Aspen Hill, Maryland 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

203
FXUS61 KLWX 200738
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
338 AM EDT Thu Aug 20 2026

.WHAT HAS CHANGED...
The chance of severe weather has continued to trend up
especially for the I-95 corridor to the Chesapeake Bay.

&&

.KEY MESSAGES...
- 1) Severe weather is possible, mainly east of the Blue Ridge
  this afternoon into early evening.

- 2) After a brief lull Friday, additional showers and a few
  thunderstorms are possible Friday night into Saturday.

- 3) Lingering chance for showers and thunderstorms on Sunday.

- 4) Drier and less humid conditions to start next week.

&&

.DISCUSSION...
KEY MESSAGE 1...Severe weather is possible, mainly east of the
Blue Ridge this afternoon into early evening.

A slow-moving cold front arcs down the Saint Lawrence River
Valley to the lower Great Lakes back into the central Great
Plains. To the south, a largely stationary front was attempting
to lift slowly northward as a warm front, and stretched from the
Carolinas back to Oklahoma. Showers and thunderstorms were
persistently trekking west to east along the northern front,
with a weak surface low and MCV/shortwave crossing Illinois.
This shortwave will be the impetus for convective development
across the Mid-Atlantic later today as the local area sits in
between the two aforementioned fronts.

Most instability this morning is rather weak and elevated in
nature, but is expected to increase rapidly in the I-95 corridor
from south to north as higher moisture air advects into the
area, as well as near the Appalachians ahead of an approaching
shortwave. For now, weak lift and CAPE is keeping the area dry
outside of a few WAA showers near western MD.

Additional development, perhaps even deeper convective cores,
is possible by as early as late morning given the anticipated
rapid increase in CAPE and lift.

Flow aloft is seasonably strong by late August standards, with a
100+ knot 300 hPa jet streak expected to pivot over the Saint
Lawrence River Valley into northern New England by evening. The
tail of this upper jet may actually strengthen and persist near
the Mason-Dixon Line early this evening, which could briefly
enhance large scale ascent locally via RRQ dynamics.

In the mid levels, the previously mentioned shortwave has
trended notably deeper and a bit slower over the last 12-24
hours. This brings the axis of the shortwave, its enhanced flow,
and PVA across the Appalachians around midday, then across
areas near and east of the Blue Ridge/Catoctin Mountains east to
I-95 and the Chesapeake Bay during peak heating. In addition to
bringing large scale ascent, this pushes effective deep layer
bulk shear into the 35- to 50-knot range later today (highest
north/northeast). Shear of this magnitude is favorable for
sustained, organized updrafts, including line segments and
supercells (discrete or line-embedded).

Thermodynamically, the atmosphere will be very moist, especially
further east. Strong low-level theta-e advection is expected to
persist through early evening ahead of the approaching shortwave
and cold front. Widespread clouds will limit heating somewhat,
especially west of the Blue Ridge where the storm threat is
notably less, but with the strong low-level moisture advection
and slow-but-steady cooling aloft, the net result should be
about 1000-2000 J/kg MLCAPE in the moist plume east of the Blue
Ridge heading into this afternoon, possibly as early as late
morning. The moist low-level advection beneath cooling aloft
may also result in re-generative CAPE and multiple rounds of
thunderstorms in any one location. This would be most likely
from near the DC metro north and east where large scale ascent
is strongest, with the added component of enhanced low-level
convergence along bay or river breezes (and eventually composite
outflow boundaries).

