Dense Fog in Virginia, July 8, 2020

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
256 PM EDT Tue Jul 7 2020

.SYNOPSIS...
High pressure remains offshore through Wednesday, pumping warm and
humid air into the region, along with daily chances for showers and
thunderstorms. Cutoff low pressure will develop to the south on
Wednesday and it may impact the area later in the week. An
upper-level trough will move into area Saturday while a cold
front passes through. The upper-level trough will hang around
for early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
This morning's cloud deck has eroded to northeast Maryland. A
cumulus field has developed where insolation has been strongest...
across central Virginia, the Shenandoah Valley and the Potomac
Highlands. Some of the more energetic updrafts appear to near
Hardy and Grant Counties.

While ample instability present (2000-3000 j/kg SBCAPE) where
heating has been the best and a widespread 1000 j/kg DCAPE,
forcing factors remain disorganized and highly mesoscale-
driven... namely boundaries from differential heating, bay
interactions, and terrain circulations. Flow weak and mean layer
lapse rates thus far unimpressive. Still believe that scattered
to numerous thunderstorms will continue to develop. Given the
available fuel, a few strong thunderstorms possible. Wind gusts
appears to be the biggest severe concern, and that's likely to
be localized. Heavy rainfall may be the greater concern given
weak flow would lead to slow movers. If storms develop along
I-95, where antecedent conditions nearly saturated, that could
be a particular concern. At this point, confidence in coverage
insufficient to justify a Flood Watch. We will continue to
monitor for that.

Since this activity will have a sizeable diurnal component,
believe there will be fairly rapid dissipation this evening.
Will keep forecast dry after midnight. Since dewpoints in the
upper 60s-lower 70s, low temperatures will remain likewise.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Wednesday and Thursday look rather disorganized from a synoptic
standpoint. There will be broad ridging across the Great
Lakes/Northeast/Mid Atlantic while an upper level low develops
over the southeast. Consequently diurnal convection remains a
possibility. However, if surface low pressure develops offshore,
which guidance is hinting at for Thursday, the local area would
be within a subsident region. Therefore, will be maintaining
chance PoPs for Wednesday, greatest in the mountains with
orographic influence, and a slight chance on Thursday. Since
there isnt a source for thermal advection, kept temperature
forecasts similar.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
The Friday through this weekend period will be fairly active as
multiple systems potentially impact our region. A coastal low will
be tracking northward along the east coast this week and will be
situated near our region to the east on Friday. There remains some
uncertainty on the exact track of this system and if it tracks
further westward, the heavy rain threat will increase for parts of
our region. As there will be continue chances for showers and
thunderstorms throughout this week and coupled with the heavy rain
we received over the past 36 hours, any additional rainfall toward
the end of the week could lead to an increased flood threat. As of
right now we are forecast to be on the west side of the storm which
will mean a more northerly flow and cooler, drier conditions which
should limit the threat for thunderstorms especially with the lack
of an upper level lifting mechanism.

The coastal low is forecast to lift to our northeast early on
Saturday. An upper level trough is forecast to drop down into our
area from the upper plains/midwest. A cold front associated with
this system will move eastward through our region Saturday into
Sunday. Ahead of this boundary, winds will become southerly leading
a return flow of warm and moist air into our region. Increasing
moisture and temps will lead to increased instability and coupled
with the lift from the front could lead to a thunderstorm threat for
this weekend. The models have the front through our region by Sunday
morning but the upper level trough overhead could lead to continue
chances for precip into the afternoon periods Sunday.

High pressure tries to build over our region briefly on Monday
before another potential low approaches from the south.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Low clouds starting to break (ceilings remain at DCA/BWI/MTN)
and convection cumulus beginning to develop. Scattered
showers/thunderstorms remain a possibility for late afternoon
into early evening. Have a few hours window (2100-2400 UTC)
where the risk is the greatest in the TAF. However, based on
coverage, the confidence in a direct impact on the lower side.
Brief IFR possible if it did.

Local fog a possibility tonight, but ultimately that will be
based on convective evolution. At this point have a brief MVFR
risk at MRB/CHO which are preferred wrt climo.

Forecast threats similar Wed-Thu, although the risk of
thunderstorms perhaps a pinch less each day.

Periods of SubVFR conditions will be possible Friday through this
weekend as multiple systems bring increase chances for rain and
thunderstorms.

&&

.MARINE...
Most mesoscale guidance suggesting that there will be a risk of
south channeling in mid section of the Chesapeake Bay/Tangier
Sound/mouth of the Potomac, with 20-25 kt present in the low
level flow. It's a bit uncertain on momentum transfer, but
believe the risk is sufficient to raise a Small Craft Advisory
late this afternoon into this evening between Sandy Point and
the Maryland-Virginia line.

Overall flow will remain weak Wednesday-Thursday, although there
are subtle signals at channeling in the Drum Point-Smith Point
and Tangier Sound area nightly.

Small Craft advisories may be needed Friday due to a northerly flow
and coastal low off to the east. SubSCA conditions expected at this
time for Saturday.

&&

.TIDES/COASTAL FLOODING...
Water levels will remain elevated over the next couple of days.
However, confidence in any inundation remains low. Caution
stages may be reached, especially during the morning cycle which
is the higher astronomical tide.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 1 AM EDT Wednesday for ANZ532>534-
     537-543.

&&

SYNOPSIS...BJL
NEAR TERM...HTS
SHORT TERM...HTS
LONG TERM...JMG
AVIATION...HTS/JMG
MARINE...HTS/JMG
TIDES/COASTAL FLOODING...HTS

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