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036
FXUS61 KLWX 211426
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1026 AM EDT Fri Jun 21 2024

.SYNOPSIS...
High pressure will remain in place offshore through this weekend. A
cold front will progress through on Monday, with high pressure
building back in by Tuesday. Another cold front will approach
from the northwest on Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Current surface and upper air analyses show high pressure
centered offshore at the surface, with broad upper level
ridging in place aloft. High pressure will remain in place
offshore today, as the upper level ridge slowly starts to break
down. With temperatures cooling off slightly aloft, terrain
driven circulations in response to daytime heating over the
Allegheny Front and Potomac Highlands may be enough to spark the
development of a few isolated thunderstorms this afternoon.
As noted by the 12Z KIAD sounding, vertical winds are pretty
weak with generally up to 10 knots observed through the entire
cloud-bearing layer. As such, any thunderstorms should remain
near the higher terrain to the west of I-81, with dry conditions
further east. It will be yet another hot day, with high
temperatures in the mid 90s for most (mid-upper 80s in the
mountains). Combining this heat with modest humidity will yield
heat indices into the upper 90s.

Any leftover storms over the higher terrain should quickly
dissipate this evening with loss of daytime heating, leading to
dry conditions for all overnight. Overnight low temperatures
will be noticeably warmer than preceding nights, with low to
mid 70s for most (upper 60s in the mountains).
-- End Changed Discussion --

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
High pressure will remain in place offshore on Saturday as
heights continue to slowly fall aloft. A shortwave trough will
track into the Western Great Lakes, causing 850 hPa flow to turn
southwesterly in response. This will draw the hottest air of the
year into the region, with 850 hPa temps soaring to around
21-23 C. When fully mixed out, that will yield high temperatures
in the upper 90s to near 100. Dewpoints will also increase into
the upper 60s to lower 70s. When combined, this will result in
peak heat index values of 100-110 across lower elevations. An
Excessive Heat Watch remains in effect for much of the area
Saturday afternoon.

A southwest to northeast oriented surface trough is expected to
develop through the center of the forecast area Saturday
afternoon. Most model guidance has showers and thunderstorms
developing within that surface trough in response to daytime
heating Saturday afternoon. Ample instability will be present
within the hot and humid airmass, but CAMs show thunderstorms
struggling to intensify within an environment characterized by
relatively dry air aloft and negligible large scale forcing for
ascent. If any stronger storms were to become established, high
levels of DCAPE will be present, making localized downbursts a
conditional possibility. Any thunderstorms should quickly
dissipate with loss of daytime heating after dark, leading to
dry conditions once again overnight. Very warm temperatures are
expected overnight, providing very little relief from the heat.
Record high minimum temperatures may be threatened Sunday, with
overnight lows only dropping into the mid-upper 70s to the east
of the Blue Ridge Saturday night. Lows in the lower 80s may even
be possible in DC and Baltimore.

A mid-level shortwave and associated area of low pressure will
track through the Eastern Great Lakes into the St. Lawrence
Valley on Sunday. Very hot conditions are expected across the
area once again, with high temperatures climbing into the mid-
high 90s for most (80s in the mountains). When combined with
dewpoints near 70, this will once again result in max heat index
values of 100-110 to the east of the Blue Ridge. Additional heat
headlines will likely be needed across portions of the area for
Sunday. Thunderstorms will be possible once again Sunday
afternoon into Sunday night as forcing for ascent associated
with the shortwave overspreads the area. Forecast soundings show
around 1000-2000 J/kg of MLCAPE, around 30 knots of 0-6 km
shear, and DCAPE values well in excess of 1000 J/kg. As a
result, severe thunderstorms capable of producing damaging winds
may be possible. SPC currently has much of the area in a
Marginal risk for severe thunderstorms on Sunday.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
The cold front will either be nearing or already over the forecast
area by Monday morning. There may be an ongoing risk of a few
showers and storms along the boundary. By peak heating, the front
may already be along or east of I-95. The greatest convective signal
is across southern Maryland, with activity clearing by evening.
Temperatures will be lower Monday owing to the clouds and falling
heights, but most areas along and east of I-81 will still reach the
lower to mid 90s.

