-- Discussion containing changed information from previous version are highlighted. --
453
FXUS61 KLWX 210133
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
933 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 FRIDAY/...
-- Changed Discussion --
High pressure system will remain close to the region, resulting
in hot and dry conditions with mostly sunny skies for the day.
During the evening, however, as the upper-level ridge starts to
break down, some convection west of I-81 could cause some
scattered showers and possibly a few thunderstorms in that area,
but they are expected to quickly dissipate.

High temperatures are expected to be in the mid 90s for most
areas and upper 80s in the mountains. However, dew points being
in the mid to upper 60s indicate that the heat indices show the
temperature feeling like upper 90s and even reach 100 in some
spots. Low temperatures will range within the upper 60s and low
70s in most areas and potentially the mid 70s in the DC and
Baltimore areas.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
-- Changed Discussion --
Friday night`s low temperatures will reach the lower to middle 70s
in the eastern two-thirds of the region. Dry conditions expected
once Friday afternoon and early evening`s thunderstorms dissipate.

The upper level ridge of high pressure will break down even further
Saturday. A light to modest westerly flow will develop within and
just above the boundary layer. High temperatures Saturday afternoon
will reach the middle to upper 90s to 100 degrees mainly east of the
Blue Ridge and interior valleys between the Appalachians and the
Shenandoah Valley. Humidity will also increase with this heat having
dewpoint temperatures reach the lower 70s. Heat indices are expected
to easily reach the 100-105 degree mark over much of the forecast
region. Areas within the metro areas could exceed 105. There is
currently an Excessive Heat Watch in effect Saturday afternoon and
Saturday evening for an area from eastern West Virginia to the
Chesapeake Bay, excluding far southern Maryland as a Chesapeake Bay
breeze expected in the afternoon could keep values below criteria.
With the high pressure ridge breaking down, showers and
thunderstorms could find it easier to develop during the heat of the
afternoon and early evening. Wind shear will be relatively weak, but
instability will not be scarce. Thunderstorm activity should be
disorganized. However, thunderstorms could 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.

Saturday night will be even warmer than previous nights with lows
dropping only into 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.
-- End Changed Discussion --

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
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.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
VFR conditions overnight through Saturday morning. Brief MVFR
conditions possible is strong thunderstorms develop with gusty winds
on Saturday afternoon and evening. Downpours could reduce visibility
to 2 miles or so while brief wind gusts from thunderstorms could
gust 35 to 40 knots.

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 --
No marine hazards overnight through Saturday morning. Small craft
advisories possible Saturday afternoon and more likely Saturday
night. As a matter of fact, special marine warnings or marine
weather statements may be warranted in thunderstorms Saturday
afternoon and evening.

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...
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

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
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...None.
-- End Changed Discussion --

&&

$$

SYNOPSIS...CJL
NEAR TERM...KLW/Guest
SHORT TERM...KLW
LONG TERM...BRO
AVIATION...BRO/KLW
MARINE...BRO/KLW
CLIMATE...CJL