Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
330 PM EDT Tuesday Mar 11 2014
high pressure will persist off the southeast coast tonight.
Strong low pressure and its associated cold front will pass
through the Middle-Atlantic States Wednesday afternoon and evening.
High pressure will build overhead Thursday before moving off the
Near term /through Wednesday night/...
the Middle-Atlantic States remains situated between hipres off the
southeast coast and lopres over eastern Canada this afternoon. A cold front
associated with the low has moved off the New England coast. Zonal
steering flow around the high had caused the southern portion of the
frontal boundary to stall over the northern middle-Atlantic region.
Looking upstream...a 995 mb surface lopres is located along the OK-Kansas
border. This low will track eastward along the stationary boundary through
the Midwest tonight. The low and its attendant strong cold front
will move through the Middle-Atlantic States Wednesday afternoon and night.
Temperatures this afternoon have reached the 70s in central Virginia...southern and western
Maryland...and eastern portions of WV under plenty of sunshine. For locations
in between...orographic cirrus has been stubborn to erode which has
kept temperatures about 10 degrees cooler. Expect more breaks in these
clouds to develop late this afternoon and evening before clouds move back
into the area overnight ahead of the cold front.
On Wednesday...the County Warning Area will be situated in pre-frontal warm sector
with anomalously deep/strong southerly flow developing over the eastern
Seaboard. Nose of low level Theta-E ridge reaches the County Warning Area and will be
accompanied by afternoon temperatures in the 60s and 70s with dewpoints in the
50s. Do not expect widespread coverage of showers Wednesday morning well
in advance of the front. Models continue to indicate the
potential for a line of convection to develop in this modestly
unstable airmass during the afternoon. Significant height falls will
occur ahead of the main upper low. The greatest height falls will
take place over the middle-Atlantic region between 18z Wednesday through
03z...which is the period that outlines the best chance for
showers and storms. After coordination with Storm Prediction Center...entire County Warning Area was
placed in a slight risk Wednesday for severe storms. The primary threat
from these storms are straight line wind damage although a few
embedded qlcs tornadoes are possible. The threat of tornadoes will
be dependent on how close the warm front boundary will be to our
County Warning Area. Latest guidance trending toward the warm front lifting north
of the Mason-Dixon line prior to the arrival of convection.
Precipitation associated with the cold front will likely move east of
the chesepeake Bay by late Wednesday evening. Models continue to show impressive
pressure rises and cold air advection in wake of cold frontal passage. Wind
advisories will likely be required for the entire County Warning Area Wednesday night with
northwest wind gusts near 50 miles per hour mixing down.
800 mb temperatures decrease from around 10c ahead of the front to between
-15c and -20c behind the cold front in less than hours. Leaned
toward the warmer side of guidance for min temperatures Wednesday night with
downsloping winds offsetting the cold air advection a bit. Wind chill advisories
may be needed late Wednesday night for the higher elevations of the
Potomac Highlands and Blue Ridge...where wind chills between 5
below and 15 below zero are forecast. Several inches of accumulating
snowfall is expected for the western slopes of the Allegheny Front.
Winter weather advisories may be needed.
Short term /Thursday through Friday/...
a tight pressure gradient between the departing storm east of New
England and cold high pressure building towards the area from the
northern Great Plains will result in cold and blustery conditions
Thursday. Most guidance suggests high temperatures will struggle to
get out of the 20s on Thursday...but a compressional downsloping
flow and higher middle-March sun angle argue leaning towards the higher
MOS guidance...which suggests highs in the 30s for most of the area.
Cold...dry and blustery conditions persist Thursday. For fire
weather threats...please refer to the fire weather section below.
Winds diminish Thursday evening...and may go calm late Thursday
night as high pressure moves overhead. High clouds move into the
area late...but a brief window of strong radiational cooling is
possible overnight. If winds become lighter earlier than forecast
during the evening...then low temperatures could be several degrees
colder than currently advertised.
