-- Discussion containing changed information from previous version are highlighted. --
297
FXUS61 KLWX 201808
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
208 PM EDT Thu Jun 20 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.

&&

.NEAR TERM /THROUGH TONIGHT/...
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A strong upper-level ridge remains overhead this afternoon, but
there is enough moisture around to pop off a few clouds in the
mid-upper levels. This hasn`t affected temperatures much overall
with coverage being fairly limited. Temperatures right now are
generally in the mid-upper 80s. We should add a few to that
before the end of the day, with most sites getting to 90 or so.
Meanwhile, dew points are only in the mid 60s, so heat indices
are pretty tolerable in comparison to what is to come into the
weekend.

High pressure will remain in control tonight, leading to
continued dry conditions. Overnight lows will be in the mid-
upper 60s for most, with lower 70s in DC and Baltimore.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
-- Changed Discussion --
Upper level ridging does begin to break down a bit by Friday.
As this occurs, the 850 hPa high will shift a bit further south,
lessening the easterly component of the wind and resultant
oceanic influence. Temperatures will be a few degrees warmer as
a result, with highs in the mid 90s for most. As the upper ridge
starts to weaken, a few afternoon thunderstorms may form over
the higher terrain of the Potomac Highlands and Allegheny Front
in response to daytime heating and resultant terrain-induced
circulations. These thunderstorms could potentially drift as far
east as the Shenandoah Valley before dissipating in the evening
hours. While not likely, if one of these were to get enough
juice behind it, low-level thermodynamics would be supportive of
some isolated strong wind gusts. Any storms that do develop
should be pretty short-lived.

Dry conditions are expected further east. Friday night will
feature warmer temperatures compared to preceding nights, with
overnight lows generally in the low-mid 70s. This will mark the
big change that will lead into the heat for the remainder of the
weekend.

Upper-level riding aloft continues to break down further on
Saturday. While this occurs, we will see flow shift out of the
west at 850 mb. This will result in even hotter temperatures
Saturday afternoon, with highs in the mid-upper 90s. It will
also turn more humid as southerly flow increases in intensity.
Dewpoints are expected to reach the lower 70s. Peak heat
indices of 100-105 appear likely across much of the forecast
area, with some even potentially exceeding 105 in the metro
areas. Heat Advisories seem increasingly likely for this time
period and will be carefully considered on the evening shift.

With the ridge breaking down, there will be less subsidence
aloft to suppress thunderstorm activity. With such a warm
airmass, we are likely to see a northeast oriented surface
trough across the center of the forecast area in response to
differential heating over the higher elevations. Mid-level flow
(and as a result, shear) will be relatively weak, at around 15
knots, but instability will be plentiful (MLCAPE values over
2000 J/kg) within the hot and humid airmass. While the activity
should be somewhat disorganized, storms may be capable of
producing localized downbursts given high levels of CAPE and
DCAPE in place. It isn`t entirely impossible for storms to get
all the way to the Chesapeake Bay if development continues
along outflow boundaries from previous storms. So have increased
POPs further east as well. Have also added some enhanced
wording to the thunderstorm forecast Saturday afternoon as a
results.

Saturday night will be warmer still, with overnight lows in the
mid-upper 70s to the east of the Blue Ridge, with lower 80s
even possible in DC and Baltimore. This level of heat overnight,
when the daytime temperatures are near 100, is especially
dangerous because there will be very little relief. Current
forecast would only have heat indices below 90 between midnight
and 8 AM Sunday.
-- End Changed Discussion --

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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The conclusion of this weekend will mark a temporary end to the
lengthy period of above average temperatures. On Sunday, 850-mb
temperatures rise to around 21-23C which dry adiabatically mixed to
the surface would support some century degree readings.
Consequently, the forecast does call for some spots reaching 100
degrees. This is accompanied increasing humidity levels as dew
points rise into the upper 60s to low 70s. The resultant heat
indices range from 100 to 107 degrees across the forecast area. If
this were to be maintained, Heat Advisories may be needed for much
of the region. An additional threat will be some strong to severe
thunderstorms, particularly during the afternoon to evening hours.
After over a week of continued heating of the surface, instability
levels should be maximized as mixed-layer CAPE values reach 2,500 to
3,000 J/kg. The guidance vary in convective coverage which suggests
there could be some capping issues.

After around 9 to 10 days without a cold frontal passage, the
guidance does bring a boundary through during the second half of
Monday. This would bring an additional threat for some severe
convection, especially for those east of the Blue Ridge Mountains.
The added clouds and thunderstorm activity does carry Monday`s high
temperatures back down into the upper 80s to mid 90s. Through
Tuesday, this boundary eventually settles off to the south across
the coastal Carolinas. Another round of hot temperatures ensue
toward mid-week as highs rise into the mid/upper 90s again. Heat
indices begin to approach advisory levels again. Another cold front
races across the local area by Thursday which helps usher
temperatures down. This would again come with another risk of severe
thunderstorms.
-- End Changed Discussion --

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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Prevailing VFR conditions are expected through at least Friday
with light southerly winds AOB 10 knots. Can`t completely rule
out a thunderstorm during the evening at MRB, but terminals
further east should stay dry.

By Saturday, thunderstorm chances increase at the terminals. Any
thunderstorm that develops Saturday afternoon could produce
locally damaging winds gusts and even some hail. Have increased
precip chances with the latest forecast, so will monitor that
threat for the terminals as we get within the TAF planning
window tomorrow.

Given Sunday and Monday will likely bring a risk of showers and
strong to severe thunderstorms, restrictions are possible at times,
particularly during the afternoon to evening hours. By Monday, the
threat for such severe weather ends up focusing mainly from the Blue
Ridge eastward. Regarding winds, Sunday`s southwesterly winds may
gust up to 15 knots before shifting over to west-northwesterly by
Monday evening behind the cold front. VFR conditions are expected
for Tuesday with winds shifting from westerly to southerly by the
second half of the day.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Light southerly flow will continue over the waters through
Friday morning. As high pressure strengthens offshore, winds
pick up out of the south, and could reach SCA criteria as early
as tomorrow afternoon. Confidence was too low for an SCA at this
time however.

On Saturday though, winds will be picking up substantially,
with SCA winds likely by the afternoon into Saturday night. Some
evening thunderstorms can`t be ruled out on Saturday either,
and if those do make it to the waters, SMWs may be necessary.

A period of southerly channeling may unfold from Sunday afternoon
through Monday afternoon. These channeling effects may warrant Small
Craft Advisories, especially for the wider waters of the Chesapeake
Bay. A cold front races through the waters by late Monday evening.
Given the threat for convection each day, Special Marine Warnings
may be needed each afternoon and evening.
-- End Changed Discussion --

&&

.CLIMATE...
-- Changed Discussion --
Very hot temperatures are expected Friday 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+)         93F
Martinsburg (MRB)            102F (1931)          94F
Charlottesville (CHO)         99F (1933)          94F
Annapolis (NAK)              100F (1988)          89F
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)          97F
Martinsburg (MRB)            102F (1933)          95F
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)         101F
Washington-Dulles (IAD)       96F (1988)          99F
Baltimore (BWI)               97F (2010)          99F
Martinsburg (MRB)            100F (1934)          96F
Charlottesville (CHO)        100F (1894)          98F
Annapolis (NAK)               98F (1988)          96F
Hagerstown (HGR)              98F (1988)          97F
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$

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