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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
349 am EDT Wednesday Oct 1 2014

Synopsis...
high pressure will build overhead through Thursday. A strong cold
front will approach the area Friday before passing through Friday
night into early Saturday. High pressure will return for the
weekend...bringing drier and noticeably cooler conditions. The high
will move offshore early next week.

&&

Near term /through tonight/...
a compact and potent upper closed low from yesterday afternoon/evening is in
the transition to weakening while drifting toward the Atlantic coast.
The subsidence /dry-descending air/ behind the center of the upper
low is coming across the central PA...W/ the southern periphery near the
Mason-Dixon line. Trapped low level moisture from the past couple of
days is interacting west/ the weak embedded zones of lift along the Lee
side of the upper trough axis.

Scattered showers have been the result across much of our mountain areas
into the northern shen valley. Even west/ dissipation of individual showers...
more have been developing in this region while being pulled toward
the east. In a kind of rare display in radar data...klwx is located
in an area west/ inbound velocities on either side. Precipitation/echos and
the general low level wind pattern west of the radar is moving
east...while there is a light onshore flow through the lowest couple
thou feet east of the radar. The onshore flow is essentially
dissipating the shower activity from the west as it arrives over the
Lee side of the Blue Ridge. This relatively drier flow will likely
keep the metropolitan areas dry this morning...outside of a brief light shower
or a few sprinkles.

Cloud heights will continue to drop and patchy fog will develop
across the area into the predawn hours. Low clouds will hang around
through the middle morning hours...moderating temperatures and keeping our highs today
again in a seasonal range - l-m70s. The southeast sections of the County Warning Area will
see some clearing later this afternoon and into the metropolitan areas...as the
ribbon of subsidence from the exiting upper low slides over the
area. This feature will also be moving over the Appalachians/Blue Ridge as
well but low level moisture will continue to be trapped in between
the two main ranges and keep low clouds and a few isolated showers
going into the overnight hours...and into Thursday as well.

Another moderated overnight periods...although lows will be a couple
degree lower than the current period - l-m50s. More patchy fog and
batches of low clouds will cover parts of the region. Blended the
bias-corrected mav/met/Euro for highs and lows for today.

&&

Short term /Thursday through Thursday night/...
after the last arm of the exiting upper low moves off the coast and
away from the area early Thursday...a brief visit from a weak in-situ
high pressure ridge will move over the area. This feature will not only
prevent precipitation for the day...it will clear out the more dense
pockets of low level moisture built-up from previous days. Still
some low-middle cloud decks dispersed about the region for a generally
partly sunny sky but certainly more sun poking through than in past
days. High temperatures not expected to be any warmer however...as a light
but steady onshore flow will moderate conditions near the
surface...despite more sun.

High pressure over New England will continue to wedge south and
west into the middle-Atlantic coast Thursday night. Low pressure
will track through the Great Lakes while a strong cold front
associated with the low moves through the Midwest. An onshore flow
is expected to increase over our area in between these two systems
and this will cause moisture to move in from the Atlantic. Also
warm and moist air will overrun the relatively cooler air at the
surface. This will cause plenty of clouds to develop. A shower or
two cannot be ruled out...but that has been left out of the
forecast since lift will be weak and the deep moisture will remain
to our west. However...there should be enough low-level moisture
trapped underneath the subsidence inversion for low
clouds along with patchy fog and drizzle late Thursday night. Min
temperatures will range from the middle to upper 50s across most
locations to the lower 60s in downtown Washington and Baltimore.

&&

Long term /Friday through Tuesday/...
low clouds along with patchy fog and drizzle are expected Friday
morning. Clouds should give way to some sunshine across central
Virginia into the Shenandoah Valley and Potomac Highlands.
However...low clouds may hang on for a while across the
metropolitan areas before giving way to some sunshine later in the
afternoon.

