Scientific Forecaster Discussion
fxus61 klwx 181956
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
256 PM EST Sat Nov 18 2017
a powerful cold front will cross the region late tonight. High
pressure will slowly build south of the region late Sunday
through Monday before passing off the East Coast on Tuesday.
Another cold front will cross the region Tuesday night, with
high pressure building back across the region for the latter
half of the week.
Near term /through tonight/...
warm front has been very slowly pushing northward across the County Warning Area
this afternoon. As of this writing it stretches from near Bel
Air Maryland southwestward to around Front Royal Virginia and west towards
Petersburg WV. It should start to move again over the next few
hours as southerly flow continues to increase just above the
surface and ongoing rain showers diminish, so we still think
warmer air will envelope the entire region this evening. Many
areas will probably see temps rise as we pass midnight and we
could be into the low 60s across much of the County Warning Area early Sunday
morning as the cold front moves in. Southerly winds have not
been very effectively mixing down even in the warm sector, with
gusts mainly in the 20 kt range, with a few gusts closer to 40
knots at the higher elevations of western Virginia and eastern WV
where the Wind Advisory starts early.
After a lull in precip expected this evening, showers will move
in ahead of the cold front later this evening into the
overnight, with potentially a line of gusty showers right on the
front. Given strong pressure surge and potential for showers
right on the front to effectively mix this wind down, decided to
start Wind Advisory earlier than the synoptic wind would argue.
Its possible that some areas see their strongest winds with the
front itself. After frontal passage, temps will crash quickly
back from high 50s and low 60s down to the 40s, with sub-
freezing expected along the Allegheny Front. These areas will
also get upslope flow, so snow is expected to start accumulating
late tonight, and a Winter Weather Advisory has been issued.
Most of the accumulation will likely be after sunrise Sunday.
Short term /Sunday through Monday night/...
gusty winds are the main story as pressure rises rapidly behind
the strong cold front on Sunday. Best wind aloft and mixing
appear to coincide mid-late morning Sunday, with most likely
region to get 45+ mph gusts being north-central MD, but given
uncertainty decided to issue for all of District of Columbia Metro and areas
further west from there. As is typical, winds will diminish as
we head south through the Virginia Piedmont, so no advisory for cho
Metro. Temps will have difficulty rising with strong cold
advection, but expect most places near I-95 to edge back into
the 50s during the day. Upslope flow will continue Sunday
morning along the Allegany front, with continued snow showers.
There may be a lull midday, with another surge in snow showers
late in the day and early Sunday evening as another shortwave
moves through. Thus, went a bit long on the advisory to catch
both bursts. Accumulations will be somewhat elevation dependent,
but local amounts of 6-7 inches aren't out of the question.
High pressure will then slowly build eastward to our south
Sunday night and Monday. Upslope flow will diminish, allowing
snow showers along the Allegany front to end late Sunday night,
and the gradient and winds aloft will both diminish as well,
allowing winds to die down. That said, we still expect it to be
breezy Monday. Otherwise, Monday will be cool but mostly sunny,
with highs in the 40s to lower 50s after a night of below
freezing temps region-wide. Not quite as chilly Monday night as
the high slips further east, but still dry and mostly clear.
Long term /Tuesday through Saturday/...
a cold front will move across the lower Great Lakes region
Tuesday and approach the mid-Atlantic region Tuesday night.
There is some uncertainty with this front as to whether it
produces precipitation or not late Tuesday and Tuesday night.
The GFS model hints of a dry frontal passage as a broad area of
high pressure covers the southeast U.S., Not allowing for a
substantial moisture return flow. The European model hints of
measurable rainfall across the eastern two-thirds of our region
with high pressure farther offshore to allow for a modest
moisture return flow. We will keep slight/chance of rain showers
and slight/chance rain and/or snow showers in the west, for
now. Temperatures will be a little chilly.
