Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
359 am EDT Thursday Apr 24 2014

Synopsis...
high pressure will build over the area through today before
gradually shifting to the east tonight. A cold front will pass
through the area Friday and another cold front is expected to move
through late Saturday through Saturday night. High pressure will
build in on Sunday as the cold front stalls to our south. Several
waves of low pressure will develop along the stalled
boundary...bringing unsettled conditions to the area next week.

&&

Near term /through tonight/...
surface high pressure will build over the region today while
ridging aloft builds overhead between an upper-level low off the
New England coast and an upper-level trough over the central
Continental U.S.. subsidence from these systems will allow for plenty of
sunshine along with dry and seasonable conditions. Maximum temperatures will
range from the 50s in the mountains to the middle and upper 60s
across most other locations.

The high will gradually shift toward middle-Atlantic coast tonight
along with the upper-level ridging. A potent shortwave and its
associated surface low will track into the Midwest during this
time. There will be some high and middle-level clouds increasing
overnight...but dry conditions will persist. Min temperatures will range
from the upper 30s across portions of northern Maryland where
there may be some radiational cooling to near 50 in downtown
Washington and Baltimore.

&&

Short term /Friday through Friday night/...
the shortwave and surface low will track into the Ohio Valley
Friday morning before moving into our area Friday afternoon. The
cold front associated with the surface low will also move into our
area during this time. A southerly flow will increase ahead of the
boundary...allowing for warm and moist air to move into the
region. Guidance has trended a bit slower with this system...which
will allow for some more moisture to make it into the area before
it moves through. Therefore...probability of precipitation have been increased and
widespread showers are expected. The best chance for precipitation
will be from middle-morning through through Friday afternoon west of
the Blue Ridge mountains...and Friday afternoon east of the Blue
Ridge mountains.

There will be enough warm and moist air for some instability to
develop as well. Therefore...a few thunderstorms cannot be ruled
out. Cloud cover and marine air should keep instability limited
enough to keep the severe threat mainly to our south.
However...should there be more sunshine that breaks out...then
severe thunderstorms would be possible due to favorable shear
profiles...especially across central Virginia.

Most models still a bit divergent in the progress/speed of the upper
level feature that is expected to slide across our region on Friday...
exiting late Friday night. A bulk of the precipitation will have already moved
through earlier in the day...W/ the back end subsidence moving any
lingering precipitation off the coast late Friday night. The drier air influx
will help drop dewpoints a few degrees from their earlier
values...but still plenty moist which will keep overnight lows in
the u40s-l50s.

&&

Long term /Saturday through Wednesday/...
over the past several years...have seen a number of late April
closed upper lows that meander and drift over a portion of either
the upper Midwest/Ohio Valley or deep south and eventually off the Atlantic
coast. Cannot remember seeing/forecasting one as large as the one
long range guidance is depicting for the better part of next week.
One of the reasons this system will likely be so vast is that it
originally will have started out as two separate upper waves that
combine over the upper Midwest.

The Friday upper wave affecting our area will slide off the middle Atlantic
coastline and stall basically off the southern New England coast this
weekend. Meanwhile...a cold and deep upper wave over the inter-mountain
west will be gearing up for an intensification once it moves east of
The Rockies this coming weekend. The stalled New England coastal low
will prevent the forward progress of the now large-scale upper
system over the northern plains sun/Mon...and also squeeze down a colder
upper vorticity down from Hudson Valley in between the two larger systems.
The continual rotation of smaller vorticity lobes around the parent
system will keep an active period of weather for roughly the entire eastern
half of the Continental U.S. For most of next week.

Closed upper lows typically present a number of forecast
challenges...and the daily depictions of this feature will be an
intriguing story for the next several days. Each phasing period of
this system could cause another round or wave of precipitation across our
region west/ periodic breaks in between. Comparing the GFS and euro's
latest versions - the GFS is a much colder and compact upper low
sliding over the upper Midwest while the Euro...though showing some
similar progress and interactions - not as strong as the GFS. The
current forecast for our area revolves essentially around a wet
middle of the week period...W/ a slow build up to it this weekend
into early next week. Temperatures under this type of regime would be well
below average but not very cold - more like a moderated environment west/
little diurnal change.