Surface high pressure will briefly build over the area Tuesday. Post-
frontal relief will mainly be in the form of lower dew points (upper
50s to lower 60s), as high temperatures will once again reach the
lower to mid 90s in the lower elevations.

The low level ridge moves offshore Wednesday, and southwesterly flow
will rapidly bring back a hot air mass. Highs will be in the mid to
upper 90s, with heat index values above 100 east of the Blue Ridge.
There is some uncertainty with the progression of the next trough.
For Wednesday afternoon, it appears any convection will be tied to
mesoscale processes, with more widespread thunderstorms developing
across the Ohio Valley. As long as the atmosphere isn`t overturned,
some of that convection could move into the area Wednesday night
given the hot and humid airmass remaining in place. If the cold
front is slower, Thursday could also be an active thunderstorm day.
However, the current consensus would place the front to the
southeast, with only some lingering showers and a few storms
associated with the upper trough. Temperatures would also be cooler
with this scenario, in the mid to upper 80s.

&&

.AVIATION /14Z FRIDAY THROUGH TUESDAY/...
Prevailing VFR conditions and light southerly winds are expected
at the terminals through this weekend. Thunderstorms may be
possible both Saturday and Sunday afternoon.

Thunderstorms are possible Monday as a cold front passes through,
especially near the metro terminals. High pressure will provide VFR
conditions Tuesday.

&&

.MARINE...
Persistent southerly flow is expected over the waters through
this weekend. SCAs are in effect late this afternoon into this
evening on the Bay and Lower Tidal Potomac in channeled
southerly flow. Similar low-end SCA conditions may be possible
both Saturday and Sunday afternoon/evening. SMWs may also be
possible in association with any thunderstorms that move over
the waters during the afternoon or evening hours this weekend.

Small Craft Advisories may be needed for portions of the waters
Monday in southwesterly flow ahead of a cold front, then with
northwesterly winds behind the front Monday night. Thunderstorms may
develop along the front Monday afternoon with a threat for strong
winds. High pressure will support lighter winds on Tuesday.

&&

.CLIMATE...
-- Changed Discussion --
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)          96F
Washington-Dulles (IAD)       98F (1988)          95F
Baltimore (BWI)              100F (2012+)         95F
Martinsburg (MRB)            102F (1931)          94F
Charlottesville (CHO)         99F (1933)          94F
Annapolis (NAK)              100F (1988)          90F
Hagerstown (HGR)              97F (1923)          94F

                                    Saturday Jun 22nd
Climate Site                  Record High     Forecast High
Washington-National (DCA)    101F (1988)         100F
Washington-Dulles (IAD)       99F (1988)          99F
Baltimore (BWI)              100F (1988)          98F
Martinsburg (MRB)            102F (1933)          96F
Charlottesville (CHO)        101F (1933)          97F
Annapolis (NAK)              101F (1988)          94F
Hagerstown (HGR)             100F (1988)          97F

                                    Sunday Jun 23nd
Climate Site                  Record High     Forecast High
Washington-National (DCA)     98F (1988)          99F
Washington-Dulles (IAD)       96F (1988)          98F
Baltimore (BWI)               97F (2010)          99F
Martinsburg (MRB)            100F (1934)          94F
Charlottesville (CHO)        100F (1894)          96F
Annapolis (NAK)               98F (1988)          95F
Hagerstown (HGR)              98F (1988)          96F
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Excessive Heat Watch from Saturday afternoon through Saturday
     evening for DCZ001.
MD...Excessive Heat Watch from Saturday afternoon through Saturday
     evening for MDZ003>006-008-011-013-014-016-502>508.
VA...Excessive Heat Watch from Saturday afternoon through Saturday
     evening for VAZ026>031-037>040-050-051-053>057-501-502-
     505-506-526-527.
WV...Excessive Heat Watch from Saturday afternoon through Saturday
     evening for WVZ050>053-055-502-504.
MARINE...Small Craft Advisory from 4 PM this afternoon to 2 AM EDT
     Saturday for ANZ530>534-537>543.

&&

$$

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