Arctic air quickly moves away from the region Friday as high
pressure moves to the east...resulting in milder temperatures closer
Long term /Friday night through Tuesday/...
high pressure will move off of the Carolinas overnight Friday and
southerly flow will fill in Saturday morning. Temperatures on Saturday
should get up into the upper 50s and lower 60s across most of the
area despite partly cloudy to mostly sunny skies. A cold front
associated with low pressure over eastern Canada will swing through
the area Saturday night. This will mainly be a dry frontal
passage...with chances for light rain confined to the Potomac
Highlands and along the Mason Dixon line Saturday evening and
Northerly flow will fill in behind the front on Sunday. High
temperatures on Sunday will be near 50 with highs on Monday about 10
degrees colder. A coastal low may impact the middle Atlantic on
Monday...but with considerable disagreement within both the GFS and
Euro ensemble members...confidence is low in timing and impact. Have
carried precipitation chances in the forecast Sunday evening through the
day on Monday. Ptype is uncertain as well...but with a very cold air
mass expected to be in place at the surface by the end of the
weekend wintry precipitation is a possibility. Generally left probability of precipitation around
30 percent and ptype as rain or snow to reflect the uncertainty with
High pressure will build back in along the East Coast early Tuesday
Aviation /19z Tuesday through Sunday/...
predominately VFR through tonight. There is a potential for low clouds to
develop on cool side of a warm frontal boundary early Wednesday morning.
According to the 12z NAM...this boundary may be positioned somewhere
between iad and mrb...putting the northern terminals in a limited setup
for MVFR ceilings.
Strengthening southerly flow ahead of a cold front and wave of lopres will
draw warm/moist air into the region on Wednesday. With these winds at the
top of the nocturnal inversion increasing to near 40 knots Wednesday
morning...there is a potential for low level wind shear conditions within the lowest 2
kft above ground level.
Strong pressure surge expected in wake of frontal passage Wednesday night. Wind
gusts near 45 knots possible during this time.
VFR conditions are forecast Thursday through Friday. Breezy
west/northwest flow of 20 to 25 knots gusting to 35 knots
Thursday. Winds diminish Thursday evening and become light and
variable early Friday morning...before becoming south around 10
knots Friday into Friday night.
IFR conditions are possible Saturday afternoon as a cold front
brings a low cloud deck to the region.
Small Craft Advisory GOES into effect Wednesday afternoon for all of the waters as southerly winds
strengthen ahead of the cold front. There is still a concern for Small Craft Advisory
winds to start a bit earlier in the southern most zones of the Maryland chesepeake
Bay with southerly channeling developing during the morning. Additionally...
there is a potential for gales Wednesday afternoon as southwesterly low level jet increases
to near 40 knots between 2-3 kft above the surface. Did not have enough
confidence to make either of these headline change given the
stability produced by warm air moving above the cooler
waters...which is not an efficient setup for mixing. These strong
winds do however have a better potential to reach the surface late in
the day Wednesday with convection just ahead of frontal passage. Thunderstorms could require
Gale Warning issued for Wednesday night with a strong pressure surge forecast in
wake of cold frontal passage producing gusty northwest winds 30-40 knots across all of
the waters. Mixing will be efficient too as lapse rates steepen in
an impressive cold air advection regime behind the front.
Gale conditions are likely across all waters Thursday in a gusty
west/northwest flow...before diminishing to below Small Craft Advisory levels
Thursday evening. Small Craft Advisory conditions are possible Friday afternoon in
A Small Craft Advisory may be necessary during the day Saturday in
breezy prefrontal southerly flow.
abundantly dry air will accompany the Arctic airmass invading the
area on Thursday...with dew points below 0f expected. This will
result in relative humidity values bottoming out in the teens
Thursday afternoon. The dry and windy conditions may result in an
enhanced fire spread potential Thursday afternoon depending on how
much rainfall the area receives Wednesday into Wednesday evening.
tidal departures remain around 0.5 feet above normal this afternoon. Do
not expect much of a change in these anomalies tonight under light flow.
The best push of southerly flow comes Wednesday...especially during the midday
and afternoon ahead of a strong cold front. While the higher of the two tide
cycles comes at night...the difference is only a matter of inches.
The PM cycle would still have a higher risk...due to when the
push of southerly flow arrives. Best estimate is that minor inundation
may result. Latest cbofs guidance shows Annapolis reaching minor
coastal flood threshold during the Wednesday afternoon high tide.
Expect a blow out to occur Wednesday night...but am unsure how it will
interact/be timed compared to the incoming tide cycle. Regardless...
once the Wednesday night tide pulls out...water should be driven out...and will
be talking about negative water level departures by Thursday.
District of Columbia...none.
Marine...Small Craft Advisory from noon to 6 PM EDT Wednesday for
Gale Warning from 6 PM Wednesday to 6 am EDT Thursday for
Small Craft Advisory from 8 am to 6 PM EDT Wednesday for