FOUS30 KWBC 311953
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Thu Jul 31 2025

Day 1
Valid 16Z Thu Jul 31 2025 - 12Z Fri Aug 01 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN MID-ATLANTIC...

...Mid-Atlantic into New England...

16Z Update...

Forecast remains on track for numerous instances of flash flooding
to impact parts of the Mid-Atlantic, including the I-95 corridor
between northern VA and southern CT, some of which could be
significant and occur during the afternoon/evening commute. The
Moderate Risk was expanded in the 16Z update to include the
remainder of southern NJ and more of central DE after evaluating
12z CAMs. A >120kt jet streak located across southeast Canada
places the Northeast and Mid- Atlantic within the favorable right-
entrance region of the upper jet, while a stationary front at the
surface was analyzed this morning to stretch from coastal southern
New England through southern PA. With forcing in place to trigger
thunderstorm development, moisture and instability are also
sufficient. SPC's mesoanalysis highlights PWs widely over 1.7" and
over 2.0" into northern VA and central MD. This equates to near the
90th climatological percentile. Potentially even more impressive
is the available instability this morning. SBCAPE values have risen
above 4,000 J/kg throughout northern VA, central MD, and into
southeast PA and with southerly 850mb flow feeding this instability
even further. A very tight gradient exists along the front with
very low instability north of the Lower Hudson Bay and southern New
England. However, this tight instability gradient may also be the
focus for repeating cells capable of containing torrential
rainfall given the mean layer flow parallel to this gradient and
out of the west- southwest at 20-30kts.

After initial convection that has already developed as of 1530Z
this morning across eastern PA into northeast MD, the expectation
is backbuilding should occur in a west-southwest to east-northeast
orientation and continue through the evening as better upper
forcing and associated surface low will sweep through tonight.
This could lead to some areas within the MDT falling between maxes
and not seeing much rain at all, but areas within these heavier
axes could see localized totals up to 5-8". 12z HREF neighborhood
probabilities for >3" in 6 hours are very high and widely above 50%
between central/southern MD and NJ. Overall, the highest HREF
signal for greater than 5" totals exist across central NJ and near
parts of MD near the Chesapeake Bay. Additionally, 1 hour
probabilities for >2" are also impressive and with scattered values
between 40-60%, highlighting the rainfall rates associated with
efficient rainfall producing thunderstorms. Should these rainfall
rates (2-3"/hr) occur over urbanized locations, the large
concentration of pavement and impervious surfaces will likely lead
to extremely rapid runoff and potentially significant flash
flooding. Even outside of the major urban centers, these rainfall
amounts could lead to widespread instances of flash flooding through tonight.

Snell

Previous Discussion...

A potent vorticity max paralleled with an exceptionally strong
upper level jet streak over southeast Canada and highly anomalous
moisture content will spin up a surface low along a frontal
boundary over the northern Mid- Atlantic and spread enhanced
rainfall to a vast portion of the East. There are two areas of
concern regarding heavy rainfall. The first area is from northeast
Pennsylvania on east through the Lower Hudson Valley and into
southern New England will have a band of heavy and efficient
rainfall. PWATs will rise to the 1.8-2.0" range and low-level WAA
ahead of the front will give rise to potential training. There is
uncertainty regarding the amount of available instability, which
could reach as high as 500 J/kg. Still, the modeled soundings
depict warm cloud layers up to 13,000ft deep, so while rates may
not be as high as farther to the south, warm rain processes could
still provide 1-1.5"/hr rates in these areas Thursday afternoon and
into Thursday night. A Slight Risk remains is in place for these
regions with locally significant flash flooding possible,
particularly for areas closer to the Poconos, higher elevations of
southern New England.

Farther south, while the Mid-Atlantic region is a little farther
removed from the jet-streak dynamics to the north, the region is
closer to the deepening surface low with strong surface-925mb
southerly theta-e advection directed into the surface front. From
northern NJ and southern PA on south to northern VA, a tropical
air-mass (depicted by 2-2.25" PWATs and mid 70s dew points) will
coincide within a warm sector that becomes increasingly unstable
(MUCAPE between 1,000-2,000 J/kg). It is worth noting that SST
anomalies along the Mid-Atlantic and Southeast coasts are well
above normal too, acting as an abnormally rich moisture source for
southerly low-level flow to draw from. By this afternoon, low-
level WAA into the front will support storms enveloping the region
with additional storms from the Appalachians tracking east. Modest
vertical wind shear aloft will also support organized clusters of
storms, some of which will have the potential to backbuild and
train over the I-95 corridor into Thursday night. Latest 12Z HREF
guidance shows 24-hr probabilities for >3" of rainfall are high
(>70%) from the DC/Baltimore area on north through northeast MD,
southeast PA, and into southern NJ. Just as concerning is this same
region sports 30-60% chances for >5" of rainfall.

Impact potential for Moderate Risk Area: These probabilities
listed above serve as signals in ensemble guidance that highlight
the potential for significant flash flooding throughout the country,
let alone over a highly populated urban corridor. Given the
atmospheric parameters in place, it is possible that the Mid-
Atlantic I-95 corridor witnesses thunderstorms that produce
rainfall rates up to 3"/hr, with perhaps the most intense storms
potentially generating 2" rainfall totals in as little as 30
minutes. QPF may seem light at first glance given the hourly
rainfall rate potential, but this is largely due to lingering
uncertainty in the placement of the heaviest rainfall. Many
individual convective models show localized totals surpassing 5"
with even some localized maxima approaching 8". Residents in the
Mid-Atlantic should ensure they have reliable means of receiving
warnings issued by their local WFOs throughout the day and into Thursday night.

...Gulf Coast into Ohio Valley...

Slow moving thunderstorms will persist today and tonight over a
vast area south of the cold front. High PWs and CAPE and
convective coverage will drive a localized flash flood risk. A
lack of organization may keep FFG exceedance coverage down, but
high rainfall rates will likely locally approach or exceed FFG.
This region is covered by a Marginal Risk for excessive rainfall
and isolated flash flooding.

...Rockies into the Plains...

Scattered thunderstorms expected to fire up from eastern
Arizona/New Mexico northward to Montana. In general, the storms
will likely lack organization for a more elevated threat for flash
flooding. The Marginal Risk was expanded to include more of WA/OR
up towards the eastern Cascades as an upper low lifts across the
region and intersects with PW values above the 75th climatological percentile.

Snell/Mullinax/Campbell
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