FXUS61 KLWX 190140

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
940 PM EDT Thu Oct 18 2018

High pressure will shift overhead tonight, then migrate off the
Mid-Atlantic coast on Friday. Strengthening low pressure over
Quebec will usher a cold front through our region Saturday and
Saturday night. Canadian high pressure will return Sunday and
Sunday night, then move offshore on Monday. Another cold front
will sweep through the area Tuesday into Wednesday, with high
pressure building over the region in its wake.


Canadian high pressure centered over the Ohio Valley will
build overhead the remainder of the night. Winds have gone
light and variable this evening, if not calm, and skies are
crystal clear. This has allowed temperatures to slide down in to
the upper 30s to middle 40s across much of the area, with the
city centers holding in the upper 40s to near 50 degrees. Given
this prime setup for radiational cooling, a Frost Advisory
remains in effect for areas along and east of the Blue Ridge,
with a Freeze Warning in effect for a majority of locations west
of there. Frost will be less likely in the downtown areas of
Washington and Baltimore, as well as over the open ridge tops
and near the Bay. Come daybreak, lows will range from the low-
mid 30s most everywhere, with lower 40s near the city centers.
Current forecast is in good condition with no major changes
needed at this time.


High pressure will move offshore Friday and a return southerly
flow will develop. The southerly flow will allow for a milder
afternoon with max temps in the lower to middle 60s for most
areas. High and mid-level clouds will increase late in the day
ahead of the next of an approaching upper-level disturbance in
the westerly flow aloft.

The upper-level disturbance will pass through Friday night,
likely triggering showers across the area. Precipitation amounts
should be light since most of the forcing will be from the mid-
levels, and there is little forcing in the lower levels due to a
strong westerly component to the low-level flow.

The disturbance will move off to the east Saturday, but a potent
cold front will approach Saturday before passing through late
Saturday afternoon into Saturday evening. A couple showers are
possible with the frontal passage. Would not be surprised to
see an isolated thunderstorm later Saturday afternoon and
evening, but did not reflect this in the forecast at this time.
This is because of a potent upper-level disturbance that will be
swinging through with the cold frontal passage. Our area will
be within left exit region of the mid-level jet, and height
falls will be potent. Strong cold advection will cause blustery
and chilly conditions overnight Saturday. Snow showers are
likely along and west of the Allegheny Front as colder air
filters in, and the first snowfall accumulation of the season is
possible across these areas. Low Saturday night will be in the
40s for most locations, but upper 20s to lower 30s along the
ridge tops.


Cool and dry conditions will be the theme across the area late this
weekend through the middle of next week.

An upper level trough will be located overhead Sunday morning, then
progress off to the east throughout the day. Subsidence in the wake
of the trough, coupled with a weakening low-level wind field will
cause any leftover upslope rain or snow showers along the Allegheny
Front to dissipate by midday. Otherwise, mostly sunny skies are
forecast across the area. With the core of the cold air located
overhead, temperatures will be well below normal. High temperatures
will range from the low 40s in the mountains, to the low to mid 50s
east of the Blue Ridge.

High pressure will crest overhead Sunday Night, leading to light
winds and cool overnight temperatures. Lows should dip below 40
degrees at most locations, with some 20s even possible in the higher
elevation valleys to the west of the Blue Ridge. High pressure will
move to our east on Monday, as a shortwave trough tracks southeast
from the Canadian Prairies toward the Great Lakes. Weak warm
advection in advance of this trough will allow temperatures to
moderate slightly for Monday, and then again on Tuesday. However,
temperatures will still be below normal with highs in the mid-upper
50s on Monday, and low 60s on Tuesday. Another strong area of high
pressure will build over the Great Lakes on Wednesday, helping to
prolong our stretch of dry weather.


High pressure building over the region will promote light winds,
clear skies, and VFR flying conditions tonight and into Friday. A
few showers are likely Friday night into Saturday. VFR conditions
are still most likely during this time, but brief MVFR conditions
cannot be ruled out. Gusty west to northwest winds are expected
behind a cold front late Saturday or Saturday night, depending on
the timing of the frontal passage. A few showers are possible
during this time as well.

VFR conditions are expected at the terminals Sunday through
Wednesday. No precipitation is expected during the Sunday through
Wednesday time period, but gusty northwesterly winds will be
possible during the day on Sunday.


Sub SCA conditions have won out this evening as winds abated
given high pressure building over the waters. This high will
shift offshore Friday, bringing a southerly component to the
wind field. The flow will strengthen ahead of a cold front late
Friday into Saturday. An SCA is in effect for the Bay and lower
Tidal Potomac River Friday afternoon and Friday night. SCA wind
gusts are possible across the upper and middle Tidal Potomac
during this time, but confidence is not high enough for an
advisory during this time.

An SCA will likely be needed for Saturday through Saturday night
both ahead of and behind the front. The strongest winds will be
behind the front Saturday night and a Gale Warning may be needed
for portions of the waters during this time. A Gale Warning may
also be needed Sunday ahead of building high pressure.

SCA levels winds appear possible on Monday as high pressure moves


DC...Frost Advisory from 1 AM to 9 AM EDT Friday for DCZ001.
MD...Freeze Warning from 1 AM to 9 AM EDT Friday for MDZ501-502.
Frost Advisory from 1 AM to 9 AM EDT Friday for MDZ003>006-011-
VA...Freeze Warning from 1 AM to 9 AM EDT Friday for VAZ025>031-503-
Frost Advisory from 1 AM to 9 AM EDT Friday for VAZ036>040-
WV...Freeze Warning from 1 AM to 9 AM EDT Friday for WVZ050>053-055-
MARINE...Small Craft Advisory from 2 PM Friday to 6 AM EDT Saturday for


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