Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Wed Mar 11 2026
Day 1
Valid 16Z Wed Mar 11 2026 - 12Z Thu Mar 12 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF
SOUTHEAST TX AND THE LOWER MS VALLEY ALONG WITH THE UPPER OH VALLEY...

16Z Update...

The 12Z HREF guidance continues to show a rather organized threat
for heavy showers and thunderstorms across areas of southeast TX
and into adjacent areas of the Lower MS Valley going through this
afternoon and evening. Anomalous PWs and a moderate to strongly
unstable boundary layer will be in place ahead of the remnants of
southern stream shortwave energy ejecting out of the southern
Plains and into the Lower MS Valley. This activity should advance
east with potential for a QLCS evolution across portions of
MS/AL/LA going into the overnight hours. Rainfall rates of up to 2
inches/hour with some localized totals of 3 to 5 inches will be
possible where any cell-training occurs. As such the Slight Risk
area has mainly been trimmed locally to account for the latest 12Z
HREF/06Z REFS consensus of guidance and the back edge of the precip
shield. Much of the flash flooding concern is expected to be urban
in nature given the fairly high FFGs, but any training convective
bands that materialize will elevate this concern locally.

Given the convective trends and locally moistening antecedent
conditions across portions of the Upper OH Valley, a Slight Risk
area was introduced which mainly impacts southern OH through
northern WV and southwest PA. Concerns for a more organized
convective threat this afternoon ahead an approaching upstream
shortwave along with cell-training will exist. Some spotty 1 to 1.5
inch/hour rainfall rates and additional localized 2 to 3 inch
rainfall amounts going through this evening may foster some
isolated to scattered areas of flash flooding.

The Marginal Risk areas elsewhere largely follow the latest multi-
model consensus. This includes the arriving atmospheric river
across western WA and northwest OR where enhanced moisture
transport will be oriented orthogonal to the coastal ranges and
the western slopes of the Cascades. IVT values peak generally near
or just over 750 kg/m/s by later today and then slowly weaken going
into the overnight period. Several inches of rain are still
expected here by early Thursday, with some spotty 5+ inch amounts
possible. Some rainfall rates generally in the 0.50" to 0.75"/hour
range will be possible during the peak IVT window where enhanced
onshore warm air advection and orographic ascent focuses over the
terrain.

Orrison

Previous discussion...

As the closed low moves across the central and eastern Texas
today...an axis of training convection should persist from the
eastern part of Texas into the lower Mississippi Valley. With very
strong synoptic forcing and increasing moisture transport we
should see convection expand in coverage and intensity over eastern
Texas by mid- to late-morning. During the development phase cell
training/backbuilding is probable over east TX, and while
convection should grow upscale and forward propagate during the
afternoon, some west to east training is possible near the
southward dropping cold front. The inherited Slight risk was
expanded off to the south and east once again to account for model
trends into parts of Mississippi and western Alabama.

Only minor nudges were needed to the Marginal risk area across the
Ohio Valley and into western New York where rainfall rate driven
localized flash flooding is possible. Areal averaged rainfall is
only 0.5"-1.5", but given the 500-1000 j/kg and convective nature
of rainfall, the possibility of localized swaths of rainfall
exceeding 2 inches persists. This should be enough to at least
locally exceed FFG across this region. Over western NY, snow melt
and locally heavy rain may combine to result in areas of excessive
runoff.

Chenard/Bann

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