Small Craft Advisory 190, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1010 AM EDT Wed Sep 22 2021

.SYNOPSIS...
A strong cold front associated with an area of low pressure over
the Ohio Valley will continue to shift eastward today into
tomorrow resulting in periods of heavy rainfall, gusty showers,
and embedded thunderstorms. The front will move through the
region Thursday as high pressure returns Friday into the
weekend.

&&

.NEAR TERM /THROUGH THURSDAY/...
Current radar imagery shows scattered showers across much of
the CWA this morning, with drier conditions over the metros.
Latest analysis continues to show a deepening upper level trough
over the Ohio Valley as high pressure resides off the New
England coastline. With the core of the surface low currently
moving through Ohio, the associated cold front sits just to our
west currently moving through western portions of West Virginia.
Meanwhile, a plume of moisture advection continues to move
northward ahead of the front which is what's causing the current
showers. Latest SPC mesoanalysis depicts PWAT values close to
2" with CAPE values running in the low 100s across the region.
With most of the forcing associated with the system residing to
our west, seems fair to say that these intermittent showers
persist for the remainder of the morning hours and throughout,
at least, the first half of the afternoon.

Activity begins to uptick late this afternoon into the overnight
ours especially for areas mainly west of US-15 as the front
slowly approaches our westernmost areas. Given the abundant of
moisture currently overhead and the sharp temperature and
moisture gradient behind the front, expect showers to become
heavier and more frequent in nature. CAPE values also uptick
slightly late this afternoon and evening given the increase in
moisture convergence. Therefore, expect a significant upslope
component into the Blue Ridge Mountains with training convection
due to strong unidirectional flow.

In regards to hazards, a Flash Flood Watch remains in affect for
areas west of US-15. No major changes to the forecast expected
at this time with precip amounts ranging between 3-5 inches with
isolated higher amounts of 6-8 inches across the ridgetops of
the Blue Ridge Mountains. With CAPE values later today and
tonight upticking to the mid 100's along with an increase in low
level wind shear, risk for severe thunderstorms persists albeit
the hydro threat is the main concern. Given the increase in the
low level wind field, an isolated tornado also cannot be ruled
out.

As the front progresses eastward during the overnight hours into
early Thursday morning, so will the convective activity.
Thursday early to mid morning could be quite interesting in
terms of hazards. With a surface CAPE values approaching 500 to
around 1000 J/Kg and low level helicity values around 100-200,
could see a QLCS-like convective line developing with damaging
wind gusts being the primary threat. However the possibility of
a few embedded tornadoes still remains on the table as well.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
...PREVIOUS DISCUSSION...
As mentioned previously, the cold front is expected to clear the
region Thursday night. In its wake, much cooler temperatures
are expected overnight, with lows dipping into the upper 40s to
mid 50s (with lower values closer to 40 out west).

Dry and seasonably cool on Friday with highs in the low to mid
70s as high pressure builds towards the region. This will
continue into Friday night as well, with temps even a touch
cooler than Thursday night.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
An amplified but fairly progressive pattern is expected through the
extended forecast period. Surface high pressure will be situated
southwest of the area Saturday, which should provide dry weather
much of the day along with slightly below normal temperatures. A
shortwave trough and accompanying weak surface front will approach
Saturday evening and cross Saturday night. Moisture will already be
limited with the front, and low level westerly (downsloping) flow
should keep any minimal shower chance near and west of the Allegheny
Front.

Surface high pressure will expand across the area Sunday and Monday.
Dry weather is expected with seasonable temperatures, with Monday
slightly warmer due to return flow.

Model solutions diverge by Tuesday in handling the timing and
amplitude of the next shortwave trough. The forecast remains dry now
due to the disparate ensemble solutions, although the 00Z guidance
suite indicates a trend toward a chance of showers and perhaps even
thunderstorms.

&&

.AVIATION /13Z WEDNESDAY THROUGH SUNDAY/...
Continued scattered showers expected across the terminals
throughout much of the day today. While current radar shows
generally light reflectivity during this time, VFR VSBYs are
present across much of the area except for CHO which has
developed fog after a round of heavier showers earlier this
morning. CIGs will continue to teeter between MVFR and IFR
during this time. Winds will remain southerly as wind gusts
uptick this afternoon into tonight. Wind gusts are expected to
range between 20-30 kts.

Heavier showers and embedded thunderstorms expected across CHO
and MRB late this afternoon through the overnight hours as the
cold front slowly moves eastward, to where IFR conditions and
perhaps periods if LIFR are expected. This line of heavier
showers and thunderstorms is expected to move eastward during
the pre-dawn hours tomorrow, impacting IAD and DCA by daybreak,
before reaching BWI and MTN by mid morning.

Strong frontal passage expected Thursday late morning with
shifting winds from NW.

VFR conditions return for Thursday night through Friday as high
pressure builds towards the area. VFR conditions expected this
weekend. Weak frontal passage will likely result in a wind shift
from south to west Saturday night.

&&

.MARINE...
Strengthening SSE winds today with solid SCA expected and Gale
gusts expected Wed afternoon and night. Gradient winds begin to
diminish after 15Z Thu but with SCA conditions persisting. As
the cold front pushes through Thursday, could see some SMWs
being needed as gusty winds will be possible in thunderstorms
along the boundary.

NW wind gusts of 20-25 kts are possible behind the cold front on
Friday. The wind should become lighter Friday night.

Winds will become southerly on Saturday but then shift to the west
Saturday night as a weak front passes. Marginal SCA conditions are
possible Saturday night into Sunday morning.

&&

.TIDES/COASTAL FLOODING...
Anomalies haven't increased much as of early this morning. Am only
expecting minor flooding at DC SW Waterfront this morning, although
Annapolis will be close.

Water levels will likely increase dramatically later today and
tonight as southerly winds strengthen. There's some uncertainty with
just how high anomalies will be, as model guidance can be too
aggressive in these regimes. However, fairly widespread minor
flooding is possible later today through Thursday morning. Have
issued a Coast Flood Watch for Annapolis since guidance indicates
close to moderate flood levels.

There's also some uncertainty on how quickly water levels decrease
on Thursday, which will be dependent on a wind shift associated with
a cold front.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until midnight EDT tonight for DCZ001.
MD...Flash Flood Watch through Thursday morning for MDZ001-003-004-
     501-502.
     Coastal Flood Watch from 3 PM EDT this afternoon through
     Thursday morning for MDZ014.
     Coastal Flood Advisory from 1 PM this afternoon to 6 PM EDT
     this evening for MDZ017.
VA...Flash Flood Watch through Thursday morning for VAZ025>031-
     036>040-501-503>505-507-508.
WV...Flash Flood Watch through Thursday morning for WVZ050>053-055-
     501>506.
MARINE...Small Craft Advisory until noon EDT today for ANZ531>534-537-
     539>543.
     Small Craft Advisory from 4 AM to 6 PM EDT Thursday for
     ANZ531>534-537-539>543.
     Gale Warning until 4 AM EDT Thursday for ANZ531>534-537-
     539>543.
     Small Craft Advisory until 6 PM EDT Thursday for ANZ530-535-
     536-538.

&&

SYNOPSIS...SOUZA
NEAR TERM...SOUZA
SHORT TERM...CJL
LONG TERM...ADS
AVIATION...ADS/SOUZA
MARINE...ADS/CJL
TIDES/COASTAL FLOODING...STAFF

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