294
FXUS61 KLWX 051419
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1019 AM EDT Wed Jun 5 2024

.SYNOPSIS...
Unsettled weather today through Thursday as a series of fronts cross
the region. Mostly dry conditions on Friday in the wake of the cold
front. A slow moving upper trough approaches the area from the Great
Lakes this weekend leading to more unsettled conditions.

&&

.NEAR TERM /THROUGH TONIGHT/...
MORNING UPDATE: Muggy conditions continue this morning, with
much of the area exceeding 70 degree dew points. Flow has turned
out of the south to south-southeast ahead of an approaching
weather system that will impact the region later today. Showers
are ongoing across portions of the region, and will likely
continue to increase in coverage in the next few hours.
Instability is already decent, even amidst the abundant
cloudcvoer, as CAPE values are in the 1000-1500 J/kg range
already. So, we should be on track with the early morning
forecast, which is below.

PREVIOUS DISCUSSION: An active weather day is likely for most
of the area today as multiple rounds of showers and
thunderstorms (possibly 3-4) move across the area. Severe
thunderstorms capable of producing damaging wind gusts, and
locally heavy rainfall leading to flash flooding are the main
threats. A weak cold front moves through the area tonight,
though isolated showers storms will linger through the
overnight.

Synoptic/Mesoscale Setup: A mid-level shortwave through will move
across the area today, bringing around 20-30 knots of deep-layer
shear. Some of the high res models indicate higher shear values are
possible during the peak convective window this afternoon/evening.

Instability will be limited for the first half of the day as
widespread cloud cover is slow to dissipate this morning. Increasing
southerly winds within the boundary layer should be strong enough to
break the inversion, leading to sufficient heating/instability this
afternoon. Given dew points in the lower 70s east of the Blue Ridge,
though mid-level lapse rates are meager, there should be at least
1000-1500 J/kg of SBCAPE.

Afternoon highs are forecast to reach the upper 70s to lower 80s,
though mid 80s are possible along/south of I-66 if cloud cover can
break earlier in the afternoon.

Flash Flood Threat: This is likely to be the biggest threat of the
day as multiple rounds of showers and thunderstorms move across the
area. WPC has expanded the Slight Risk of flooding to encompass most
areas along/north of I-66, the northern Shenandoah Valley, and
Alleghenies. Widespread rainfall amounts of 1-2 inches is likely,
with isolated amounts up to 3-4 inches is possible.

A very important feature to watch is the subtle surface boundary
located from Central VA to around Norfolk early this morning. The
increasing southerly flow will push this boundary north into our
area this afternoon, likely somewhere along or north of the Potomac
River. Storms today are likely to be efficient rain producers given
the deep warm cloud layer, a moist airmass, sufficient shear, and
low-level convergence into the previously mentioned boundary. While
mean storm motion is likely to be fast, around 15-20 knots, the
multiple rounds of heavy rain could cause issues in urban areas.

The 00Z guidance shows the potential for locally heavy rainfall, but
exactly where that occurs remains to be seen. The heavy rain
footprint various still varies quite a bit from model to model,
which is expected in this type of disorganized/cluster storm mode.
Given the sensitivities of the DC and Baltimore metros, a targeted
Flood Watch has been issued from 12PM today through tonight to
account for possible flash flooding due to multiple rounds of
storms. Elsewhere, FFG values are high enough and convection spaced
out enough that a Flood Watch is not warranted at this time.
However, the current watch may be expanded later today depending on
future model data.

Severe Threat: SPC has added a Marginal Risk (level 1 of 5) of
severe storms for most areas along/east of I-81 and north of I-64.
Damaging wind gusts will be the main threat, and a tornado or two is
also possible. Saturated soundings, with a deep warm cloud layer and
modestly steep low-level lapse rates (7-8 C/km) favor wet
microbursts in any organized line segments/clusters of storms. The
tornado threat is evident in the favorably curved low-level
hodographs in the model soundings, though it is limited by weaker
shear and lower instability.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
The leading mid-level shortwave moves east of the area Thursday
morning, as a weak cold front continues its slow trek through the
area. This boundary is likely to produce a broken line of showers
and thunderstorms during the late morning to afternoon, mostly along
and east of the Blue Ridge. Additional showers could develop in the
Alleghenies. Very warm afternoon temperatures in the low to mid 80s,
with some spots possibly reaching the upper 80s.

