Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
808 PM EDT Thu Apr 17 2025

Day 1
Valid 01Z Fri Apr 18 2025 - 12Z Fri Apr 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR
SOUTHERN MINNESOTA AND IOWA...

The Marginal Risk area was shifted south based on the 18z HREF and
radar/recent ML CAPE trends. Precipitable water values up to 1.25"
lie across the region, and effective bulk shear is roughly 50 kts.
When combined with fairly unidirectional southwest flow with
height, organized convection with some periods of training and
mesocyclones have been observed so far. New convection development
in southeast NE implies that 700 hPa temperatures of 9C are acting
as the effective mid-level capping inversion. When combined with
widely scattered convection in southern MN and a forecast
increasing in MU CAPE in and near IA, figured the Marginal Risk of
excessive rainfall should continue into the overnight hours as
there is some risk of cell mergers. Hail is leading to higher
radar-derived rainfall estimates than observed, much like KS/MO
last night. As 700 hPa temperatures warm, the southern edge of the
convective footprint should lift slowly northward through IA
overnight, though the speed of the northward shift is uncertain as
convection could retard the shift to some degree. Backbuilding,
training, and merging storms will be possible before the storms
attempt to congeal into a line, which will limit the potential for
training with time early Friday morning. Much like yesterday,
hourly rain totals to 2" are possible, which may very well be the
overall totals as convection attempts to forward propagate. The
biggest flash flood concern would be within urban areas, as it has
been quite dry over much of this region during the past couple of
weeks.

Roth


Day 2
Valid 12Z Fri Apr 18 2025 - 12Z Sat Apr 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHEASTERN
OKLAHOMA THROUGH CENTRAL MISSOURI...

20Z Excessive Rainfall Discussion Update...
Models are in fairly good agreement amongst themselves with respect
ot the beginning of a prolonged moderate to heavy rainfall from
Friday into the early morning hours on Saturday. Changes made to
the Excessive Rainfall Discussion were fairly subtle to account
for a slight westward shrift in the QPF placement but didn't not
reflect any significant change in forecast reasoning.

Bann

Previous Excessive Rainfall Discussion...
A well-established and persistent southerly LLJ will be parked
across much of the middle of the country Friday into Friday night.
Deep tropical moisture originating from the Caribbean will flow
into the Texas Gulf Coast and along a very slow moving frontal zone
draped across the Plains to the upper Great Lakes. Showers and
thunderstorms will likely break out during peak heating across the
region starting during the late afternoon hours, but the strongest
storms and greatest rainfall are more likely to occur overnight
Friday night. The strong cold front to the north will run into
opposing flow from the LLJ, causing its progression south and east
to slow as it approaches Oklahoma and Missouri from the northwest.
This will act as a wall, resulting in the LLJ air mass to ride up
and into the front, then track along the frontal interface to the
northeast. This classic frontal scenario will strongly support
training thunderstorms that individually may move quickly to the
northeast as the LLJ intensifies to 50 kt over northwestern
Arkansas Friday night. However, the steady influx of moisture will
support backbuilding along the front, resulting in a line of
training storms across this area. The rain will be somewhat offset
from the area of greatest historic rainfall from 2 weeks ago, and
the smaller streams and creeks have fully drained from that event,
but the bigger rivers, including the Missouri, Arkansas, and
Mississippi continue to drain, and this new influx of rainfall
could back up as the new rainfall meets already full rivers. The
bulk of the event will focus into the Day 3 period. With greater
instability available for the storms to produce heavy rainfall
during this period, it's likely that where the storms are most
persistent, there will be widely scattered instances of flash
flooding, especially near the Tulsa and Joplin metros.

