083
FXUS61 KLWX 211919
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
319 PM EDT Fri Jun 21 2024

.SYNOPSIS...
Heat and humidity continue to increase through the weekend as high
pressure sits overhead. Mostly dry conditions are expected with
isolated showers and thunderstorms mainly focused over the
mountains. A cold front will work across the region Sunday into
Monday bringing a reprieve in the high heat and humidity. High
pressure briefly builds back over the area Tuesday before an
additional cold front sweeps through the region Wednesday and
Thursday next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A persistent subtropical ridge aloft remains over the Southern
Plains eastward into the Mid-Atlantic and southeastern U.S. This
sprawling 594-597 dm anticyclone has maintained an area of above
average temperatures across the local area. Early afternoon
temperatures have risen into the upper 80s to low 90s, locally
approaching 95 degrees in a few spots. Humidity levels have come
down a bit as the deepening boundary layer continues to mix in
some lower dew point air from above 850-mb. The resultant heat
indices continue to sit in the 90s.

Recent GOES-16 visible satellite imagery show much of the
cumulus fields have concentrated over the mountains,
particularly the Allegheny Front. Any convective attempts have
been squashed by the subsidence aloft. However, some activity
could track down from south-central Pennsylvania this afternoon.
Some modest radar returns are dropping down toward the point
where I-68/I-70 intersect in western Washington County.
Lightning activity continues to diminish as this activity pushes
southeastward toward the Mason-Dixon Line. Have maintained
thunderstorm chances through dusk before the threat wanes.

A toasty evening and night looms ahead with forecast low
temperatures in the upper 60s to mid 70s. Some patchy fog may
develop across the Allegheny mountain valleys.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
Oppressive heat continues into the weekend with widespread
mid/upper 90s likely, locally to near 100 degrees. Although the
mid/upper ridge begins to buckle which allows heights to
decrease, the lower portion of the atmosphere will begins to
heat up. From this afternoon (Friday) into the weekend, 850-mb
temperatures are expected to go up by around 2 to 4 C. This net
increase in low-level temperatures will bring daily conditions
into the more hazardous levels. Heat indices rise to around 105
degrees on Saturday, locally near the century mark over areas
west of the Blue Ridge. Consequently, Heat Advisories have been
issued area a vast majority of the area from 11 AM until 8 PM.
Although 110 degree heat indices are not completely out of the
question, model soundings show some vertical mixing which could
carry humidity levels down a bit during peak heating. However,
there are a broad range of potential solutions in how much dry
air ends up mixing in. Regarding convective threats, the
guidance shows some isolated activity firing off the mountains
which slowly pushes toward U.S. 15. Expect this all to decay by
sundown which again leads to a warm and muggy night across the
area.

Heights are expected to further lower as the approaching trough
and cold front begin to show their influences over the region.
Heat-related products will likely be needed again on Sunday late
morning into the early evening. The combination of upper 90s
temperatures and upper 60s/low 70s dew points will support heat
indices into the 103 to 107 degree range. One question that
looms is how much cloud cover works its way eastward toward the
Mid-Atlantic region. Depending on how extensive these clouds
are, it could slightly hinder reaching the upper 90s threshold.
Convection should be much more prevalent, particularly during
the second half of the day and into Sunday. This could bring
both a severe and flash flooding threat. For the latter, while
the area has moved into a D0 to D1 drought, precipitable water
values of around 2 inches should support that threat for
localized flooding issues. The mentioned cold front begins to
enter the Alleghenies late Sunday night before pushing into the
I-95 corridor by daybreak. This will allow for a continued
threat for showers and thunderstorms during the overnight hours.
This is accompanied by lows in the low/mid 70s, locally to
around 80 degrees in the urban hubs.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Overall it`s a variable and increasingly unsettled pattern
throughout the extended period. A cold front will continue east of
the area Monday with a brief reprieve in the heat and humidity
Tuesday as high pressure settles overhead. A stronger cold front
midweek will bring an additional threat for strong to severe
thunderstorms to the region.

Current 12z guidance continues to hint at a front along or just east
of the Blue Ridge Monday morning into Monday midday. A few
thunderstorms may accompany the boundary as it passes through. With
peak heating Monday afternoon into the early evening, additional
showers and thunderstorms are possible with the greatest concern for
severe weather east of I-95 and down into southern MD/northern neck
of VA. Primary threats with thunderstorms Monday looks to be wind.
All convective activity looks to quickly wane Monday evening into
Monday night as dry air filters back into the region. This is in
association with a brief area of high pressure building back into
the region. As for temperatures, Monday expect highs to run slightly
cooler due to residual cloud cover as the front passes through.
Highs will range from the upper 80s and low to mid 90s east of I-81
with upper 70s and mid 80s further west. Overnight lows will fall
back into the mid to upper 60s with upper 50s over the mountains.

Surface high pressure briefly builds back over the area Tuesday
giving us a brief reprieve in humidity as drier air settles in.
Dewpoints will fall back into the mid to upper 50s and low 60s with
light west to northwest breezes. The drier air combined with a
downslope component to the wind will allow high temperatures to
surge back into the low to mid 90s especially east of the Allegheny
Front. Overnight lows Tuesday night will fall back into the upper
60s and low 70s.

Unfortunately, high pressure pushes offshore Wednesday allowing for
increased southwesterly return flow to funnel back in. This will
allow temperatures to surge back into the mid to upper 90s with heat
index values up and over the 100 degree mark east of the Blue Ridge
Wednesday afternoon. Some uncertainty remains in regards to
temperatures due to the progression of the next shortwave trough and
incumbent cold front from the Ohio River Valley set to approach the
region. 12z synoptic/ensemble guidance suggest fairly spotty
convection Wednesday afternoon and evening with more widespread
thunderstorm activity developing across the Ohio River Valley and
advancing east toward the region Wednesday night. The front and
upper level trough slow a bit into Thursday leading to continued
thunderstorm chances especially in areas east of the Blue Ridge.
With that said, extra cloud cover and a front overhead will lead to
cooler temperatures in the mid to upper 80s. Drier weather returns
Friday and Saturday as high pressure builds back over the region.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
VFR conditions are expected through early Saturday across the
area terminals. While dry today, a few showers and thunderstorms
could impact the western terminals by Saturday afternoon. Any
restrictions would be brief in nature. Sunday brings the main
threat for showers and thunderstorms which would lend itself to
sub-VFR conditions at times. This would especially be the case
the second half of Sunday into the night. Some of these storms
could be severe in nature. Prevailing southerly winds eventually
shift to southwesterlies on Sunday. Any wind shifts with the
cold front likely hold of until early Monday.

Prevailing VFR conditions are expected Monday outside of a few
thunderstorms along and east of the corridor terminals late Monday
morning into Monday afternoon. Convection will be spotty in nature
and mainly tied to the cold front passing through. High pressure
returns to the area Monday night into Tuesday bringing continued VFR
conditions. Additional showers and thunderstorms are likely
Wednesday into Thursday as another cold front passes through
the area.


&&

.MARINE...
Aside from the middle/upper tidal Potomac, all marine locations
are in Small Craft Advisories until 2 AM tonight. Southerly
channeling will raise gusts up to 20 knots at times through the
duration of the advisories. Additional southerly channeling
effects are possible Saturday evening/night, and again on Sunday
as the influences of convection begin to impact the waters.
Special Marine Warnings are possible for any of that convection,
with the threat persisting through the overnight hours on
Sunday.

Small Craft Advisories may be needed for portions of the waters
Monday into Monday night as a cold front passes through. South to
southwesterly flow will lead to channeling over the open waters
before switching to the west and northwest Monday night into Tuesday
as high pressure builds in. A few thunderstorms may develop along
the front Monday afternoon especially over the middle and lower
waters. The primary threat with thunderstorms will be strong winds.
Sub-SCA level winds return with high pressure Tuesday into
Wednesday.


&&

.CLIMATE...
Very hot temperatures are expected through Sunday. Several
records could be in jeopardy, as seen below. Below is a list of
record high temperatures for Jun 21st, 22nd, and the 23rd and
the year the record was set, and the current forecast high
temperatures for those days. RERs are only issued for DCA, IAD,
BWI, and MRB, but other sites are shown for reference.

                                     Friday Jun 21st
Climate Site                  Record High     Forecast High
Washington-National (DCA)     99F (2012)          94F
Washington-Dulles (IAD)       98F (1988)          95F
Baltimore (BWI)              100F (2012+)         94F
Martinsburg (MRB)            102F (1931)          93F
Charlottesville (CHO)         99F (1933)          93F
Annapolis (NAK)              100F (1988)          89F
Hagerstown (HGR)              97F (1923)          93F

                                    Saturday Jun 22nd
Climate Site                  Record High     Forecast High
Washington-National (DCA)    101F (1988)          99F
Washington-Dulles (IAD)       99F (1988)          99F
Baltimore (BWI)              100F (1988)          99F
Martinsburg (MRB)            102F (1933)          98F
Charlottesville (CHO)        101F (1933)          97F
Annapolis (NAK)              101F (1988)          94F
Hagerstown (HGR)             100F (1988)          98F

                                    Sunday Jun 23nd
Climate Site                  Record High     Forecast High
Washington-National (DCA)     98F (1988)          98F
Washington-Dulles (IAD)       96F (1988)          97F
Baltimore (BWI)               97F (2010)          98F
Martinsburg (MRB)            100F (1934)          94F
Charlottesville (CHO)        100F (1894)          96F
Annapolis (NAK)               98F (1988)          94F
Hagerstown (HGR)              98F (1988)          95F

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Heat Advisory from 11 AM to 8 PM EDT Saturday for DCZ001.
MD...Heat Advisory from 11 AM to 8 PM EDT Saturday for MDZ003>006-
     008-011-013-014-016>018-501>508.
VA...Heat Advisory from 11 AM to 8 PM EDT Saturday for VAZ026>031-
     037>040-050-051-053>057-501-502-505-506-526-527.
WV...Heat Advisory from 11 AM to 8 PM EDT Saturday for WVZ050>053-
     055-502>504.
MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ530>534-
     537>543.

&&

$$

SYNOPSIS...EST
NEAR TERM...BRO
SHORT TERM...BRO
LONG TERM...EST
AVIATION...BRO/EST
MARINE...BRO/EST
CLIMATE...LWX