Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
937 PM EST Wednesday Dec 4 2013
a warm front will pass through the region tonight into Thursday
morning. A cold front will approach the region late Thursday
before slowly passing through Thursday night through Friday night.
High pressure will build overhead Saturday...but the next low
pressure system will impact the area Sunday and Monday.
Near term /through Thursday night/...
a warm front across West Virginia and southwest Virginia will
move northward tonight. Additional moisture will allow for low
level stratus clouds to form...accompanied by pockets of drizzle
or patchy fog overnight and early Thursday. Low temperatures
overnight will be 10 to 15 degrees above normal. Winds will be
light and variable or from the southeast.
Just as the warm front moves to our north and a ridge of high
pressure over the eastern Ohio Valley builds into the region
Thursday...the next cold front will be quick to move in behind
it by Thursday night.
But before the cold front arrives...we can anticipate enough of
an influence from the ridge...breaks in the clouds due to some dry
air...and a moderate advection of warm air to allow for our
temperatures to reach the upper 60s across the southern two-thirds
of the region on Thursday. There may be a few spots across the
Shenandoah Valley and Piedmont that reach the lower 70s. Highs
should be at least 10 to maybe 15 degrees above normal for
Thursday. This may depend highly on how fast clouds can break in
the afternoon. Winds will be south to southwest at 5 to 10 miles per hour.
The cold front will move from northwest to southeast across the
eastern one third of the region Thursday evening before slowing
its forward progress across the remainder of the region overnight
Thursday. Enough warm and moist air at the surface as well as a
few thousand feet above the surface to keep precipitation in the
form of rain. Thursday night temperatures will be mainly in the
50s with exception to the northern Potomac Highlands and along the
Mason Dixon line of northern Maryland where temperatures will
start in the 50s before gradually falling into middle 40s after
cold frontal passage.
Short term /Friday through Friday night/...
12z model guidance in decent agreement with the position of the cold
front near I-95 at 12z Friday. With a blocking ridge just off the southeastern
coast...the cold front will be slow to pass through the area on Friday.
The Ana-frontal structure of the front will lead to widespread rain
on Friday and Friday night. Probability of precipitation are categorical both periods but the
heavier precipitation is expected Friday night when middle- and upper-level lift
from the trough and jet streak spread over the County Warning Area. There is still
some uncertainty with how quickly the colder air filters in to the
region behind the front. There is a chance that precipitation mixes with or
changes to snow before ending Friday night. The greatest potential for
any light accumulations would be along the Allegheny Front.
Long term /Saturday through Wednesday/...
high pressure briefly moves over the region Saturday into Saturday
evening with drying conditions and slightly below normal
temperatures. By late Saturday night...low pressure developing to
the south and west will be close enough to begin producing some
precipitation late Saturday night into Sunday morning. The primary low
track will be well west of the area and over the Great Lakes. A
40-50 knots southerly 850 mb jet positioned downstream of the low will
draw warm air up the eastern Seaboard. However...cold high pressure to
the north will lock low-level cold air in place...causing the
precipitation to start as a wintry mix across all areas from south to
north. With this classic cad setup in place...there is a potential
for a period of freezing rain Sunday into early Monday for the
entire area. At this time...the potential for a significant wintry/icing
event increases the farther north and west of the cities you go. Precipitation
will gradually change to all rain in a warming southerly flow as
the low passes to the west of the area Sunday night and Monday.
Rain and warmer temperatures Monday as the low slowly moves
across the eastern Great Lakes and into southeastern Canada. The
models continue to come into better agreement on this entire
scenario...but still with some timing and vertical temperature
profile differences...which will affect accumulations.
The GFS is quicker than the European model (ecmwf) in moving precipitation out of the area
later Monday into Monday night. A changeover to snow is possible
in the higher elevations out west depending on how much precipitation
falls after colder air moves back in behind the storm. High
pressure with a much colder airmass builds into the region into
the middle of next week.
Aviation /02z Thursday through Monday/...
additional moisture during the last 24 to 36 hours...light
winds...and an approaching warm front have provided a set up for
low stratus clouds and patchy fog across most terminals overnight
and early Thursday. Some drizzle may also occur in spots as ceilings
lower from MVFR to IFR...maybe even LIFR for a portion of tonight.
Visbis will also lower from 6sm to 2 or 3sm around midnight at the
While winds are northeast and light at dca and mrb...calm at BWI
and mountain...they are more southeasterly around 5 kts at iad and cho
as of 02z. Expect winds to become primarily south at all terminals
Thursday and averaging 5 to 10 kts with a few higher gusts around
20 kts in the afternoon. LIFR and IFR conditions will improve to
MVFR and then VFR late Thursday morning into Thursday afternoon.
An approaching cold front Thursday night will bring additional low
clouds and even some light rain to the terminals...reaching mrb
late Thursday. VFR conditions will become MVFR in rain activity.
We could even encounter IFR conditions again Thursday night with
the low clouds and rain setting the stage for low ceilings and visbis.
Cold front will slowly move south of the terminals on Friday. Widespread
rain expected behind the front with a prolonged period of MVFR-IFR
restrictions possible through Friday night.
VFR likely Sat into Sat night...then sub-VFR likely sun-Monday night
with low ceilings/visibilities in precipitation.
low stratus clouds...patchy fog and some drizzle expected tonight
ahead of and along an approaching warm front tonight. Light and
variable winds will become southerly and increase Thursday.
Clouds could break for some sunshine during the afternoon with
warm advection becoming pronounced.
Small Craft Advisory conditions may occur Thursday afternoon.
A cold front will be slow to move through waters on Friday. North-northwest winds
eventually pick up once the front moves south of the area Friday
night...with Small Craft Advisory conditions possible.
Small Craft Advisory conditions possible especially southern area waters Sat-sun am
in northwest flow behind departing cold front. Brief lull sun as pressure
gradient relaxes...then southerly flow ahead of approaching low pressure
leads to increase in winds/waves Sun night into Monday.
an unsettled weather period is in store for the middle-Atlantic region.
The going forecast for Friday is a perfect heavy rain setup for
areas southwest of here. Locally...expected rainfall is on the order
of 0.75-1.50 inches...which creates rises of several feet on many of
our rivers. Flooding cannot be ruled out with the Friday rains...but
it is unlikely unless rainfall is heavier than currently expected.
However...more precipitation is projected over the region Sunday
into Monday...and this will likely be heavier than the first round.
GFS ensemble mean rainfall for both events combined is 3.0-3.5
inches...with some members showing over 4 inches. Given little
recovery time in between events...this amount of rainfall...if it
occurs...could be sufficient to generate flooding on both small
streams and on the mainstem rivers Sunday through early next week.
If frozen precipitation falls...the melting of it factors into the equation
too. There is still a great deal of uncertainty regarding where the
heaviest precipitation sets up...and whether it stays in one place
for a while. So this situation will continue to be monitored in the
District of Columbia...none.