Finally, PWs will approach or exceed 2 inches in the moist axis
near and east of I-95 this afternoon into early evening. Deep
warm cloud layers in the high PW airmass will yield heavy to
extreme instantaneous and sub-hourly rainfall rates in the
stronger precip cores. Individual cell motion is expected to be
progressive (30-40 mph) given enhanced flow through the column
and the potential for modest cold pool development with any
persistent cells, clusters or line segments. However, the wavy
slow-moving front approaching from the north may lie close to
parallel to the mean steering flow at times, especially when
modulated by outflow from potential multiple rounds of storms.
This, coupled with the bay breeze which often takes on a similar
orientation, plus the urban footprint that has seen heavy rain
recently raises a (primarily urban/poor drainage) flash flood
risk over the I-95 corridor in northeast Maryland, including the
Baltimore area. The threat is a bit more conditional further
south and west into areas like Annapolis and the DC/Capital
Beltway region; these areas also feature urban sprawl and
inherently lower FFG, but the training potential is more
conditional based on favorable outflow or cell interactions
given slightly further displacement from the synoptic front(s)
and bay breeze.

Multiple hazards are possible today, and are outlined below.

Damaging Wind...
Straight-line/downburst winds are likely the highest threat
today, at least from an areal coverage standpoint. Tall, skinny
CAPE profiles with modest pockets of mid-level dry air yield
DCAPE of 600-800 J/kg, which would be marginally favorable for
strong to severe downdrafts. Background mid-level flow may
approach 40 knots around 700 hPa. With the high PW air and
modest DCAPE, water-loaded downdrafts via precip drag would be
possible especially with any bowing line segments or collapsing
mesocyclones/mesovorticies. The lack of steeper lapse rates
(hence modest DCAPE) should keep most wind gusts within the
strongest storms in the 55-70 mph range, with more widespread
40-55 mph potential. Given heavy precip and late summer, leafed-
up trees, would anticipate scattered to numerous instances of
tree and branch falls and subsequent power outage potential. The
risk is highest near and east of I-95 where storms are most
likely to become linear and be a bit more intense, since they`re
further east in the moist axis with a little more time to heat.
To the west, especially west of the Blue Ridge, less moisture
and heating should keep the coverage of more intense storms low.

Tornadoes...
A conditional risk for a brief spin-up is evident for most areas
east of the Blue Ridge given the magnitude of low/mid-level flow
and high PWs/lower LCLs. Relatively greater tornado potential
may exist closer to the front near the I-70 corridor east of
Frederick where surface winds will be backed and low-level SRH
will be higher, especially in close proximity to the bay breeze
along the western shore of the Chesapeake Bay into northeast MD.
Potential would be further maximized by (1) discrete supercells
occurring earlier when the synoptic low-level winds remain
backed and SRH is higher on a broad scale, and (2) with any
favorable QLCS surge/mesovortex interactions with residual
outflow or the bay breeze. Although unlikely due to a lack of
stronger 0-1 km shear/SRH and steeper lapse rates, robust 3CAPE
near the bay could result in a conditional risk for a strong
tornado if all of the above factors were to coalesce.

Flash Flooding...
We have been in a wetter pattern recently, but the majority of
the region is still in or just coming out of drought. Poor
drainage and urban areas are susceptible to heavy rain,
especially the intense sub-hourly rates possible today,
regardless. Additionally, areas near and just northeast of
Baltimore have seen multiple episodes of heavy rain over the
last few weeks. Individual storms should move pretty swiftly,
but the slowing southwest end of any lines or clusters would
orient parallel to the nearby surface front, bay breeze, and
mean steering flow. This brings a risk of flash flooding to
northeast MD in particular, with a more conditional risk further
south and west into the greater DC metro.

Hail...
Although lapse rates aloft will be modest at best given the
moist profiles, supercellular shear and modest mid-level cooling
will be favorable for at least some marginal/spotty severe hail
potential. Should mid-level shear/cooling be more potent or
cells be more discrete, the hail risk could increase, but this
seems unlikely given the otherwise tropical-esque airmass.

The threat for severe weather and flooding appears highest
between 2 PM this afternoon and 8 PM this evening, progressing
west to east in one or two rounds.

By later this evening, the departure of large scale lift and the
loss of daytime heating should end the severe/flood threat.


