Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
936 PM EDT Tuesday Mar 11 2014
high pressure will persist off the southeast coast into Wednesday.
Strong low pressure and its associated cold front will pass
through the Middle-Atlantic States Wednesday afternoon and evening.
An upper level trough will move across the area Wednesday
night...then Canadian high pressure will build through Thursday
night before moving off the coast Friday. An upper level trough
will persist over the region this weekend.
Near term /through Wednesday night/...
as of 01z...Lee cirrostratus have eroded making for a clear
evening except for scattered middle level clouds over the Mason-Dixon which
is associated with a warm front ahead of the developing low over
the Midwest. A northern stream trough is swinging from the Great
Plains to the Midwest with surface low development over MO (999mb mslp
at Columbia mo). Lightning has become more abundant over the past
couple hours over the show-ME state with strong wind gusts as
well. Precipitation with this system is breaking out across the Midwest.
Meanwhile...a southern stream trough is over MS/la. These two systems
will phase over the Kentucky late tonight and then track northeast to
the Mason-Dixon line by Wednesday afternoon.
So radiational cooling will dominate this evening and with the
central middle-Atlantic currently between the two systems...cloud
cover may not come in for several more hours. Therefore...will
update for lower temperatures tonight...but it will still be mild
relative to this winter.
Headlines...considered a high wind watch for northwestern zones for
Wednesday afternoon/evening...but the best chance for 50 knots gusts
will come from convective activity so left that be for now. Snow
is expected near the lower threshold of Winter Weather Advisory
for the five zones west of the Allegheny Front (three inches).
Based temperature on a blend of local 12km WRF-nmm and mav/met.
This is a strong cold front an cold air advection will be
strong behind it...allowing wind gusts to be maximized Wednesday
afternoon through night.
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 /01z Wednesday 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 for southern Maryland waters Wednesday morning for southerly
channeling... then for all waters Wednesday afternoon as southerly winds strengthen
ahead of the cold front. 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 smws.
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 through at least
Thursday morning in a gusty west/northwest flow...before
diminishing to below Small Craft Advisory levels Thursday evening. Scas are likely
Thursday night...then possible Friday afternoon in southerly flow.
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 astronomical
prediction this evening. This level will merely be maintained
overnight from southerly flow around 10 knots.
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