384
FXUS61 KLWX 150554
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
154 AM EDT Sat Aug 15 2026

.WHAT HAS CHANGED...
Updated to add a Key Message about dense river valley fog.

&&

.KEY MESSAGES...
- 1) Dense river valley fog early this morning.

- 2) Severe thunderstorms and some instances of flash flooding
  are possible on Sunday.

- 3) Seasonably warm conditions and daily shower and
  thunderstorm chances are expected through Monday, before
  another round of storms are possible on Thursday.

&&

.DISCUSSION...
KEY MESSAGE 1...Dense river valley fog early this morning.

A combination of clearing skies, abundant low-level moisture in
part due to recent rain, and calm winds is resulting in areas of
fog this morning in the river valleys of western MD, eastern WV
and into the northern Shenandoah Valley in VA. Some of this fog
has become dense with visibility down to one quarter mile. Just
across the border in PA, some areas are seeing visibility as low
as just a few hundred feet.

Fog will likely persist/expand through daybreak before slowly
burning off. For now, a Dense Fog Advisory is in effect until 9
AM for Allegany, Garrett, Hampshire, and Mineral Counties where
dense fog is most widespread. But, this advisory may very well
need to be extended/expanded south and east.


KEY MESSAGE 2...Severe thunderstorms and some instances of
flash flooding are possible on Sunday.

Some residual convective activity may be ongoing on Sunday
morning. This remains to be seen and will be guided by the
degree of overnight instability and boundary placement. How the
atmosphere recovers through the day should ultimately dictate
the degree of severe thunderstorm threat during the core diurnal
heating hours. The ingredients are certainly present for a more
impactful event if meteorological parameters all come together.

Looking a bit more closely at this event, a broad mid/upper
ridge persist across the Four Corners eastward to the
southwestern Atlantic. Amplified flow north of this expansive
anticyclone will continue to be a conduit to active convection,
many in the form of mesoscale convective systems (MCS). As
mentioned above, a remnant MCS might still be ongoing early
Sunday across the region. While above average heights persist,
expect some non-zero height falls to cross the area in response
to an approaching shortwave. Within this northwesterly flow
aloft, vertical shear is ample for the time of year, generally
around 30 to 35 knots. Depending on model choice, mixed-layer
CAPE values approach 1,500 to 2,500 J/kg. The combination of
these two will easily support a risk of severe weather. While
damaging wind gusts are the most likely threat, other severe
types loom if storms can remain discrete in nature.

Additionally, a Slight Risk for excessive rainfall continues
along much of the Alleghenies eastward along/north of I-66/U.S.
50. Precipitable water values rise markedly, possibly into the 2
to 2.25 inch range. While a single thunderstorm will be capable
of torrential downpours, the bigger threat would be for repeat
and/or training convection. The latter potential exists which
supports the flash flood risk. Although drought continues over
much of the region, many areas have seen a one category
improvement over the past week. Convection will likely be
ongoing into Sunday night with the cold front not passing
through until early Monday.


KEY MESSAGE 3...Seasonably warm conditions and daily shower and
thunderstorm chances are expected through Monday, before another
round of storms are possible on Thursday.

High pressure building across southern Quebec down into New
England will favor a mostly quiet day for the area today. An
onshore component to the winds should favor a mix of clouds and
sun through the day. Highs will rise into the mid/upper 80s with
any shower/thunderstorm chances confined to the Potomac
Highlands across the central Shenandoah Valley toward I-64.
Recent high-resolution models are showing an MCS racing across
the Ohio Valley late today. Some of this could leak across the
Alleghenies, but confidence is low given a fairly mixed model
signal.

As Sunday`s cold front tracks offshore, upper-level zonal flow
bringing drier air from the Great Lakes persists over the Mid-
Atlantic through midweek, creating a break in the rain and
thunderstorms. While temperatures & dewpoints drop a few degrees by
Tuesday in the wake of the front, temperatures should gradually
increase again in the latter half of the week as high pressure
builds overhead.

Meanwhile, an upper-level trough will begin to form over the
Northern Plains and then track across the Great Lakes late in the
week, with a strong cold front potentially moving through the Mid-
Atlantic between Thursday and Friday. Medium-range algorithms are
already highlighting a potential for severe thunderstorms with this
system. However, severe probabilities currently vary widely across
models, with CSU ML probabilities highlighting only a marginal 5%-
15% probability as of the 12Z model runs while the WxNext2 Mean AI
outlook is already showing a 30%-45% probability. Deterministically,
the GFS/ECMWF also have not quite aligned on the timing and
placement of the trough 7 days out. Should this setup
materialize, storms capable of heavy downpours and damaging
winds would be possible. As drought conditions continue to
improve area-wide, localized flooding becomes more of a concern
with any thunderstorms that form in the region. For now, this
will be a feature to continue to monitor as the synoptic setup
becomes more defined.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
Given low T/Td spreads at MRB and CHO, have inserted TEMPO for
IFR 09Z-13Z. Confidence is slightly higher at CHO where rain
occurred Friday.

Otherwise, VFR conditions are expected for today as high
pressure builds over New England into southern Quebec. Any
shower chances are likely confined to those west of the Blue
Ridge and south of I-66. A decaying MCS could move through late
tonight, but there is plenty of uncertainty at this point.
Sunday presents itself as a much more active weather day which
would include some convective-driven restrictions. Ahead of the
approaching cold front, southerly winds pick up in strength with
afternoon gusts to around 15 to 20 knots. Convection could loom
into Sunday night and Monday morning which would support
additional sub-VFR conditions. Conditions turn drier for later
in the day as the cold front moves off toward the southeast.

Prevailing VFR conditions are anticipated at terminals Tuesday
through Wednesday. Light northwest winds shift southerly on
Wednesday.

&&

.MARINE...
Background wind fields remain below advisory threshold through
early Sunday. Ahead of a cold front, southerly winds increase in
earnest which will carry gusts to around 20 to 25 knots. This
likely requires Small Craft Advisories from Sunday afternoon
into the overnight hours. Additionally, there will be an
increasing risk of strong to severe thunderstorms, many of which
may need Special Marine Warnings. Sunday likely presents itself
as the most hazardous day to be on the waters, particularly
from the afternoon onward. Convection may continue into Monday
morning before the storm threat and winds drop off through the
day.

Winds stay below SCA thresholds on Tuesday. Beginning Wednesday
afternoon, wind gusts pick up to marginal SCA thresholds before
diminishing overnight. Northwest winds shift southeast by Wednesday
morning.

&&

.TIDES/COASTAL FLOODING...
Water levels are largely expected to stay below Action Stage
through Saturday. The potential exception could be Annapolis,
which could reach Action Stage at the higher of the daily tides.
Water levels will start to increase in southerly flow on
Sunday, with Annapolis potentially approaching Minor Flood
Stage.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Dense Fog Advisory until 9 AM EDT this morning for MDZ501-502-
     509-510.
VA...None.
WV...Dense Fog Advisory until 9 AM EDT this morning for WVZ050-503-
     504.
MARINE...None.

&&

$$

DISCUSSION...BRO/DHOF/CJL/SRT
AVIATION...BRO/DHOF/SRT
MARINE...BRO/DHOF/SRT
TIDES/COASTAL FLOODING...LWX