KEY MESSAGE 2...After a brief lull Friday, additional showers and
a few thunderstorms are possible Friday night into Saturday.

Although a stalled front will remain nearby on Friday, a lack of
shortwaves aloft or other lifting mechanisms should generally
keep shower and thunderstorm coverage and intensity lower.

By Friday night into Saturday, a surface low develops near the
VA Tidewater and moves northeast along the coast/front. Also,
another mid/upper wave approaches from the west. Depending on
the exact track and strength of each of these features, and how
they interact, this could bring higher potential for showers
and thunderstorms to the region. Enhanced flow downstream of a
deepening trough over the Midwest results in an environment
conditionally favorable for strong storms if enough instability
is realized.


KEY MESSAGE 3...Lingering chance for showers and thunderstorms
on Sunday.

Guidance has trended a bit slower with the eastward progression of
a highly amplified upper trough that will be in place across the
eastern CONUS on Sunday. Most solutions now have the trough axis
just to our west at peak heating, as opposed to earlier solutions
which had it directly overhead. With the trough axis just upstream
and the potential for a shortwave to rotate through the base of the
trough and move toward our area, chances for thunderstorms on Sunday
appear to be trending upward. With temperatures cooling aloft, weak
low-level winds, appreciable deep layer shear, and a relatively
straight hodograph shape, any stronger storms that occur may be
capable of producing some hail, and potentially some locally gusty
winds as well.


KEY MESSAGE 4...Drier and less humid conditions to start next
week.

The upper trough will progress off to our east and high pressure
will build to our north on Monday and Tuesday. Northwesterly winds
ahead of the approaching high will advect drier air into the area at
low levels (dewpoints falling into the 50s). This will result in
dry conditions locally. Guidance begins to diverge a bit by
Wednesday, but some solutions shower higher dewpoint air and
associated chances for showers and thunderstorms returning again.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Early morning satellite and surface obs show widespread mid-
level cloud cover. VFR bases should persist well into the
afternoon, expect perhaps in the vicinity of convection. TS
potential will increase by around 18Z and persist through 00Z
especially for terminals near and northeast of the DC metro.
Some storms could result in briefly very low visibility and
strong, erratic winds. These is some potential for multiple
rounds or backbuilding storms, especially in NE MD.

Low (IFR) ceilings or fog likely develop this evening in the
wake of rain (except perhaps near KCHO which may be most
removed from the higher precip coverage). A modest northerly
wind in the wake of a southward pushing cold front overnight may
cause conditions to lift and improve for the metros after 06Z.

Friday offers a relative lull in more widespread rain/TS probs,
but sub-VFR is possible both early in the morning with low
clouds or fog, and/or in the vicinity of storms. Prevailing VFR
conditions and northwesterly winds are forecast for both Sunday
and Monday. A stray afternoon shower or storm can`t be ruled
out Sunday afternoon.

&&

.MARINE...
Southerly winds are expected to increase ahead of an approaching
cold front. The highest likelihood for persistent gusts around
20 knots (outside of storms) is over the wider waters of the
middle Chesapeake Bay, but felt there is enough background flow
to expand the SCA to cover all waters this afternoon into early
evening (2PM to 8PM EDT). This also coincides with the highest
thunderstorm potential and Special Marine Warnings.

A brief and marginal northerly push of wind is possible late
tonight behind a cold front, but confidence was not nearly high
enough in magnitude nor duration to hoist a headline. Otherwise,
winds shift back to the south Friday into Saturday, though with
a bit tempered confidence due to waves of low pressure and a
stalled front nearby. Winds should be relatively light outside
of any storms, for which the potential peaks Friday night into
Saturday. Sub-SCA level northwesterly winds are forecast for 
both Sunday and Monday. A stray shower or storm can`t be ruled
out Sunday afternoon.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this
     evening for ANZ530>543.

&&

$$

DISCUSSION...DHOF/KJP
AVIATION...DHOF/KJP
MARINE...DHOF/KJP