Scientific Forecaster Discussion
fxus61 klwx 120237
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
937 PM EST Mon Nov 11 2019
low pressure will track northeastward from the Ohio Valley to
New England through Tuesday, bringing its associated cold front
across the mid-Atlantic by Tuesday afternoon. Arctic high
pressure will follow Wednesday into Thursday. A cold front will
likely move from the Midwest to off the East Coast Thursday into
Friday. Low pressure may develop offshore of Georgia and the
Carolinas this weekend as another area of Arctic high pressure
builds over northern New England.
Near term /through Tuesday/...
very little change in thinking this evening. What follows is
mostly the afternoon discussion...
Arctic cold front currently stretches through the Ohio Valley
into southern NY, with low pressure developing over western
PA. Very mild air has been drawn into the area ahead of this
system, with temperatures still in the 50s to near 60.
Meanwhile, mid 30s are found immediately behind the front.
The low will move off to the northeast overnight, and the cold
front associated with the low will move into the area from the
northwest after midnight. A digging upper-level trough will also
slide east, and a potent upper-level jet combined with the
frontal passage will be enough for some rain to break out around
midnight near the Potomac Highlands, and toward morning farther
east across the Shenandoah Valley and Metro areas. Much colder
air will begin to move into the area behind the front, and this
will cause rain to mix with and change to snow across the
Allegheny Highlands toward morning. Have issued a Winter Weather
Advisory for western Grant and western Pendleton for a
combination of higher elevation (better snow accum) and
favorable positioning for residual upslope. Gusty northwest
winds will cause temps to fall sharply into the 30s (20s in the
mountains), but temps in the 40s and 50s will persist ahead of
the boundary through tonight.
The cold front will pass through the rest of the area Tuesday
morning. Expect a relative minimum in precipitation in the Lee
of the Appalachians to the Blue Ridge due to downslope flow.
However, good mid level forcing, an anafrontal structure, and a
coupled upper jet will likely result in a blossoming of
precipitation east of the Blue Ridge. Despite rapid cold
advection, it will still take some time for low levels to
support snow, so we'll be threading a needle in terms of seeing
any flakes before dry air wins out and precipitation ends. It
could be that the best chance of snow falling ends up east of
I-95, where better forcing will result in precipitation hanging
in the cold air longer, and heavier rates to potentially aid
temperatures falling. Am still not expecting much if any
accumulations in the lower elevations, especially since ground
temperatures will be very warm. Precipitation should be exiting
southern Maryland during the early afternoon.
Blustery and cold conditions are expected during the afternoon
with some partial clearing, as temperatures level out in the 30s
to lower 40s. As the mid level trough axis pivots across, snow
showers...and perhaps squalls...will continue along and west of
the Allegheny Front with totals of 2-4 inches in favored
locations. With Froude numbers well over one and saturation/weak
instability in the dendritic growth zone, would expect some snow
showers to move east of the ridge too, with the most favored
areas being western Maryland and eastern West Virginia. However,
there could be some snow showers as far east as the Metro areas.
The most intense showers could produce a quick coating but most
areas won't see accumulation.
Short term /Tuesday night through Wednesday night/...
high pressure will build toward the area Tuesday night. Any
leftover snow showers should dissipate during the evening, and
even upslope snows should steadily dwindle, with only potential
flurries by late night. Very cold conditions are expected with
min temps in the teens and 20s and wind chills in the lower to
middle teens. Wind chills could drop below zero in the higher
elevations. Will let later shifts evaluate if the higher
alleghenies will meet Wind Chill Advisory criteria of -10f. See
climate section below for records.
High pressure will build across the area Wednesday and Wednesday
night, so winds will diminish. The main story is the continued
well-below normal temperatures, with highs in the 30s to around
40, and another night of lows in the teens and 20s.
Long term /Thursday through Monday/...
mostly dry conditions and well below normal temperatures are
expected to persist through the long term period. A weak
shortwave trough embedded in northwesterly flow will approach
the area from the Ohio Valley on Thursday. Most guidance doesn't
produce any precipitation, but a brief rain or snow shower
can't be ruled out in the zone of warm advection ahead of the
system. At the very least, the shortwave should produce a fair
amount of cloud cover across the area, with high temperatures
only reaching into the 40s.
By Friday, a trough is expected to dig southward from the
Mississippi Valley toward the southeast US and eventually become
cutoff from the upper level waveguide. This cutoff trough will
slowly drift off the Georgia/SC coast this weekend, inducing
cyclogenesis off the coast. Meanwhile, a potent trough is
expected to swing across northern New England on Friday. A
strong area of high pressure will build over the northeast for
the weekend in the zone of subsidence following the
aforementioned trough. This high will act to keep the area of
low pressure off the Carolina coastline suppressed to our south
through much of the weekend. Even though the majority of both
deterministic and ensemble guidance keeps the system centered
well to our south, it will be a feature that bears watching as
we move through the upcoming week.
Aviation /03z Tuesday through Saturday/...
VFR conditions are expected through this evening. A period of
rain will develop late tonight (09-10z) into early Tuesday
morning before ending from late morning through midday (15-18z).
While a period of brief -rasn or even -sn is possible, it will
be brief if it occurs, so shouldn't be too impactful with air
temperatures above freezing. With precip likely fairly light,
left conditions more in the VFR range, though an interval of
MVFR can't be ruled out. A sharp wind shift to the northwest is
expected behind the cold front early Tuesday morning. Gusts
around 20 to 30 knots are expected behind the boundary.
If we lose VFR, they will return by Tuesday afternoon and
continue through Wednesday night. A snow shower is possible
Tuesday afternoon and evening. Winds will diminish some Tuesday
night, but some gusts around 20 knots are still possible before
winds diminish more later Wednesday and Wednesday night as high
pressure builds in.
VFR conditions are expected at the terminals Thursday and
an Small Craft Advisory is in effect for all waters, firstly for southerly flow
tonight. A cold front will pass through the waters Tuesday
morning. A sharp wind shift to the northwest is expected with
gusts around 20 to 30 knots. An Small Craft Advisory is in effect for all of the
waters Tuesday. There's some potential for a few near-gale
gusts, but don't have the confidence for a warning at this time.
A period of rain could possibly mix with snow before ending as
it turns sharply colder through the day. Small Craft Advisory starts to dwindle
later Tuesday night, but portions of the Bay may hold on to Small Craft Advisory
conditions Wednesday morning, before winds become lighter later
in the day as high pressure builds overhead. Sub Small Craft Advisory level
winds are expected through the latter portions of the week.
the Arctic air plunging southeast across our region later this
week will threaten a few records. Here are the current record
lows and record cold highs.
Site low 11/12 low 11/13 high 11/13 low 11/14
dca 24/1926 22/1911 31/1911 19/1920
BWI 18/1957 22/1911 32/1911 18/1986
iad 22/1988 21/1963 38/1996 13/1986
District of Columbia...none.
WV...Winter Weather Advisory from 3 am to 6 PM EST Tuesday for
Marine...Small Craft Advisory until 6 am EST Wednesday for anz530>534-
Small Craft Advisory until midnight EST Tuesday night for