Not much cooling in wake of the front for Thursday night as lows
settle in the upper 50s to mid 60s. High pressure starts to build in
from the west Friday as westerly winds around 5-10 mph continue.
Aside from a few showers in the Alleghenies, mostly dry conditions
are forecast Friday. Afternoon highs will be closer to normal, in
the upper 70s to low 80s. Seasonal lows Friday night in the 50s,
with low 60s along the immediate Western Shore and Potomac River.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
An upper level trough is expected to remain over the area for the
duration of the extended with the main feature of interest being a
strong cut-off low descending into the Northeast. Guidance has
struggled getting a handle on the cut-off low with significant
variance between models and run-to-run. While there is general
agreement on taking the low into SE Canada/NE US, some guidance
brings it a bit further south, and there is a range of possible
shortwaves rounding the base of the low through the weekend.

At the very least, the general expected location of the upper low
will yield unsettled conditions for our area over the weekend and
into early next week, with the focus of diurnal shower and t-storm
activity being the northwestern portions of the CWA. Sunday has the
greatest potential for precip as a cold front associated with the
low moves through Sunday into Monday.

High temperatures will be in the mid 70s to low 80s for most, a bit
cooler in the 60s for the mountains. Lows will generally be in the
mid 50s to mid 60s, though a few degrees cooler Sunday night with
the passage of the cold front.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
Clouds are generally starting to lift to MVFR at this time, and
should become VFR by noon or so. Expect multiple rounds of
showers and thunderstorms to move across the area throughout the
day and into this evening. The strongest storms will produce
periods of sub-VFR conditions and strong wind gusts in excess of
35 knots. A 4-5 hour window of most likely VCTS was kept in the
06Z TAFs. Storms are expected to be widely scattered, but still
difficult to time exactly when the worst conditions will occur
(and how many times any terminal could see impacts).

Showers and storms decrease late this evening, though showers remain
in vicinity through most of tonight. Low clouds are likely to build
in again at BWI and MTN tonight, and could reach MRB, IAD, and DCA
though confidence is low at this time.

A weak cold front crosses the waters Thursday morning, causing winds
to turn southwest, then west by Thursday afternoon. Another round of
showers and thunderstorms is likely, though mainly along and east of
I-95.

High pressure builds into the region Thursday night into Friday,
bringing dry conditions and continued west winds.

Predominantly VFR conditions are expected over the weekend, but
showers and thunderstorms in the afternoon/evening could bring
restrictions to the terminals. MRB is the most likely to see precip
on Saturday. A cold front moves through Sunday into Monday with
greater coverage of convection, so restrictions could be more
widespread.

&&

.MARINE...
Multiple rounds of showers and thunderstorms will move across the
local waters throughout the day, and into this evening. Special
Marine Warnings may be needed for any strong storm that could
produce wind gusts in excess of 35 knots. A waterspout or two is
also possible.

Increasing southerly winds tonight into Thursday morning will likely
result in SCA conditions in the open waters of the central and
southern Chesapeake Bay, along with the Lower Tidal Potomac.

A weak cold front crosses the area Thursday morning, causing winds
to veer southwest, then west Thursday afternoon. At this time, winds
are forecast to remain below SCA level. However, occasional gusts to
around 15-18 knots is possible Thursday afternoon. Additional
showers and thunderstorms are likely to cross the waters Thursday
afternoon/evening.

Westerly winds around 5-10 knots Friday, with dry conditions.

Westerly winds over the weekend are expected to remain below SCA
criteria. Showers and thunderstorms on Sunday could bring stronger
gusts to the waters.

&&

.TIDES/COASTAL FLOODING...
Increasing southerly flow has resulted in rising tidal anomalies,
with a few sensitive sites already reaching minor flood. Annapolis
and Straits Point likely reach minor again during Wednesday night`s
high tide, and other sites may approach or exceed minor as well.
This will be even further enhanced by the presence of the New Moon.

A weak cold front crosses the area early Thursday, with the
resulting west winds causing tidal anomalies to drop.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flood Watch until 10 PM EDT this evening for DCZ001.
MD...Flood Watch until 10 PM EDT this evening for MDZ011-013-014-
     503>506.
     Coastal Flood Advisory until noon EDT today for MDZ508.
VA...Flood Watch until 10 PM EDT this evening for VAZ053-054-506-
     526-527.
WV...None.
MARINE...Small Craft Advisory from 2 AM to noon EDT Thursday for ANZ532-
     533-537-540-541.
     Small Craft Advisory from 10 PM this evening to 2 PM EDT
     Thursday for ANZ534-543.

&&

$$

SYNOPSIS...KRR
NEAR TERM...CJL/KRR
SHORT TERM...KRR
LONG TERM...CAS
AVIATION...CJL/CAS/KRR
MARINE...CAS/KRR
TIDES/COASTAL FLOODING...CAS