Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Wed Apr 30 2025

Day 1
Valid 01Z Thu May 01 2025 - 12Z Thu May 01 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHEAST TEXAS AND NORTHWEST LOUISIANA...

01Z Update...

Updates to the previous ERO were based upon recent runs of the
HRRR, the 18Z HREF, and current observation trends. 

The greatest heavy rain and flash flooding concerns through the
evening hours focus along the southwest flank of ongoing
convection and a coincident slow-moving boundary from the ArkLaTex
southeastward back through the Waco area. Current radar shows
storms training along the boundary, with rainfall rates of 1-2
inches/hour within some of the stronger cells. Cell-training and
heavy rainfall will remain a concern for at least the next few
hours as deep layer flow is aligned with the boundary. The Moderate
Risk was adjusted to align with the training convection, which
agrees well with the higher HREF neighborhood probabilities for
additional accumulations of 3 inches or more during the
evening/overnight.

Farther to the north across Arkansas, the threat for heavy
rainfall and flash flooding is diminishing where the convective
line has become more progressive.

Areas back to the north and west, where the rainfall has ended,
were removed from the outlook.

Pereira


Day 2
Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN SOUTHERN
OKLAHOMA, FAR NORTH TEXAS, AND FAR WESTERN ARKANSAS...

The most significant change for this outlook period was to
introduce a Slight Risk primarily over southern Oklahoma, where
previously the probabilities were below the Marginal Risk (5
percent) threshold. This was due to an emerging consensus in model
guidance that thunderstorms will blossom on Thursday Night ahead of
a digging shortwave in an area of relatively strong upper level
divergence. Although the mid levels will be drier than in recent
days, leading to lower PWs overall, strong instability will be in
place which should support organized convection with more intense
rain rates at times. A key uncertainty at the moment is the overall
structure of convective clusters and lines, and whether they will
be more progressive or include some training. However, the
potential for heavy rain rates in an area that has received a lot
of rainfall already in recent days does raise the risk of flash
flooding. Hourly rainfall on the order of 1-2 inches will be
possible in this environment, and in areas with already wet ground
conditions this would be more than sufficient to cause flash
flooding.

Elsewhere, the broad Marginal Risk that was already in place was
maintained with few changes. A broad plume of moderate to strong
instability will favor scattered lines and clusters of
thunderstorms in advance of a surface low and trailing cold front
with fairly high instantaneous rain rates. The key uncertainty
would be the persistence of those rain rates and the degree to
which any training could occur, but the overall environment with
PWs around 1.3 to 1.4 inches raises the probability high enough to
warrant the broad Marginal Risk. The only exception to the
continuation of the risk area was over much of IL, IN, and MI,
which should largely be behind the cold front by 18Z and so most of
the QPF on model guidance would be associated with post-frontal
showers that would tend to have lower rain rates overall.

Lamers


Day 3
Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE
SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...

A flash flood event may be ongoing at the beginning of the period,
with convection developing on Thursday Night in southern Oklahoma
and continuing into Friday morning. This may continue beyond 12Z
Friday (into this Day 3 period) in SE OK and AR in the region of
strong upper-level divergence. The existing Slight Risk over TX was
expanded northward to account for this scenario. Whether that round
of convection wanes due to more extensive cloud cover and limited
daytime heating, or progresses southeast and becomes reinvigorated
during the afternoon, the threat of heavy rainfall and flash
flooding should expand southeast on Friday into the Lower
Mississippi Valley (S AR, N LA, W MS) and much of C and E TX. This
would be via progression of the early round of storms, or
redevelopment in the afternoon and evening hours.

Strong instability will remain over much of the warm sector with
CAPE values potentially in excess of 3000 j/kg in some areas, and
PW values are expected to increase above 1.5 inches as SSE flow
increases off the Gulf. The combination of strong instability and
high PWs will favor very heavy rain rates in organized convection,
and this is supportive of the broad Slight Risk on the new
outlook. At the moment, model guidance is showing the most
consistent signal for heavy rain from Texas Hill Country in C TX
east- northeast into the Piney Woods region of E TX. Given the
overall environment, it is possible an upgrade to a Moderate Risk
may be needed at some point.

Lamers


Day 1 threat area:
https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area:
https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area:
https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt