Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu May 21 2026
Day 1
Valid 01Z Fri May 22 2026 - 12Z Fri May 22 2026
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN PLAINS SOUTHWARD INTO TEXAS...
01Z Update...
The main area of concern for excessive rainfall remains in Texas
given the low level moisture and dynamics...especially where on-
going convection intersects/interacts with deeper moisture and
strengthening low level flow. Elsewhere...the risk for excessive
rainfall should be diminishing with loss of daytime heating.
Bann
...Southern Plains...
16Z update... A modest expansion of the western and southern
portions of the boundary were made to reflect model trends and the
HREF and REFS FFG exceedance of 25 to 70% for parts of Texas. A
substantial westward expansion of the Marginal Risk into West Texas
was made to account for a cluster of thunderstorms and a bowing
segment that is progged to advance southeast through west-central
Texas during the afternoon/evening hours. Rain rates of 1.5 to
2.5+ inches/hour were depicts in the CAMs and will be going over
some locations that have very low FFGs thus elevating the threat
for local flash flooding concerns.
Campbell
The combination of several ingredients coming together across
eastern Oklahoma and north Texas will be the driver for rounds of
training convection expected across the area. At the surface, the
nose of a low level jet will advance northward over Oklahoma,
spiking the PWATs over 1.5 inches this morning. Surface convergence
around a trough will support low level lift and thunderstorm
formation. An approaching upper level trough will increase the
divergence and lift in the upper levels. The result will be a
rather narrow corridor at first with mostly showers but some
thunderstorms by early afternoon, as the day is approaching peak
heating. Cold pools and speed convergence will focus the
convection, allowing training to become more likely as the storms
track northeastward. A secondary wave of thunderstorms will move
northward across north Texas, which will increase the areal
coverage of convection into the early evening hours. The
strengthening upper level shortwave driving this secondary wave
will become the primary forcing, allowing the complex of weakening
storms tonight to push northward and end the rainfall threat from
south to north. While the greatest flash flooding threat may be in
a rather narrow corridor from north Texas north and east through
Tulsa, sufficient Gulf moisture could still allow for flash
flooding in other areas as well.
The inherited Slight Risk was split with this update owing to
better CAMs agreement that from north Texas through Oklahoma will
be the best area for flash flooding risk, but less rainfall is
expected across east central Texas, between Houston and the
Metroplex, allowing for a downgrade to the risk in that area.
...Upper Texas Coast into Southwest Louisiana...
16Z update... Heavy rainfall was observed along the Texas and
Louisiana coastline this morning with several locations reporting
a few inches of accumulation already. This has helped prime soil
saturation and increase sensitivity for the next round of
convection expected later today. A modest northward nudge was made
to the Slight Risk across southeast Texas.
Campbell
A large complex of storms currently over South Texas will push
northeast up the Texas Coast through the day today. The area will
have abundant Gulf moisture, instability, and some limited support
from the upper levels. Thus, any storms will be capable of very
heavy rainfall, but the time of the heavy rainfall in any one area
may be limited as the complex progresses northeastward. The area
was hit hard with very heavy rain yesterday, resulting in rainfall
totals over 5 inches near Freeport. As this second round of storms
moves through this morning, convergence into the storms, sea breeze
effects, and the saturated soils from yesterday's rains could
result in additional instances of flash flooding. The storms
continue into the Houston Metro around midday, introducing an urban
factor to the flash flooding threat. The storms then turn more
northward through the afternoon, ending the flash flooding threat
for the day along the coast. To summarize, the Slight Risk, albeit
a low- end one, remains in place mostly for the potential for very
heavy rains with PWATs approaching 2 inches and the favorably wet
soils overcoming the fast movement of the storms to result in flash
flooding.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu May 21 2026
Day 2
Valid 12Z Fri May 22 2026 - 12Z Sat May 23 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY AND THE SOUTHERN Appalachians....
21Z update... Storms will be firing along and ahead of a frontal
boundary lifting northward through the Ohio Valley. With
instability increasing and PW values surging up to 2-2.5+ std. dev
above normal local enhancement will be expected, especially
northern Kentucky and Ohio. Recent rains have lowered local FFGs to
around 1-1.5 inches which may easily be surpassed. Therefore this
part of the Ohio Valley was upgraded to a Slight Risk.
Further south, east-southeasterly low level winds will lead to
local enhanced lift along the higher terrain from northeast Georgia
to far western North Carolina. During this time PWs of 2+ will be
pooling over the region in proximity to the stalled front. Areal
averages of 1 to 3 inches forecast for this narrow strip however
very isolated higher maximums possible and a Slight Risk was
raised.
Campbell
The inherited Marginal Risk was left largely unchanged for many
areas with this update, save some trimming across both Oklahoma and
Missouri due to reduced signal for heavy rainfall in those areas.
Two areas in particular stand out as higher-end Marginal Risks,
where a Slight may be needed with future updates:
Across a portion of the Ohio Valley from northern Kentucky through
southern Ohio, efficient moisture advection will spike PWATs from
around 1.25 inches early in the day Friday to 1.75 inches by the
afternoon. With this added moisture, so too will instability
increase. Rain in the form of showers and thunderstorms following a
warm front, the same wave that will impact the Slight Risk areas on
D1/Thu will increase in coverage at peak heating Friday afternoon.
Mostly stratiform rain will lead the convection in the morning,
helping saturate the soils that in some areas of the Ohio Valley
remain nearly saturated from prior days' rains. Then during the
afternoon as instability and moisture rapidly increase and the
cloud cover and relative stability push north into northern Ohio
and Pennsylvania, areas of showers and storms will impact northern
Kentucky into southern Ohio. There remains considerable uncertainty
as to how much instability can develop given the short time frame
between the morning's stabilizing rains with the warm front and the
following convection. Further, there is uncertainty as to how the
storms will organize, with some training needed to produce anything
more than isolated instances of flash flooding. Thus, for now, the
Marginal was maintained, but is likely the higher of the two areas
for a potential Slight risk upgrade with future updates.
The other area of the large Marginal that is drawing additional
scrutiny is the area from north Georgia into the Carolinas. Here
moisture and instability will be plentiful, supporting clusters of
heavy-rain-producing thunderstorms. Winds are generally
unidirectional which may support training. However, lack of forcing
with upper level ridging in place will act as a counter to allowing
the storms that form to organize. Thus, clusters of disorganized
convection will be unlikely to produce much in the way of flash
flooding. Further, much of this area is in severe drought, and
soils are extremely dry. Sandy soils would work against flooding as
well into the Piedmont, so that highly unfavorable hydrology should
also work to effectively keep any flooding threat in check.
Wegman
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu May 21 2026
Day 3
Valid 12Z Sat May 23 2026 - 12Z Sun May 24 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF EASTERN
TEXAS AND WESTERN LOUISIANA...
21Z update... The latest guidance had small increases in the QPF
for counties near the Rio Grande in South Texas therefore the
southern boundary of the Slight Risk was extended to the border
with Mexico. This also required a small nudge of the Marginal Risk
boundary near the border as well. Much of the coastline had
received 4 to 6 inches of rain recently and soils are saturated.
This particular period has the potential for additional few inches
to fall over these sensitive areas and are fairly confident that
this will fall on the higher end of the Slight Risk. There is
enough spread in the placement of the highest QPF totals (offshore
or not) that confidence was not there to introduce a Moderate Risk
for much of the Texas Coastline. This part of the state will
continue to be monitored with future updates on whether there is a
need for an upgrade.
Campbell
Perhaps the most active day of the Days 1-3 period will be on
Saturday across eastern Texas into Louisiana. Plentiful moisture
and instability will be drawn north off the Gulf, spiking PWATs to
around 2 inches in some areas, and instability could peak over
4,000 J/kg right at convective initiation midday/early afternoon
Saturday. Lines of intense thunderstorms will develop in this
extremely favorable air mass during the afternoon. The storms will
generally track northeastward. However, as they move north,
additional clusters are likely to form as both moisture and
instability rapidly recover. In the upper levels a series of small
shortwaves, but nonetheless potent ones given the highly favorable
atmosphere will allow for new clusters of storms to form across
southeast Texas soon after the prior rounds move off to the north.
Once daytime heating wanes in the evening, it appears likely that
additional storms that form will predominantly move southeastward
towards the Gulf as they follow the instability. This could lead to
additional periods of heavy rainfall over many of the same areas
hit with heavy rains Saturday afternoon. Further, these storms
moving against the prevailing windflow could slow down the storm
movement, increasing both interaction potential between cells as
well as the duration of the heavy rainfall, this threat appears
particularly probable in the Houston metro area. With CAMs input
over the next day or so, a Moderate Risk may need to be considered
in and around the Houston metro, perhaps extending southwest along
the Texas coast with future updates.
