Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
1052 PM EDT Wednesday Oct 1 2014
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 Thursday/...
upper lopres continues to lift northeastward this evening. Broad/diffuse hipres
affecting County warning forecast area at this time. A rather substantial patch of low cloud
stretches from the eastern wva pnhndl southeastward across Virginia. Bases/ceilings gnly
3000-4000 feet. There/S not much flow to speak of at this point.
Dont really forsee this area mixing out overnight...however suspect it
wont be reinforced either. Instead...itll be trapped under
subsidence invrsn /evident in lwx 00z raob/...perhaps lowering through
As long as these clouds are around...the fog threat-- at least dense
fog-- will be delayed. Thats not the case across Maryland... where skies are
mostly clear and fog has already begun developing. Will be revamping clouds/weather
grids to account for going trends.
With ridging in place Thursday...a mix of sun and clouds are
expected once clouds mix out. Further west...terrain circulations may
cause a cumulus field to develop across the Virginia Highlands.
Short term /Thursday night through Friday night/...
main story of the period will be a strong cold front swinging
through the area. A large trough will dig into the upper Midwest
during the day Friday...assuming a negative tilt as it pivots across
the Great Lakes/Ohio Valley Friday night. 500 mb low cuts off in the
process and moves roughly from Chicago east to Pittsburgh overnight.
Strong and deep lift ahead of this trough along a surface cold front
will drive a band of showers moving across the area from west to
east Friday afternoon and Friday night. The latest guidance has
slowed the timing of the front somewhat so most of the rain looks to
be overnight Friday night...especially east of the Blue Ridge.
The overnight timing of the frontal passage would mean lower
instability levels and thus a lower chance of thunderstorms overall.
However the aforementioned lift and strong height falls/cooling
aloft may still result in a line of heavier showers with isolated
lightning strikes despite Low Cape. NAM/GFS/ECMWF now in better
agreement with the strength of the southerly low-level jet as well
preceding the cold front...but again the overnight timing would tend
to work against mixing the majority of this wind to the surface. Any
heavier bands of showers could mix some of this wind down but
overall the severe threat appears too low to warrant a mention in
the severe weather potential statement for now. Will continue to monitor.
Long term /Saturday through Wednesday/...
the front pulls east of the area Saturday morning leaving in its
wake a much cooler and drier airmass over the weekend. As the trough
axis pivots over the area...favorable trajectories for upslope will
be likely with showers possible over the western slopes. Some
snowflakes may be mixed in as well given 850 mb temperatures crashing to
between 0 and -2 c Saturday night. Spotty freezing temperatures will
be possible along the ridges as well overnight Saturday night.
The airmass will quickly moderate as high pressure moves east
resulting in a low-level southwesterly flow by early next week. The
next storm system and chance of rain will move across the area from
west to east Monday night through Tuesday.
Aviation /03z Thursday through Monday/...
ceilings have dropped to MVFR at mrb. Elsewhere...VFR prevails for now. Expect
flgt restrictions to develop overnight due both to lowering ceilings as well as
fog development. Tafs as of 00z placing more emphasis on visibilities...W/
IFR at climatology favored cho/mrb and MVFR at iad. However...latest trends
suggesting low ceilings may become more of a player. Will monitor for
possible adjustments. Either way...we/ll have IFR conds /or perhaps lower/
in place for start of morning push west of The Hubs...and perhaps
encroaching on iad/dca/BWI as well.
Conditions will improve towards Thursday midday and VFR conditions
expected through Thursday afternoon.
Low ceilings/visibilities to IFR or lower possible Friday-Sat am in moist southerly flow
10-20 kts and gusty ahead of strong front. Low level wind shear possible Friday night.
Models coming into better timing/strength agreement lending to
moderately high confidence during this time. VFR returns Sat PM and
persists through the weekend...winds turning west 10-20 kts behind
light winds on the waters tonight...W/ spds below 10 knots. While there wont
be a strong directional push...a northeasterly component to the drift would
NE-east flow will continue on the waters Thursday. Winds 10-15 kts
are expected. The tighter gradient and therefore stronger gusts
are across the ocean waters. A isolated gust of 18-20kts is
possible across Tangier Sound into Thursday afternoon.
Small Craft Advisory conditions very likely in southerly channeling ahead of strong
cold front Friday. A band of heavy showers which may enhance gusty
winds will approach the waters from the west Friday night. The front
clears the waters Saturday with gusty northwest flow to Small Craft Advisory levels
possible through the remainder of the weekend.
tidal anomalies continue around a foot to 1.25 feet. We/ve met the
minor tidal inundation threshold at pptm2 and slim2...and
Annapolis is on pace to do the same. Coastal flood advisories in place for
all three counties.
Based on current departures...as well as the scope of the tide...
suspect that Baltimore...SW District of Columbia...and Alexandria will also reach
minor /advisory/ criteria...though do not expect a long resident
flood. Have added to the advisory for these areas midnight-5am.
Do not see big changes in the weather regime Thu-Fri. Winds will
mainly be light /or lighter/ with an onshore component. While
departures may not continue quite as sharp as they are now... do
think that they will remain elevated until a cold front crosses lt
Based on the tidal departures we have...seeing whats at the Bay
Bridge Tunnel this evening...using cbofs guidance /accounting for the
bias/...and recognizing that the nighttime tide is the higher of
the two...we/ll be extending the coastal Flood Advisory for the
western shore of the Bay from St Marys to Anne Arundel for another
25 hours. Am more confident on the nighttime tide...but the day
cycle stands a decent chance too.
District of Columbia...coastal Flood Advisory until 5 am EDT Thursday for dcz001.
Maryland...coastal Flood Advisory until 4 am EDT Friday for mdz014-017-
Coastal Flood Advisory until 5 am EDT Thursday for mdz011.
Virginia...coastal Flood Advisory until 5 am EDT Thursday for vaz054.