The strong cold front will move into the Ohio Valley Friday before
passing through our area Friday night into early Saturday. A
southerly flow ahead of the cold front will usher in plenty of
moisture across our region. Latest guidance has slowed down the
timing of the frontal passage...and this makes sense given the
fact that there will be plenty of ridging ahead of this system
over the western Atlantic. Therefore...the probability of precipitation have been slowed
down a bit...with the best chance for showers occurring late
Friday through Friday evening in the Potomac Highlands...Friday
night in the Shenandoah Valley...and overnight Friday into early
Saturday for the Washington and Baltimore metropolitan areas. The
synoptic pattern suggests that a qlcs would develop due to the
strong forcing from this system...but instability looks to be weak
and it will be moving through at an unfavorable time Friday night.
There will be strong winds aloft from the low-level jet that could
mix down...but with little to no instability it appears that the
threat for severe weather is minimal. Will continue to monitor
because just a little more instability will significantly enhance
the threat for gusty winds with a qlcs Friday night.

The cold front will move away from the area Saturday and high
pressure will return for Saturday afternoon through Sunday. A west
to northwest flow behind the cold front will usher in drier and
noticeably cooler conditions during this time. An upslope component
to the low-level flow will cause more clouds along and west of the
Allegheny Front with the possibility for a few showers. A couple
snow showers are possible Saturday night along the ridge tops.

The high will move off the coast early next week and a southerly
flow will bring milder conditions during this time. Another cold
front may approach during the middle portion of the week. Showers
are possible ahead of this system...especially Tuesday.
However...timing and details remain uncertain this far out.

&&

Aviation /07z Wednesday through Sunday/...
trapped low level moisture continues to create patchy areas of
low-medium range visibility fog. Only isolated dense fog observation
appearing...through a few more may pop-up in the next couple of hours.
The main concern is from patchy low ceilings...especially across the Piedmont
and shen valley. Lots of random ceiling heights across the area as a
turbulent and complex upper low rotates by just to our north. This
feature will remain north of the area today...moving toward the Atlantic
coast but keeping a wave of low-level moisture centered over the
middle-Atlantic through the day. Shower activity will become more isolated
heading into the Post-dawn hours...and ground fog will dissipate off -
as will the low decks by middle morning. A light east-northeast surface wind will
continue throughout the day and into tonight. More patchy fog and
some low clouds possible again overnight tonight.

An onshore flow will cause low clouds overnight Thursday along
with patchy fog and drizzle. IFR conditions are expected during
this time. The low clouds will hang around Friday morning before
slowly breaking Friday afternoon. A strong cold front will pass
through the terminals Friday night...bringing showers and possibly
a thunderstorm. West to northwest winds are expected behind the
front Saturday and Sunday.

&&

Marine...
an upper low over central PA will continue to move east today. High
pressure off the Atlantic coast is directing a steady onshore surface wind over
the waters. This flow will continue...both direction and range of
speeds...through the rest of the day - below Small Craft Advisory criteria. Dry weather and
only a light east-southeast onshore flow tonight into Thursday...as a high pressure
ridge moves across the area and prevents much in the way of precipitation.

A strong cold front will approach the waters Friday before passing
through late Friday night into Saturday. Southerly winds will
increase ahead of the front. Winds will shift to the west-
northwest behind the front later Saturday morning through Sunday.
A Small Craft Advisory will likely be needed for portions of the
waters ahead of the front Friday afternoon into Friday night...and
also behind the front Saturday and Sunday.

&&

Tides/coastal flooding...
an onshore flow has caused tidal anomalies to increase to around
one-half foot above normal early this morning. The anomalies
should not change much since the onshore flow will persist
today...but it will be light. The high tide this afternoon and
early evening will be the lower of the two...so minor tidal
flooding is not expected. However...the high tide cycle later this
evening into late will be the higher of the two. Minor tidal
flooding is possible for sensitive areas along the western shore
of the Maryland Chesapeake Bay and the shore of the upper tidal
Potomac River.

The onshore flow will continue for Thursday through Friday night.
The flow will strengthen and turn more southerly Friday afternoon
into Friday night ahead of a cold front. Minor tidal flooding is
possible near high tide...especially during the late night and
early morning tide cycle. Water levels should be highest during
the high tide late Friday night into early Saturday morning.

&&

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

&&

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

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