&&

Aviation /07z Thursday through Monday/...
VFR conditions are expected through tonight. Northwest winds will
gust around 15 to 20 knots this morning through middle-afternoon
across the eastern terminals. High pressure will allow for light
winds tonight.

Southerly winds will increase ahead of a cold front Friday with
gusts around 15 to 20 knots expected in the afternoon. Showers are
expected...especially during the afternoon. Ceilings/visibilities will lower
to MVFR levels and possibly IFR levels in bands of heavier
showers.

A quiet weather weekend in store...as one upper level system moves off
the coast Friday night - allowing high pressure to squeeze in over the
area Sat/sun and lasting through late Monday. However...the Friday/Friday night
system will hang out off the New England coast and help steer a cold
upper low into a large and potent low pressure system over the northern
plains. These features will eventually combine into a larger system
that will slowly drift toward and over the area for much of next
week...starting likely Tuesday and Onward. Periods of rain and
periodically lowered flight categories will likely be common into
the middle and latter parts of next week.

&&

Marine...
a Small Craft Advisory is in effect for the waters today. There
may be a lull in the winds early this morning across the upper
tidal Potomac River...but increased mixing should cause gusts
around 20 knots later this morning into early this afternoon. The
pressure gradient will decrease this afternoon and winds will
diminish during this time from west to east. The Small Craft
Advisory may be able to be cancelled later this afternoon,

High pressure will allow for lighter winds tonight. No marine
headlines are expected. A southerly flow will increase ahead of a
cold front Friday. A Small Craft Advisory is in effect for the
waters Friday afternoon. Showers will develop during the
afternoon.

A surface low is expected to pass by the northern part of the Bay Friday
night...but then winds will pick up again behind the low. Winds will
then continue to gust into Sat...W/ periods of breezy channeling
winds over the Bay/tp into next week. A breezy Sat will give way to
a quiet start to next week...as high pressure moves in - then out by
late Monday/early Tuesday. Onshore flow will then begin ahead of another
system for middle next week.

&&

Fire weather...
a northwest flow has caused dewpoints to drop several degrees
compared to Wednesday. Therefore...the relative humidity will be
significantly lower today with min relative humidity values around 15 to 25
percent this afternoon. Gusty northwest winds are still expected
across northeastern Maryland so a red flag warning is in effect.
Elsewhere...winds may gust around 15 to 20 miles per hour this morning into
early this afternoon...especially east of the Blue Ridge
mountains. However...winds should diminish this afternoon as high
pressure builds overhead. Therefore...no fire weather headlines
are in effect.

Winds will be light tonight with a strong nocturnal inversion.
Southerly winds will increase Friday and showers will develop
ahead of a cold front. The best chance for showers will be Friday
morning and Friday afternoon west of the Blue Ridge mountains...and
Friday afternoon into Friday night east of the Blue Ridge
mountains. Rainfall amounts will average between one-quarter and
one-half inch across most locations. However...locally higher
amounts are likely since there will be a convective element to the
precipitation.

&&

Tides/coastal flooding...
a southerly flow will strengthen ahead of a cold front Friday
afternoon through Friday evening. This will cause tidal anomalies
to sharply increase. The flow will turn offshore behind a cold
front after midnight...causing tidal anomalies to decrease by
Saturday morning. Minor coastal flooding is possible near times
of high tide Friday night...especially if the arrival of the
offshore flow is delayed a bit.



&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...red flag warning from 10 am this morning to 6 PM EDT this
evening for mdz005>007-010-011-014.
Virginia...none.
WV...none.
Marine...Small Craft Advisory until 6 PM EDT this evening for
anz530>543.
Small Craft Advisory from noon to 6 PM EDT Friday for
anz530>543.

&&

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