High pressure will make an attempt to build in behind the
weakening and departing cold front. Here again, there are
discrepancies as to how far east the high will build in the mid-
Atlantic and whether or not the preceding cold front will stall
along the East Coast, allowing for a possible low to develop
along it. The GFS model is weaker than the European with the
building area of high pressure, while the GFS tries to develop a
low along a stalled front. For now, we will lean toward dry and
chilly conditions with high pressure building into the region
Wednesday through Saturday.
Aviation /20z Saturday through Thursday/...
mrb has gotten stuck north of the warm front and has gone IFR
with low cigs from the rain. Expect this to lift later this
afternoon or evening, but timing it out is tricky given
persistetnce of light rain and low level cold air. Elsewhere,
gusty south wind and VFR cigs/vis with a stray shower is the
rule through this evening. Later tonight, powerful cold front
moves through with possible MVFR restrictions in showers, and
gusts possibly up to 40 knots at frontal passage. VFR returns
behind the front for Sunday and Monday, but gusty winds expected
to continue Sunday with gusts still possibly reaching 40 knots,
particularly BWI/mountain (much less chance at cho). Winds diminish
Monday as high pressure builds in.
VFR conditions overall Tuesday through Wednesday night. Any
rain showers could produce a brief period of MVFR conditions
Tuesday night. Winds southwest 5 to 10 knots Tuesday and Tuesday
night. Winds becoming northwest 5 to 10 knots Wednesday and
Small Craft Advisory all waters at present as storm system approaches. Small Craft Advisory should
be the rule tonight as the potenti low level jet will still have
trouble mixing down. However, with the cold front passage late
tonight, we could see gusts to 40 knots, particularly if a line
of showers develops right on the front, so started the gale
tonight. Best gale period is probably going to be mid-late
morning Sunday, however, with diminishing winds thereafter. Small Craft Advisory
will likely need to continue after the gale ends Sunday night
and Monday, then sub-Small Craft Advisory may finally return Monday night for a
Small craft advisories possible Tuesday ahead of a cold front.
No marine hazards expected Tuesday night through Wednesday
night. Winds southwest 10 to 15 knots gusts up to 20 knots
Tuesday. Winds southwest around 10 knots Tuesday night. Winds
becoming northwest around 10 knots Wednesday and Wednesday
strait's point peaked below flood stage this afternoon. The
lower of the two astronomical tides is tonight so despite
gradually increasing anomalies it shouldn't hit again this
evening. Highest anomalies are in the northern part of the Bay
so expect solid minor tidal flooding at
Annapolis/Baltimore/Havre de Grace. District of Columbia SW Waterfront may come
close as well but not enough confidence for a headline just yet
but will monitor. It's a bit odd to see Havre de Grace hitting
minor but not strait's point, probably because winds are more
west of south near the mouth of the Bay pushing water out, while
winds north of southern Maryland are more southerly trapping water
further north. Anomalies will begin to decrease as winds become
westerly late tonight across the entire area, but water levels
may not fall quickly enough to prevent flooding during tomorrow
morning's high tide cycle.
District of Columbia...Wind Advisory from 4 am to 3 PM EST Sunday for dcz001.
Maryland...Wind Advisory until 3 PM EST Sunday for mdz501.
Wind Advisory from 4 am to 3 PM EST Sunday for mdz003>006-011-
Coastal flood advisory from 7 PM this evening to midnight EST
tonight for mdz508.
Coastal flood advisory until 9 PM EST this evening for mdz014.
Coastal flood advisory until 10 PM EST this evening for mdz011.
Virginia...Wind Advisory until 3 PM EST Sunday for vaz025-026-503-504-507-
Wind Advisory from 4 am to 3 PM EST Sunday for vaz027>031-040-
WV...Wind Advisory until 3 PM EST Sunday for wvz501-503-505-506.
Wind Advisory from 4 am to 3 PM EST Sunday for wvz050>053-055-
Winter Weather Advisory from 4 am to 11 PM EST Sunday for
Marine...Small Craft Advisory until 6 PM EST this evening for
Gale Warning from 6 PM this evening to 6 PM EST Sunday for