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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Thu Sep 24 2026

Day 1
Valid 12Z Thu Sep 24 2026 - 12Z Fri Sep 25 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHWEST, AND THE SOUTHERN AND CENTRAL PLAINS...

Heavy rain will remain in the picture for much of the southwest,
as mid-level shortwave troughing moves northeast from the Baja
Peninsula, eventually ejecting into the Southern to Central Plains.
Notable low-level southerly-to-southeasterly flow, around 20kts,
will continue to supply abundant moisture from the Gulf into the
Plains and Southwest. PW values are expected to remain around 1.5",
which still remains above the 95th percentile for the area
climatologically, although the highest moisture flux appears more
localized over the southeast parts of New Mexico. Sufficient
instability (MUCAPE ~1300J/kg) will also be present to support
thunderstorms capable of producing locally heavy rain rates. In New
Mexico, per the 00Z runs of the HREF and RRFS, rainfall rates
appear most favorable to exceed 1"/hr, with medium-to-high chances
of exceedance (~50% and ~65% respectively). 3-hour FFG exceedance
probabilities among the two also suggest higher-end chances of
exceedance; around a 65-70%.

Antecedent conditions are also very favorable for continued
instances of flash flooding. Given multiple days of Moderate EROs
over New Mexico and precipitation continuing last night, soils
remain saturated, contributing to very low FFGs. Even modest
rainfall amounts at this point will have the potential to produce
flash flooding. As such a high-end Slight Risk remains in place for
southeast New Mexico, with the surrounding area over the Southwest
maintained in the inherited Slight.

Minor expansions were made to the inherited Slight Risk area to
include northern Kansas. The aforementioned mid-level shortwave
will eventually reach the Central Plains, and with a long fetch of
southerly flow bringing moisture originating from the Gulf, a
broad area of precipitation is expected through the day. While
global models remain bullish with QPF, including localized totals
of 2-3 inches, limited instability should largely favor a more
stratiform and longer- duration rainfall regime. As a result,
hourly rainfall rates are expected to remain modest despite notable
24hr QPF, particularly in Nebraska. This is supported by the
relatively muted HREF and RRFS FFG exceedance probabilities. As
such, the Slight Risk was only expanded to include more of Kansas,
where HREF and RRFS FFG exceedance probabilities appear more
favorable.

Blanco-Alcala
 

Day 2
Valid 12Z Fri Sep 25 2026 - 12Z Sat Sep 26 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM NEW MEXICO
INTO THE PLAINS, AND OVER SOUTHEAST NEW ENGLAND...

...Southwest...

A lingering mid-level disturbance, along with sufficient
instability and moisture influx from the Gulf, will support another
day of showers and thunderstorms across portions of the Southwest.
Antecedent conditions will remain a concern following multiple
rounds of rainfall, particularly across portions of New Mexico
where soils remain saturated and FFGs remain low. While the overall
coverage and intensity of convection may be reduced compared to
previous days, locally heavy rainfall rates will remain possible
where stronger convection develops. Given the increasingly
sensitive hydrologic conditions, even modest additional rainfall
may result in localized flash flooding. The inherited Marginal was
maintained with only minor adjustments.

...Central Plains...

The mid-level shortwave will continue to progress across the
Central Plains, maintaining broad ascent and precipitation from
portions of Kansas northeastward through Nebraska and Iowa. Locally
appreciable rainfall total remain possible, though limited
instability may again favor a substantial stratiform component and
temper rainfall rates. A Slight Risk was considered across portions
of Nebraska into Iowa. However, uncertainty regarding the coverage
and magnitude of heavier rainfall precludes an upgrade at this
time, and the Marginal Risk was maintained. An upgrade may become
necessary in later forecast cycles as a consensus hopefully becomes
more clear, and confidence in mesoscale details increase.

...Northeast...

A Marginal Risk has been maintained for parts of the Northeast as
the details of a potent coastal low continue to be ironed out.
There is still uncertainty pertaining to the time and location of
the low track, but there is a consensus forming in heavy rainfall
occurring along coastal Massachusetts, Rhode Island, and
Connecticut. However, placement of the low along with its central
pressure vary. With global guidance remaining in disagreement,
enough uncertainty was raised to preclude any upgrade at this
time, with continuity of the previous forecast being favored.

Blanco-Alcala
 

Day 3
Valid 12Z Sat Sep 26 2026 - 12Z Sun Sep 27 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE NORTHEAST...

The potential for heavy rainfall increases substantially in the
Northeast in the Day 3 period. The coastal low will bring a
generally prevailing southeast to easterly flow around a low
centered around Long Island, aiding in a substantial influx of
moisture onshore into the Northeast and parts of the Mid-Atlantic.
Current QPF guidance suggest widespread totals of 2-4" with
isolated totals as high as 5" in the 24-hour period. The
significant amount of expected rainfall over the long period into
the urban corridor of eastern MA will likely cause widespread
hydrologic concerns, including flooding in urban areas including
Boston, and poor drainage locations. Additionally, given increased
confidence regarding the heavy rain area, the now "higher-end"
Slight Risk area was maintained as inherited.

Further west the focus of the possible heavy to excessive rainfall
remains over the Midwest/Central Plains on Saturday as a shortwave
energy ejects northeastward from the Rockies. The envelope of
solutions span the EC on the north side and the GFS favoring a more
southerly solution corresponding to Iowa and Missouri/Kansas
respectively. A few of the globals remain rather bullish on the
amount of expected rainfall, particularly the ECMWF, while
ensembles continue to focus on this area as well. As such, the
Marginal Risk in Iowa was maintained where flash flood guidance is
lowest after the recent rainfall.

Blanco-Alcala
 

Day 4 and Day 5
Valid 12Z Sun Sep 27 2026 - 12Z Tue Sep 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF NEW ENGLAND
ON SATURDAY...

New England...
There is a Slight risk of excessive rainfall in proximity to a deep
coastal low that slows its forward speed near southeast New
England. Heavy rainfall will begin to taper off during the period
in terms of areal coverage, rainfall rates and daily rainfall
amounts as the upper system fills. Model consensus has been
building around the idea that the surface system heads inland
compared with pivoting and heading back out to see. The cumulative
effect of rainfall over the long period into the urban corridor of
eastern Massachusetts will cause hydrologic concerns, including
some flooding in poor drainage locations. For that reason, opted to
keep the Slight risk on Day 4. Opted for a Marginal risk given the
additional amounts shown by the models on Monday...but that may
ultimately depend on the hydrologic state by then.

...Southwest US...
After a relative lull on Sunday with only a fairly focused
Marginal Risk area confined to parts of the Intermountain region,
the infusion of mid- and upper-level moisture with a tie to Polo in
the eastern North Pacific basin is expected to lead to blossoming
coverage of rainfall and increased risk of intense downpours on
Monday across the Southwestern states. With uncertainty about the
time of arrival...started with a Marginal Risk area.

...Northern Plains/Upper Midwest...
A trough in the northern stream will push a cold front across the
northern tier of states in the Upper Midwest on Monday. Moisture
anomalies are modest and the upper system appears to be
progressive...both of which will limit the concern for excessive
rainfall. But there are enough ensemble members...especially within
the EC ensemble suite...to make a low- end Marginal warranted.

Bann
$$
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