739
FXUS61 KLWX 261516
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1116 AM EDT Wed Jun 26 2024

.SYNOPSIS...
Excessive heat and humidity are expected across the region today
ahead of a strong cold front set to cross the area early Thursday
morning. The cold front will bring the return of strong to severe
thunderstorms across portions of the area later this afternoon
and into the late evening hours. High pressure briefly returns
from the north late Thursday into Friday bringing with it
slightly cooler temperatures. Heat and humidity return this
weekend along with increased thunderstorm chances as a series of
fronts push through the region.

&&

.NEAR TERM /THROUGH TONIGHT/...
Overall not much change to the current forecast. Still monitoring
the influx of moisture pushing north this afternoon and winds aloft
to determine the extent of our severe weather threat later today.
The current 12z KIAD this morning shows plenty of dry air in
the mid to upper levels and even at the surface. PWATS this
morning are around 1.43 inches with low level RH values ta 45
percent and mid level RH values around 55 percent. 12z KRNK
sounding to the south is a bit drier with PWATS around 1.14
inches.

As a result, expect an excessively hot and humid afternoon ahead of
a strong shortwave disturbance/cold front that is set to cross
tonight into early Thursday morning. Convective activity looks to be
a bit more organized although some uncertainty remains with the
better forcing forecast north of the area up across PA/NJ.

Severe THREATS:This convective activity is due largely in part
to a lee trough ahead of the main boundary leading to increased
height falls and a quickly moistening thermodynamic profile
which should initiate convection mid to late afternoon (between
3-7pm). The Storm Prediction Center continues to highlight the
majority of the forecast area north of Interstate 64 in a Slight
Risk (Level 2 out of 5) Wednesday afternoon and evening. The
primary threats with these thunderstorms will be damaging winds
and large hail. The threat for damaging winds (70 mph or more)
continues to increase per the latest 12z guidance although some
uncertainty remains to how widespread this threat will be
resolved with two rounds of convection set to take place.
Additionally, there is a secondary threat of an isolated tornado
mainly along the PA/MD line and back into portions of western
MD given the back flow. This threat has lowered somewhat in the
last few model runs, but something that we are still
monitoring at this time. Isolated instances of urban and poor
drainage flooding are also possible given increased PWAT values
above 1.5 inches. With the antecedent dry conditions expect a
bit more runoff as water will have a bit of a harder time
percolating through.

Ahead of the front, south to southwesterly flow will help boost
afternoon temperatures into the mid to upper 90s. Some urban
locations especially from DC points south and east may push up and
over 100 degrees. Of course this will be determined by how much
low level moisture influx there is over the region and how early
convective initiation takes place. Models have dewpoints
quickly getting back into the low 70s this afternoon with
precipitable water values on the order of 1.8-2.2 inches. The
confidence for Heat Advisories still remains low although a few
areas may touch criteria for a brief period of time. Overall
heat index values this afternoon will run between 98 to 104
degrees during the peak of the afternoon. Will continue to
monitor the need for any such heat-related products.

SEVERE TIMING:Expect convective initiation to kick off along
the lee side trough later this afternoon before propagating east
of the metros tonight (between 3-7pm). Storms will feed off of
CAPE values between 1500-2500 j/kg. This is especially true east
of the Alleghenies where the bulk of the instability looks to
remain untapped from limited convective debris. 0-6 km shear
values will be on the order of 30-40 kts with lapse rate 6-7
degrees C/km favoring well organized storms. This is backed up
in several hi-res CAMS and model soundings. Convection is slow
to wane as we get into tonight and Thursday morning as the cold
front slowly pushes through. The severe threat at this point may
be on it`s way down due in part to the initial round of
thunderstorm activity during the afternoon and evening hours.
We`ll have to continue to monitor this timeframe though as some
of the latest guidance has trended toward isolated activity
mainly east of the Blue Ridge this afternoon with the next round
blowing up west of the Alleghenies this evening and propagating
east overnight. Lows tonight will fall into the mid to upper
60s with low to mid 70s along and east of I-95.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As a cold front exits early Thursday, some shower activity could
linger into Thursday morning/afternoon. Temperatures lower behind
the cold front Thursday into the 80s to low 90s, but dewpoints
look to remain in the low-mid 60s during the day, dropping into
the upper 50s overnight with lows temps in the 60s to near 70
east of the mountains.

A quick-moving area of high pressure passes to our north on Friday.
Mostly dry conditions are expected during the day, but some showers
could develop in the evening/overnight around the Shenandoah Valley
and Virginia Piedmont as the center of high pressure swiftly moves
offshore by the evening and a shortwaves passes to the northwest.
Temperatures look relatively cooler, topping out in the upper
80s. Lows hover in the 60s to mid 70s.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As high pressure further retreats offshore into the western
Atlantic, a return to a warm advection dominant regime ensues. Above
average mid-level heights persist over the weekend as forecast
heights range from 590 to 594 dm. A net warming of the troposphere
will yield another period of hot and humid conditions across the
area. A tongue of moisture is set to transport northward in response
to the prevailing southerly flow. Multi-ensemble solutions favor dew
points into the mid 70s which appears to be a tad aggressive. With
forecast high temperatures in the low 90s over the weekend, the
addition of high dew points would yield heat indices into the 100 to
105 degree range. If this degree of moisture transport comes to
fruition, the region could teeter near Heat Advisory thresholds.
This all comes with increasing convective threats as the atmosphere
becomes more unstable due to the upticks in heat and moisture.

Looking more closely at the threat for thunderstorm activity,
Saturday`s setup looks a bit more unpredictable as it will be
dependent on low-amplitude impulses in the west-southwesterly flow
regime. Heading to the second half of the weekend, storm-scale
organization should improve with the approach of a cold front. This
boundary is currently slated to track through the local area
sometime on Sunday. The added lift from the front coupled with
modest height falls aloft should help enhance the coverage of
showers and thunderstorms. Any severe aspect of this activity would
be too difficult to predict this far out in time.

In the wake of the frontal passage, winds shift to northwesterly by
late Sunday before turning more northerly on Monday. This ultimately
ushers in a cooler and drier air mass for the start of the work
week. This shift to below average temperatures also coincides with
the first day of July. On Monday, forecast highs are in the upper
70s to low 80s, accompanied by dew points in the 50s. This pattern
holds for the start of the work week before the heat and humidity
return by mid-week. The Climate Prediction Center Week 2 outlook
does suggest some return of excessive heat during the mid/late week
period next week.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions are expected to start today with winds out of the
W/SW. A cold front arrives tonight, preceded by a lee trough
this afternoon to kick off convection. Guidance has trended a
few hours slower with initiation of showers and thunderstorms
this afternoon, around the 19-21z time frame for the terminals.
A lull in convection is likely between the lee trough afternoon
convection and the arrival of the front tonight, but the timing
isn`t certain. Approximated about a 4 hr duration for the
decrease in coverage around the terminals, around the 23z-4z
time frame. Don`t currently explicitly mention TS in the second
round arriving 3-4z due to low confidence on how much instability
exists after the first round and loss of daytime heating. Latest
guidance has showers exit around 9z for most of the area.

Some showers and thunderstorms could linger into Thursday
afternoon, with the greatest chance for CHO. Winds turn NW
behind the cold front by late morning. Dry conditions follow and
continue into Friday as high pressure passes to the north. Some
shower activity is possible again Friday afternoon/evening but
should be isolated, and again most likely near CHO if so.

As a warm front pushes through the area on Saturday morning, this
brings a return of the heat and humidity. Daily showers and
thunderstorms can be expected, particularly on Sunday as a cold
front approaches from the west. While some restrictions are possible
at times on Saturday, the better shot for any impacts to the
terminals would be the frontal convection on Sunday. Initial south
to southwesterly winds give way to northwesterly flow by Sunday
evening into the night.

&&

.MARINE...
Small Craft Advisories continue for the Chesapeake Bay and tidal
Potomac early this morning as marginal criteria winds continue
to be observed. SCA my be canceled early if winds continue to
diminish.

Additional Small Craft Advisories are in effect this afternoon
and evening as a slow moving cold front pushes through the
region. The front will yield strong to severe thunderstorms
which may initiate the need for Special Marine Warnings over the
waters. Initial activity looks to push toward the waters around
21z before crossing the waters 00z. Additional showers and
perhaps a few thunderstorms will follow late tonight into early
Thursday morning. This frontal system exits by Thursday morning
with sub-advisory level winds expected through Thursday night.
Winds over the waters may approach SCA criteria Friday morning,
diminishing in the afternoon.

Behind a warm front that tracks through Saturday morning, southerly
flow returns across the area. This leads to some southerly
channeling effects over the Chesapeake Bay. With forecast wind gusts
up to around 20 to 25 knots, Small Craft Advisories may be needed
from Saturday morning through midday Sunday. Behind a cold front, a
shift to northwesterlies may bring near advisory caliber winds to
the waters late Sunday. Besides the background wind fields,
convection may lead to some Special Marine Warnings. This is
especially the case on Sunday as an approaching cold front increases
thunderstorm activity.

&&

.TIDES/COASTAL FLOODING...
Prolonged southerly flow has led to a large ramp up in tidal
anomalies which currently average around 0.75 to 1.25 feet. These
elevated water levels will bring Annapolis and Fort McHenry into
Action stage this morning. However, do not expect any location to
see a rise toward minor flooding. A cold frontal passage tonight
should help lower the tides into Thursday. A shift to east-
southeast on Friday could lead to another spike in water
levels.

&&

.CLIMATE...
Hot temperatures are expected on Wednesday. Several records
could be in jeopardy. Below is a list of record high temperatures
for June 26th, the year the record was set, and the current forecast
high temperatures for that day. A plus sign after the date signifies
the record was set multiple times, with the most recent year indicated
below. RERs are only issued for DCA, IAD, BWI, and MRB, but other
sites are shown for reference.

                                    Wednesday Jun 26th
Washington-National (DCA)    101F (1952)          99F
Washington-Dulles (IAD)       95F (1998)          98F
Baltimore (BWI)               99F (1954+)         99F
Martinsburg (MRB)            102F (1943)          92F
Charlottesville (CHO)         99F (1998)          99F
Annapolis (NAK)               98F (1952)          94F
Hagerstown (HGR)              98F (1954+)         94F

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 1 AM EDT Thursday for ANZ530>534-
     537>543.

&&

$$

SYNOPSIS...EST
NEAR TERM...EST
SHORT TERM...CAS
LONG TERM...BRO
AVIATION...BRO/CAS
MARINE...BRO/CAS
TIDES/COASTAL FLOODING...
CLIMATE...