Elsewhere, the Marginal across portions of the Northeast was
removed with this update as the warm front over the area will have
zero instability to work with, resulting in a long duration
stratiform rain. Some elevated convective elements may sneak their
way that far north, but the otherwise light rainfall should allow
any rain to soak into the soils with no flash flooding expected.
Conversely, a Marginal Risk was introduced for a portion of north
Georgia into the Carolinas. Following likely rain from storms on
D2/Friday, additional widespread storms are expected again Saturday
afternoon into the evening over many of the same areas. As on
Friday, forcing in this area will be very limited. The storms will
generally track east-southeast towards the coast, with enough
movement that interactions are generally unlikely to initiate
additional convection. Any lines of storms should align
perpendicular to the flow reducing the training risk. Finally,
despite any rains on Friday, the ongoing severe drought should
still greatly mitigate most of the flash flooding threat. Given the
plentiful moisture and instability the storms will have to work
with, they will still likely produce very heavy rain in their
cores, raising resultant flooding potential into the Marginal
category.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
354 AM EDT Thu May 21 2026
Heavy rain will continue to make weather headlines going into
Sunday and Memorial Day from eastern Texas to the Deep South with
multiple mesoscale convective systems developing. These will be in
response to enhanced ascent from a weak upper trough in the
southern stream flow, coupled with a weak front in the region and
plenty of moisture and instability. A Slight Risk for excessive
rainfall will be valid on Sunday from eastern Texas to western
Louisiana, where the potential exists for a few inches of rainfall
with the most persistent storms over soils that will be becoming
increasingly saturated. The coverage becomes less concentrated
going into the Day 5/Monday time period, so a broad Marginal Risk
area is valid across much of the South and extending eastward to
include portions of southern Virginia. Scattered showers and
thunderstorms are likely to develop, and some of these could be
locally heavy. More precipitation returns to the Pacific Northwest
to start next week with a cold front moving in and a building upper
level trough, and this moves inland through the middle of the week.
A gradual warming trend can be expected for the Mid-Atlantic and
the Northeast to start next week after a few days of cooler
weather from cold air damming. Slightly below average temperatures
are likely across the Gulf Coast region given the widespread
rainfall that is expected. With the upper low developing over the
northwestern U.S., a return to colder conditions is also likely
here going into Tuesday and Wednesday, with some higher elevation
snow likely for portions of the northern Rockies. The greatest
positive anomalies will likely be over the northern Plains and
Upper Midwest with the upper ridge axis Monday through Wednesday,
with readings up to 20 degrees above average.
Hamrick
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
354 AM EDT Thu May 21 2026
Heavy rain will continue to make weather headlines going into
Sunday and Memorial Day from eastern Texas to the Deep South with
multiple mesoscale convective systems developing. These will be in
response to enhanced ascent from a weak upper trough in the
southern stream flow, coupled with a weak front in the region and
plenty of moisture and instability. A Slight Risk for excessive
rainfall will be valid on Sunday from eastern Texas to western
Louisiana, where the potential exists for a few inches of rainfall
with the most persistent storms over soils that will be becoming
increasingly saturated. The coverage becomes less concentrated
going into the Day 5/Monday time period, so a broad Marginal Risk
area is valid across much of the South and extending eastward to
include portions of southern Virginia. Scattered showers and
thunderstorms are likely to develop, and some of these could be
locally heavy. More precipitation returns to the Pacific Northwest
to start next week with a cold front moving in and a building upper
level trough, and this moves inland through the middle of the week.
A gradual warming trend can be expected for the Mid-Atlantic and
the Northeast to start next week after a few days of cooler
weather from cold air damming. Slightly below average temperatures
are likely across the Gulf Coast region given the widespread
rainfall that is expected. With the upper low developing over the
northwestern U.S., a return to colder conditions is also likely
here going into Tuesday and Wednesday, with some higher elevation
snow likely for portions of the northern Rockies. The greatest
positive anomalies will likely be over the northern Plains and
Upper Midwest with the upper ridge axis Monday through Wednesday,
with readings up to 20 degrees above average.
Hamrick
