Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
957 am EDT Wednesday Jul 23 2014

Synopsis...
a cold front from the Great Lakes will slowly approach the region
through tonight. The boundary will slowly pass through the area
Thursday into Thursday night and high pressure will return for
Friday. High pressure will move off the coast during the weekend
and a cold front will pass through the area early next week.

&&

Near term /through tonight/...
a cold front is located along Lake Ontario and Lake Erie across central
in/Illinois this morning while the middle-Atlantic is under the influence of a
Bermuda high.

Hot and humid conditions expected today with temperatures already around 80
as of 9am...82 at dca...80 at BWI and dewpoints around 70. This has
led to around 500 j/kg this morning which will increase as temperatures
reach the low to middle 90s this afternoon. A subtle Lee trough is
located across the region but should become more defined by the
afternoon. A quiet morning is expected as the Bermuda high to the
east is dominating with little forcing aloft. Per the 12z iad
radiosonde observation...0-6km shear is less than 10kts and a stable layer exists
at 650mb which will inhibit updrafts that do try and form along
the Lee trough. That being said...cumulus field is still expected to
form by this afternoon. As the day continues...the cold front will
approach The Highlands and shear profiles will increase. An upper
level trough will also dig into the Midwest increasing flow aloft this
afternoon and evening. Showers and thunderstorms will begin to
form this afternoon from pbz to rlx and will track eastward
reaching the middle-Atlantic late this afternoon and evening. Strong
to severe storms are possible this evening as conditions become
more unstable aloft. Damage winds are the primary threat from the
Potomac Highlands to the I-95 corridor.

Tonight...before the cold front arrives from the north...temperatures will
still be fairly warm. 80s will still dot the map well after sunset but
as the front moves in from the northwest...temperatures will slowly drop over the
northwestern part of the area as drier northwest winds take down temperatures and
dewpoints.

&&

Short term /Thursday through Friday/...
an upper-level trough will remain over the region Thursday through
Friday. A cold front will slowly pass through the area Thursday
into evening. More showers and thunderstorms are possible ahead of the
front...especially across central and southern portions of the
County Warning Area where moisture will be deeper. Drier air will work its way
into most areas overnight Thursday.

High pressure will build over the region Friday. A scattered to broken cumulus
deck should develop during the afternoon due to the upper-level
trough overhead. Terrain circulation could lead to an isolated
shower west of the Blue Ridge mountains...but the forecast remains
dry for now thinking that low-level dry air should suppress
convection for most areas. Maximum temperatures will be near climatology...ranging
from the 70s in the mountains to the middle and upper 80s across most
other locations...and it will feel noticeably less humid compared
to recent days.

&&

Long term /Friday night through Tuesday/...
high pressure will slowly move off the coast Friday night into
Saturday and it will remain over the Atlantic Ocean for Sunday. A
return southerly flow will allow for the humidity to return
during the weekend. A couple showers and thunderstorms are
possible Saturday afternoon and evening...but convection should be
limited in coverage due to the lack of a strong lifting mechanism.

However...a potent shortwave is expected to dig through the Great
Lakes and into New England Sunday. This will increase shear
profiles and carve out a trough of low pressure both at the
upper levels and lower levels of the atmosphere. Showers and
thunderstorms may increase in coverage for these reasons Sunday. A
potent cold front will likely pass through the region
Monday...bringing the chance for more showers and thunderstorms
before cooler and drier air moves into the region during the
middle portion of next week.

&&

Aviation /14z Wednesday through Sunday/...
an upper ridge keeping the airmass overhead will keep the area under
moist and humid conditions throughout the day. The typical terrain
circulations occurring over the Appalachians to the west of the metropolitan
areas...when there is a large offshore high as there is currently.
The mountains receive periods of low level convergence west/
moisture...helping to make scattered batches of middle-level decks.
Mostly clear further east...as it will remain so in the coming hours.

Only the daytime heating of the middle-late morning will begin developing
scattered cumulus cloud decks across the region and eventually help
west/ isolated showers/thunderstorms as the day progresses. The convective
potential increases later today...as the winds in the column
increase - albeit slightly and slowly. Incoming 20-30kt low level
shear should be enough to create increased convective potential over
more of the area later tonight. A cold front will slowly arrive out
of the northwest late tonight and into the overnight...turning surface flow out
of the northwest and north...though still fairly light. This will also
slowly end the convective potential...making ongoing activity turn
into showers.

Subvfr ceilings and visibilities are possible from patchy low clouds and fog
Thursday morning. A cold front will slowly pass through the
terminals Thursday into Thursday evening. Showers and
thunderstorms are possible ahead of the front...with the most
widespread coverage being across the central and southern
terminals. Subvfr conditions are possible in any showers and
thunderstorms.

Drier air will move in behind the cold front Thursday night
through Friday. Patchy br may cause subvfr visibilities early Friday
morning...especially across kcho where drier air will be lower to
move in. High pressure will build overhead Friday into Friday
night before moving off the coast during the weekend. A southerly
flow will cause more moisture to return along with an increased
chance for showers and thunderstorms...especially Sunday afternoon
and evening.

&&

Marine...
light winds over the waters at this time...W/ only some southerly channeling
across the Middle-Bay region. This regime will stay in place for much
of the day...even as pressure gradient increases. A few near-Small Craft Advisory gusts
may appear later this afternoon. Winds will shift to the northwest
behind a surface trough tonight. Winds will be below Small Craft Advisory
criteria...10-15kt or less. Thunderstorms are possible this
afternoon and evening across the waters. Thunderstorms will be
capable of producing gusty winds and hail.

A cold front will slowly pass through the waters Thursday into
Thursday evening. Showers and thunderstorms are possible during
this time...especially across the middle portion of the Bay and
the lower tidal Potomac River. A pressure surge behind the front
may cause northerly winds to gust near Small Craft Advisory criteria overnight
Thursday into Friday morning across the Maryland portion of the
Chesapeake Bay and the lower tidal Potomac River.

High pressure will build over the waters Friday and winds will
weaken and perhaps even turn onshore during the afternoon. High
pressure will move off the coast Saturday and Sunday and a
southerly flow will return during this time. The southerly flow
will bring more moisture...increasing the chance for showers and
thunderstorms...especially Sunday. A potent cold front will move
toward the waters Monday...bringing the chance for more
thunderstorms.

&&

Tides/coastal flooding...
a southerly flow has caused tidal anomalies to be around one-half
to three quarters of a foot above normal. The maximum high tide will
be early this morning...but water levels should remain just below
minor flooding thresholds. The positive anomalies will continue
this afternoon and evening...but the high tide will be the lower
of the two so minor flooding is not expected. The flow will
turn offshore behind a surface trough late tonight into
Thursday...so anomalies are expected to decrease during this
time.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...none.

&&

$$
Synopsis...bjl
near term...has/gms
short term...bjl
long term...bjl
aviation...bjl/gms
marine...bjl/gms
tides/coastal flooding...lwx

National Weather Service Glossary of Abbreviations