Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Thu Jul 9 2026
Day 1
Valid 16Z Thu Jul 09 2026 - 12Z Fri Jul 10 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE CENTRAL HIGH PLAINS TO THE MID-ATLANTIC...

...16Z Update...

Only a few changes were needed with this update. The latest
guidance suggests a higher threat of exceeding flash flood guidance
across central New Jersey up to the Jersey suburbs of NYC, due to
strong storms moving through this afternoon into this evening. For
that reason, and due to urban concerns along I-95, the Slight Risk
was extended northeastward to Staten Island.

A higher-end Slight was extended west along I-66 into the northern
Shenandoah Valley with this update, as well as into central Jersey,
near Trenton. Expect increasing chances of flooding with
backbuilding storms in these areas this afternoon. Average soil
moisture in this area from recent heavy rains will also support a
bit higher threat for flash flooding.

No changes were made to the Slight Risks across the mid-Mississippi
and lower Ohio Valleys. MCSs expected in these areas moving over
areas of saturated soils from heavy rains will also lead to widely
scattered instances of flash flooding late this afternoon through
this evening.

Wegman

...Previous Discussion...

...Mid-Atlantic...
Moisture and instability will increase today ahead of a mid-level
shortwave tracking through the area during peak heating. PW values
are forecast to exceed 2" alongside SBCAPE over 2000 J/kg, a
favorable profile for heavy rainfall rates. While convection is
expected to eventually consolidate into a forward propagating
convective line or cluster, scattered thunderstorms should develop
initially from northern VA into southeast PA. Cell mergers are
possible with this initial activity and as the upstream convective
line/clusters arrive.

With both the REFS and HREF showing locally heavy rain and 15%+
FFG exceedance probabilities, the Slight risk was expanded
westward into more of southern PA and portions of WV. The highest
EAS probabilities from the HREF/REFS point to an axis across
northeast MD, southeast PA, southern NJ and DE as the most favored
corridor for concentrated heavy rainfall. Neighborhood
probabilities from both ensemble suites indicate a 50-80% chance of
locally exceeding 3" of rain, a 15-30% chance of exceeding 5", and
a 30-50% chance of 2"'hr rainfall. Given the impressive CAPE/PW
overlap, wherever clusters manage to merge and briefly train, some
flash flooding is likely and consider this a higher end Slight
risk. Locally significant urban flash flood impacts are possible
if heavier cells end up over a sensitive urban location.

...Mid MS Valley into the OH Valley...
Morning convection moving across MO is expected to weaken, however
robust redevelopment is expected by this afternoon ahead of a
shortwave extending from MO into IL. While cells should generally
remain progressive as they track eastward, brief cell mergers could
locally elevate the flash flood threat. The 00z HREF and REFS are
in good agreement that southeast MO, southern IL and western KY
will see the most concentrated heavy rainfall axis. As a
strengthening low level jet boosts moisture transport this
evening, the southern tail of the convective line may hang up,
allowing for a period of backbuilding and training. HREF/REFS
neighborhood probabilities show a 40-70% chance of rainfall locally
exceeding 3" along this axis. Scattered flash flooding appears
likely given this setup, although just enough southward propagation
of convection may cap rainfall totals and keep the flash flood
risk from being even higher.

...Central High Plains...
Mid-level shortwave energy combined with upper jet support will
produce convection across the High Plains from CO to SD this
afternoon. Deep layer shear and instability should be sufficient to
support slow moving supercells capable of pockets of intense
rainfall and localized flash flooding. With storm motion vectors
quite weak, most high res models seem too quick in moving this
activity off to the east (a common bias). Convection will
eventually shift east by tonight, and a majority of the 00z high
res models support some upscale growth as it moves into western and
central KS. Portions of eastern CO, southwest NE and
western/central KS have been upgraded to a Slight risk with
isolated to scattered flash flooding possible.

...Arizona...
Convection developing over the higher terrain may propagate
southwestward into the lower deserts during the late afternoon and
evening hours. Uncertainties persist regarding storm propagation
speed (whether cells will move too quickly to produce any
flooding) and the exact spatial extent/magnitude of instability.
While the overall threat looks more robust later this week into
the weekend, we will maintain the inherited Marginal risk.

Chenard


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt