Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Wed Aug 19 2026
Day 2
Valid 12Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...

...2030Z Update...

The main update for this forecast cycle was to modestly expand and
then extend the inherited Slight Risk over the Mid-Atlantic
northward to include more of the I-95 corridor from Philadelphia to
the New York City metro. As outlined below, a line of
thunderstorms is expected to develop along the eastward moving cold
front as it reaches the I-95 corridor. The now available CAMs show
considerable rainfall potential (local amounts of 3-6") from the
Virginia Tidewater north-northeastward through DelMarVa and into
far southeastern PA, most of NJ, and the NYC metro. This is backed
up by moderate (40-70%) HREF/RRFS probabilities of rain totals
greater than 3", and low end (20-30%) probabilities of greater than
5". There remains some uncertainty with the timing/progression of
the cold front and when storms form along/ahead of it relative to
the I-95 corridor before moving eastward, but again the latest CAM
guidance did show notable agreement at least for the northern
extent of the risk. The risk remains just to the east further south
from Baltimore to DC except for some of the more eastern
suburbs/exurbs and towards Annapolis/Chesapeake Bay. In the
Southwest, the forecast remains unchanged for the inherited
Marginal Risk covering additional Monsoon activity.

Putnam

...Previous Discussion...

...Mid Atlantic...

Shortwave over the Ohio Valley will continue its progression to
east, then northeast as it migrates into the Central and Northern
Mid Atlantic by Thursday afternoon. At the surface, a cold front
will remain steadfast on its advancement through the Piedmont and
the eastern sections of PA by late-Thursday morning leading to a
favorable scenario for a line of thunderstorms to develop along and
east of the front itself. Increased dynamical forcing and
favorable diffluent flow within the RER of a 130kt jet over the
Northeast U.S. will lead to a blossoming of afternoon convection
within proximity of the front itself. Latest CAMs are quite
aggressive on their interpretation of both the magnitude and
coverage of the heavy convective signals, especially for areas
extending from Northern NJ down through eastern PA, the Delmarva,
and the VA Tidewater. Thunderstorm initiation will be very close to
the metro areas of DC-Balt-Philadelphia with the majority of CAMs
developing cells almost overhead before advancing steadily
eastward, away from the I-95 corridor. The threat will be close for
the urban centers with the Philadelphia metropolitan area the
closest for more substantial impacts from convection when you
correlate with the mean QPF output in the region.

PWATs between 2-2.3" coupled with a strong theta_E ridge in place
ahead of the front promotes a formidable environment capable for
producing significant rainfall rates in any convective core that
materializes. Analysis of multiple soundings from both the global
and hi-res suite at 00z indicated a textbook signal for heavy
rainfall with a tall, skinny CAPE signature with moisture layers
present from surface to the tropopause. Typically these types of
environments favor, at least scattered flash flood prospects in
areas where thunderstorms reach their mature phases, which seems to
align with areas along and east of I-95. The most notable heavy
rain depictions favor NJ/DE/VA Tidewater for the time being as the
frontal progression times well with the late-afternoon period, a
time frame when diurnal instability has reached its maximum. The
signal is pretty close to the NYC metro, but the heaviest may lie
just south of the metro area, so it will bear watching the trends
over the next 24 hrs to see if a higher risk is necessary for New
York to western LI. For now, a MRGL risk encompasses this
particular zone with areas just south within the added SLGT risk.
The DC/Balt metro area is currently in a MRGL with the new SLGT
just to the east over Prince Georges and Anne Arundel counties.
Areas east of the Chesapeake are firmly into the SLGT risk, as it
stands as the signal is the most robust in that particular
corridor.

...Southwest U.S. into the Southern Rockies...

Relatively textbook monsoonal pattern across the west will lead to
scattered thunderstorm development across much of the Desert
Southwest extending north into parts of the Great Basin and through
the Southern Rockies. As of now, the signal is mainly seasonal with
regards to the PWAT anomalies and general expectation as the most
notable areas will likely be in those terrain focused zones across
AZ (Mogollon Rim), NM (Sangre de Cristos and Sacramento Mtns.), and
UT (Zion National Park). Low probs for >1" on the latest
ECENS/GEFS suites and 01z NBM allow for a standard MRGL risk
issuance for the area as the setup is genuinely favored for pulse
convection and slow cell motions tied to the terrain as a broad
597dm ridge centers over the Four Corners and makes little
progression in the D2 window. No changes were necessary to the
previous forecast given the consistency within guidance over the
last series of model runs. Most favorable areas of impact will
include the more flashy streams and arroyos across the Southwest,
as well as the slot canyon areas of southwest UT and the burn scars
across each of NM and AZ.

Kleebauer


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt