Small Craft Advisory 94, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
301 PM EDT Wed Apr 28 2021

.SYNOPSIS...
A Bermuda high will continue to shift farther offshore
through Thursday, which will allow a cold front to approach the
region. The front will cross the area Thursday night, then high
pressure will gradually build back into the region for the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
High pressure is located off the southeast coast today with a
cold front stretching across the Great Lakes. Temperature are
well above normal and approaching records (see climate section
below).

Forecast soundings suggest that we remain capped most of the
afternoon. However, at least some instability should build
thanks to moderate mid level lapse rates. If this cap can break,
we could have a few showers and storms late this afternoon and
evening, mainly over the Appalachians and in a pressure trough
east of the Blue Ridge to the I-95 corridor. Given weak forcing,
potential for too much dry low level air, and relatively weak
shear, am not expecting coverage or intensity to be too great at
this time. If a stronger shower or storm were to develop,
however, it could produce gusty winds given an inverted-V
boundary layer. Will need to keep an eye on upstream activity
in the Ohio Valley/PA moving toward our area this evening as
well, but most models keep these storms away from our region.

A vort max approaching from the west tonight could spawn some
additional showers, mainly along the Mason-Dixon region and
Appalachians. Low temperatures will stay very mild tonight
thanks to clouds, a light south wind, and higher dewpoints in
the 50s-60s.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
By Thursday, the front will have stalled just to our north
across Pennsylvania and Ohio. There will be plenty of moisture
and convergence along the frontal zone, so showers should be
ongoing. The biggest question is just how far south into our
area showers will impact the morning hours: highest chances near
the Pennsylvania border, with dry conditions likely across
central Virginia/southern Maryland. An upper level trough will
dive southeastward across the Midwest during the afternoon and
evening, causing low pressure to develop along the front and
eventually push the front southeastward across the area.
Precipitation may still largely be limited to northern and
western areas through the afternoon and early evening. With the
upper level trough approaching, wind fields will increase, but
instability will be inhibited by ongoing showers, cloud cover,
and poor lapse rates. However, some gusty/organized showers and
embedded storms could develop as forcing increases late in the
day, which is highlighted by a Marginal severe weather risk from
SPC. The other issue to watch will be the potential for locally
heavy rain due to the stalled front, training precipitation, and
unseasonably high precipitable water values. At this time, the
highest rain totals are expected north and west of our CWA. High
temperatures will stay in the 70s across northwestern areas,
but temperatures should be able to push into the mid 80s again
near and south of DC where clouds aren't as thick.

The cold front will push across the area Thursday night. Showers
and storms may tend to fall apart with the loss of daytime
heating, but at least some showers will be possible across the
entire area.

On Friday, strengthening low pressure will be moving across New
England, while strong high pressure builds across the Midwest
behind the departing cold front. This will result in strong
northwest winds potentially reaching Wind Advisory criteria,
especially across northern/western areas. There remains some
uncertainty in the strength of the low level wind fields, as
well as whether the strongest winds will overlap the deepest
diurnal mixing (two surges: post frontal early in the morning,
and with a vort max/secondary front during the late
afternoon/evening). Winds could remain elevated through the
night.

Most areas will be dry by Friday, although there could be some
upslope rain showers along the Allegheny Front, as well as an
isolated shower near the Pennsylvania border as the upper trough
crosses the area. Temperatures will return closer to normal.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Guidance continues to depict a split flow regime across the CONUS
with a deep trough exiting New England Saturday. A somewhat deeper
trend has been noted with this trough over the last few model
cycles. There is some lingering uncertainty with the fate of an
upper low in the southern stream and its ejection across the lower
MS Valley late in the weekend into early next week, but locally most
sensible weather will be reliant on the more closely positioned
northern stream this weekend.

Noticeably cooler temperatures and blustery conditions are forecast
Saturday in the wake of a surface cold front attendant to the
aforementioned upper trough exiting New England. A warming trend is
anticipated Sunday into Monday as high pressure building in behind
the front shifts offshore.

By early next week, the upper low moving across the lower MS Valley
begins to drift back into phase with the northern stream. At the
surface, a sprawling frontal zone will likely arc from the Northeast
across the Midwest and into the Plains. As the front slowly presses
southeastward, multiple waves riding along it will spark shower and
thunderstorm chances. Given increasing heat, humidity, and shear
overlaying the approaching frontal zone, thunderstorms that develop
could become severe, especially if the waves forcing the storms
cross around peak heating. Uncertainty remains, though, given at
least part of this setup will be reliant on the ejection of an upper
low, something that models often struggle with in the medium
range.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
VFR conditions continue with southwesterly winds through tonight.
A few gusts to around 20 kt are possible this afternoon. While
conditions are expected to remain mostly dry, there's a slight
(~20%) chance of a shower or thunderstorm generally between
22-03Z this evening. Did not have the confidence to include that
in the TAFs due to weak atmospheric triggers. A few additional
showers could occur late tonight with the greatest chance at
MRB. However, coverage/confidence is not high enough to include
a mention in the TAF yet.

A cold front will approach Thursday and cross Thursday night.
Gusty southwest winds will pick back up during the day. It's
questionable how far south and showers or isolated thunderstorms
get during the day, with the highest chances at MRB (including a
period of MVFR ceilings). The metro area terminals will have
the highest rain and possible thunder chances during the
evening, but by this time, coverage may be more scattered. A
brief period of MVFR ceilings could occur, but confidence is
low.

Winds will become northwesterly behind the front, and gusts
could reach 30-40 kt Friday into Friday evening.

Mainly VFR expected this weekend. NW wind gusts of 20-30 kt psbl
Sat.

&&

.MARINE...
Southwesterly winds will continue through Thursday due to high
pressure off the southeast coast. An SCA remains in effect for
all waters this afternoon and evening, but it's marginal due to
very warm air over the cooler waters. A stray shower or
thunderstorm could also approach the waters this evening,
which could cause some gusty conditions. SCA conditions could
linger a good part of the night on the wide waters of the mid
Bay, so extended this SCA in this area.

Small Craft Advisory conditions may be a little more common
Thursday, but mixing issues will continue, with the highest
gusts more likely near land. A cold front will cross the waters
Thursday night. Showers and a few thunderstorms will be
possible along the front. SCA conditions will also likely
continue, especially after winds shift to the northwest. Two
surges of strong winds are expected: one early Friday morning
behind the front, and another late Friday afternoon/evening
behind a secondary front. At least brief gale force winds are
possible, so have hoisted a Gale Watch for a large portion of
the waters. Have held off for the southern Maryland waters for
now since winds may not increase until Friday evening and may be
marginal. Elevated winds likely continue through Friday night.

A gusty northwesterly wind will likely persist through at least the
first half of Saturday behind a departing cold front. The wind
should abate Saturday night as high pressure moves across, then may
increase a bit again late in the weekend as the high moves
offshore.

&&

.TIDES/COASTAL FLOODING...
A southerly flow will keep water levels elevated through
Thursday. Minor tidal flooding is possible in sensitive areas
like DC SW Waterfront and the Upper Chesapeake Bay, but
caution/sub-flood levels look more likely. Water levels will
decrease for the end of the week behind a strong cold front.

&&

.CLIMATE...
Records for today/Thursday across the area climate sites:

TODAY'S HIGHS
DCA: 92 (1957)
IAD: 88 (1990)
BWI: 90 (1957)

THURSDAY MORNING LOWS
DCA: 70 (2017)
IAD: 65 (2017)
BWI: 67 (2017)

THURSDAY HIGHS
DCA: 91 (2017)
IAD: 89 (2017)
BWI: 91 (1974)

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Gale Watch from Friday morning through late Friday night for
     ANZ530>533-535-538>541.
     Small Craft Advisory until 9 PM EDT this evening for
     ANZ530>533-535>542.
     Small Craft Advisory until 2 AM EDT Thursday for ANZ534-543.

&&

SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...DHOF
AVIATION...ADS/DHOF
MARINE...ADS/DHOF
TIDES/COASTAL FLOODING...ADS
CLIMATE...LWX

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