Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 270803

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
403 AM EDT Tue Jul 27 2021

A weak cold front will stall out over central Virginia into southern
Maryland this afternoon. Another weak cold front will approach
from the north Wednesday while an upper-level disturbance passes
through. A stronger cold front will pass through late Thursday
into early Friday and high pressure will build overhead for
Saturday before moving offshore Sunday. A reinforcing cold front
is likely to move through by Monday.


07Z surface observations show a frontal boundary stalled over
the region. Based on the dew point gradients, this baroclinic
zone sets up roughly along I-70. The combination of weak
gradients and mostly clear skies have resulted in sporadic fog
formation. Some of the lower lying areas along I-81 have seen
occasional dense fog with similar conditions along Virginia
state route 15 from Culpeper to Warrenton. Do not anticipate
such occurrences to lead to Dense Fog Advisories. Otherwise, a
mild and humid air mass remains in place overnight accompanied
by dry conditions.

Northwesterly flow aloft remains a fixture from the Great Lakes
into the northeastern U.S. The stalled boundary will gradually
sag southward as a cold front toward central to southern
Virginia. This frontal zone should act as the primary driver for
any convective activity this afternoon. The 00Z HREF depicts a
sharp north/south oriented instability gradient. Within the warm
sector, 1,000 to 2,000 J/kg of mixed-layer CAPEs are indicated
from central Virginia into southern Maryland. The threat could
extend a bit farther north if the boundary were slower to
adjust southward. With vertical shear only running in the 10 to
15 knot range, do not anticipate much storm organization with
any updrafts. Any shower and thunderstorm will wane into the
overnight. While humidity levels drop off a bit, today will
remain hot with high temperatures in the low 90s. Dry conditions
persist into tonight with mild lows running around 3 to 6
degrees above climatology.


On Wednesday, a slow moving backdoor front will be on the
doorstep of the Mason-Dixon Line. Based on the latest guidance,
this boundary may eventually stall leading to additional chances
for convection. The 00Z ECMWF in particular lights up this
boundary with strengthening updrafts, especially to the north of
I-66 and Highway 50. Compared to the 00Z high-resolution models,
this solution may be a tad robust. Will continue to maintain a
chance of afternoon/evening thunderstorms with this boundary in
play in conjunction with modest levels of instability. High
temperatures may drop by a degree or two, but conditions remain
seasonable for late July standards. Conditions should dry out
into the night with lows in the mid/upper 60s, locally into the
low 70s around the DC and Baltimore metros, as well as southern

Much uncertainty continues to plague the forecast for Thursday.
A strengthening mid-level shortwave tracks through the Mid-
Atlantic region with some timing differences among the global
solutions. The trend has been to slow the progression of an
upstream cold front which would delay convective activity.
Additionally, the thermodynamic environment is not well resolved
either with not all models depicting substantial CAPE values.
However, given the stronger mid-level flow coupled with mid-
summer diurnal heating, strong to severe thunderstorms remain
possible. If the threat were to materialize a bit later,
thunderstorms could be ongoing into portions of the night.
Isolated flash flooding cannot be ruled out either given the
high precipitable water environment. Portions of the region are
a Marginal risk for severe thunderstorms and/or flash flooding.
High temperatures may come down a couple degrees depending on
how much cloud cover develops. Winds begin to shift over to
northwesterly overnight as the cold front pushes through.


A dominant upper-level ridge will continue across the
western/central US through the weekend. Upper level west/northwest
flow will continue throughout the long term as shortwaves ride the
edge of the ridge. These shortwaves will allow for an upper-level
trough to become more defined late in the weekend across the

Guidance has trended slower with the cold front passage
Thursday/Thursday night, so have kept low end chance POPs for
southern portions of the FA to account for any lingering showers or
thunderstorms Friday.

High pressure will build in Friday night into Saturday bringing
cooler conditions for a couple of days and notably lower humidity.
Lows in the 40s to near 50 are possible across the cooler locations
of the Allegheny Front Friday night. Otherwise, lows will be in the
50s for most aside from the immediate metros where lows will be in
the 60s. High temps remain below normal Saturday before temperatures
moderate a few degrees Sunday as the high moves offshore. A
weak frontal boundary will bring the opportunity for at least
isolated rain showers and thunderstorms Sunday.


While some fog formation has taken place, these should stay away
from the area terminals. Likewise, VFR conditions are likely to
be maintained today. The only exception may be KCHO with the
frontal zone sagging southward. Any reductions in flight rules
should be temporary. Wednesday and Thursday will bring a better
chance for showers and thunderstorms which would favor periods
of sub-VFR conditions. Winds remain light through mid-week
before southwesterly flow strengthens on Thursday ahead of the
approaching cold front.

A shower or storm is possible near KCHO Friday as a frontal boundary
lingers nearby. Most other terminals should be dry with a northwest
wind. Remaining VFR Saturday for all terminals under a northwest


Through early Thursday, marine winds stay under SCA levels.
There is some increase in southwesterly winds on Thursday which
may require a SCA for portions of the waters. In terms of
thunderstorms, some of the lower marine zones may see a Marine
Weather Statement or Special Marine Warning this afternoon and
evening. A better chance for thunderstorms exists on Wednesday
and Thursday which would require Special Marine Warnings as

An isolated thunderstorm is possible over the southernmost waters
Friday, but most locations will be dry. Winds could approach SCA
levels Friday behind the front, mainly over the Chesapeake Bay.




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