parts of the County Warning Area early this
morning in association with weak shortwave energy. Most of these
should exit the area shortly after daybreak...and there may be a bit
of a lull into the middle morning...but can/T totally rule out an
isolated shower...mainly over northern areas.

Very subtle atmospheric forcing today. Low pressure will be slowly
tracking along a nearly-stationary front through the Great Lakes.
Return flow to be established locally...with Theta-E advection east
of the Blue Ridge. Vorticity maxima over the Ohio and Tennessee valleys
will make a run at the area this afternoon/evening...but model
depictions are slightly different. Terrain and Lee troughing may be
as much of a factor in storm initiation. Instability should build during
the afternoon as temperatures rise through the 80s to perhaps around
90...however middle-level lapse rates will not be very steep. Amount of
cloud cover could also be a factor. Models continue to advertise
deep layer shear at or above 30 knots...so if enough instability is realized...some
strong to severe storms could develop. The primary threat would be
localized damaging wind gusts as storm structures may be transient.
Continued highest probability of precipitation in northern County Warning Area closer to forcing/height
falls...although coverage is low confidence.

While there should be an overall eastward progression and diurnal
weakening of activity this evening...models maintain elevated instability
overnight...and middle/upper level trough will be approaching toward
daybreak. Therefore some showers and storms may linger well into the
overnight period. Min temperatures in the Lower/Middle 60s west and upper
60s/lower 70s east.

&&

Short term /Wednesday through Thursday night/...
middle level trough axis will be lifting north during the morning
hours. Surface cold front will be approaching from the northwest but
likely won't enter the area during the daytime hours. Timing and
coverage of precipitation quite uncertain without strong forcing
mechanisms. If forecast soundings are correct in decreasing instability
behind the trough passage in the afternoon...convection may be
rather limited. Have maintained low chance probability of precipitation throughout day.

On Wednesday night...the cold front will stall out over the middle
Atlantic just to our south. Support aloft will wane as the trough
passes so expect convection to quickly dissipate or move away.
With the front stalled in the area...guidance is beginning to hone
in on a wave of low pressure moving east along the front from the
middle Mississippi Valley Thursday or Thursday night.
However...timing is still much in question...so generally keep
chance probability of precipitation around with likelys to the south closer to the stalled
front. With the front to the south and increased cloud
cover...temperatures should be cooler on Thursday.

&&

Long term /Friday through Monday/...
through the long term the pattern will remain fairly static with a
broad trough over Canada...and a strong ridge out west. This will
keep funnelling disturbances southeast and east across our area as
a frontal boundary remains stalled out over the region. Showers
and thunderstorms will be possible each day with the front nearby
but timing disturbances will be increasingly difficult as we go
further out. Right now one system may affect US Saturday...and
another Monday...but confidence on timing is not high.
So...generally have kept temperatures a bit below normal...with a chance
of showers and thunderstorms in the forecast each day. As we get
closer...we should have better clarity as far as days which will
turn out mostly rain-free...and those which will be pretty wet.

&&

Aviation /08z Tuesday through Saturday/...
VFR expected through the morning hours...although a passing light
shower or sprinkle will be possible at mrb through daybreak.
Southerly winds will increase during the midday with occasional gusts to
20 knots. Thunderstorm coverage and timing low confidence today. Likely
to be largely diurnal in nature...favoring 21-01z time frame...but
can/T rule out activity before/after. Brief MVFR or IFR conditions
and strong wind gusts could accompany any storm.

Disturbance crosses area late tonight/Wednesday morning...so a shower or
storm will remain possible...but lower potential for ceiling/visibility
reductions. Questionable how much of this activity
lingers/redevelops Wednesday afternoon...low chance of impacts.

Main concern through extended is potential for scattered
thunderstorms each day. Winds will be uncertain given the front
will continue to meander in the area after arriving Wednesday.
Wave of low pressure may provide a more concentrated period of
rain and IFR ceilings/visible at some point Thursday...Friday or Saturday
but low confidence right now on when this is...or if it even
happens.

&&

Marine...
S/southeast channeling occuring over the waters now with gusts to 15 knots.
Will monitor the need to start Small Craft Advisory early should gusts increase.
Otherwise will go into effect for all waters at 10am with southerly
gusts of 20-25 knots possible. Thunderstorms will develop in the area
from late afternoon into the evening with the potential for strong
wind gusts...and some smw/S are possible. Gradient winds will
diminsh overnight. SW flow through Wednesday around 10 knots...although may
be slightly stronger toward Tangier Sound. Lower chance of storms
through Wednesday.

Winds expected to remain below Small Craft Advisory criteria through the
extended...though will need to monitor as front will be near area
each day. Showers and thunderstorms are also possible each day...which
could impact marine concerns.

&&

Tides/coastal flooding...
water levels will be increasing tonight through middle week. The
water levels may approach minor thresholds around SW District of Columbia high tide
this morning...and then again Wednesday along the western Chesapeake
during the higher high tide...which is the am cycle /midngt through
noon/.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...Small Craft Advisory from 10 am this morning to 4 am EDT
Wednesday for anz531>534-536-537-539>543.
Small Craft Advisory from 10 am this morning to 8 PM EDT this
evening for anz530-535-538.

&&

$$
Synopsis...rcm
near term...ads
short term...ads/rcm
long term...rcm
aviation...ads/rcm
marine...ads/rcm
tides/coastal flooding...

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