Guidance has favored a very small (roughly 5-10 mile) trend
towards the northwest with each run, so the Slight Risk area was
expanded a row of counties north and west with this update, but
otherwise no big changes were made. The greatest uncertainty lies
in the southwestern extent of the axis of heaviest rainfall, so a
small adjustment to the Red River (TX/OK border) was also made with
this update. In coordination with PAH/Paducah, KY and ILX/Lincoln,
IL forecast offices, the Marginal Risk was also expanded south and
east to account for the new rainfall interacting with still
draining rivers from 2 weeks ago into far southeastern Missouri and
southern Illinois as well.

Wegman

Day 3
Valid 12Z Sat Apr 19 2025 - 12Z Sun Apr 20 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN
OKLAHOMA THROUGH MUCH OF SOUTHERN MISSOURI...

20Z Excessive Rainfall Discussion Update...
The 12Z suite of model guidance largely supports the placement and
amounts of QPF..although there has been a subtle westward shift
over the course if the past few model runs...especially noticeable
from parts of northeast Texas/southeast Oklahoma into Arkansas.
Trimmed a bit territory from outlook areas in this
region...although did not want to remove given the propensity for
convection to build into the deeper moisture and instability south
of the front with time even though the model QPF was focused in a
narrow channel. Pulled the southern end of the Moderate risk area
southward given the increase in WPC deterministic back towards the
Red River. Overall...the large scale synoptics have not changed
significantly so much of the previous discussion remains valid.

Bann

Previous Excessive Rainfall Discussion...
In coordination with the impacted forecast offices, a Moderate
Risk upgrade was introduced with this morning's ERO update. A
stationary front separating very warm and moisture-laden air to the
south from abnormally cold air to the north will stall out from
being a cold front in previous days. Weakening northeasterly flow
of cold, dry air on the north side of the front, spurred on by a
Canadian high over the Great Lakes will contrast with an ever
increasing supply of hot and humid air straight out of the Gulf
being advected northward on up to 50 kt winds across Texas and into
the frontal interface from north Texas through Oklahoma and
Missouri. Meanwhile in the upper levels, a potent shortwave,
previously an upper level low will eject eastward out of the
mountains and into this warm and humid air mass. This will result
in cyclogenesis across Oklahoma by Sunday morning. Until that
happens though, the front will be largely stationary, with the warm
humid air tracking northeastward on one side, and cool, dry air
tracking southwestward on the cold side. The abundance of moisture
and cold pools from Friday night's storms will support continued
shower and thunderstorm development south of the front. As the
shortwave approaches, additional shower and thunderstorm
development will occur, and with stronger storms due to the added
forcing as the surface low develops to the west.

The southerly flow of humid air will cause the storms to run into
the Ozarks of Oklahoma, Arkansas, and especially southern Missouri
on Saturday. The mountains will further uplift the moisture,
resulting additional locally heavy rainfall. The Ozarks are known
for being particularly flash flood prone due to the mountains
helping funnel the water quickly into the river valleys, so this
prolonged period of heavy rainfall is likely to cause some of the
worst impacts to occur in these regions.

As the storms track northeastward across Missouri, the front is
likely to direct them into the St. Louis metro. In coordination
with LSX/St. Louis forecast office, the Moderate Risk's
northeasternmost extent includes the St. Louis metro. While the
storms may not be quite as strong as compared with areas further
south and west by the time they reach St. Louis, the urban corridor
and confluence of major rivers in the area may exacerbate urban
flooding concerns as repeated rounds of heavy rain likely move over
the city.

Some of the guidance (RRFS/UKMET/CMC) are favoring the southern end
of the ERO risk areas across Texas and Oklahoma as being the area
with the heaviest rain. Should that occur, then, the Dallas-Fort
Worth Metroplex will also be at higher risk for flash flooding, and
the Moderate Risk may need to be expanded southwestward with future
runs. There were 2 camps of guidance, largely split between foreign
and domestic, favoring the southern and northeastern ends
respectively. To account for these factors, the Slight and Marginal
Risk were expanded northeastward along the frontal interface to the
IL/IN border for the Slight, and well into northern Ohio for the
Marginal.

Wegman


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