Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Thu Apr 17 2025

Day 1
Valid 16Z Thu Apr 17 2025 - 12Z Fri Apr 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
MINNESOTA AND WISCONSIN...

16Z Excessive Rainfall Discussion Update...
Maintained the previously issued Marginal Risk area in the Upper
Midwest. The 12Z HREF and associated neighborhood probabilities
suggested a subtle westward expansion was in order...where some 2
inch accumulations in an hour were now depicted. The Twin Cities
remained very close to the precipitation axis despite run to run
shifts....with the concern for isolated excessive rainfall
concerns being greatest as a result of the urbanization.

Bann

Excessive Rainfall Discussion...

A Marginal Risk upgrade was introduced with this update for
portions of Minnesota and Wisconsin. Surface troughing associated
with a developing low over the Plains will strengthen today as a
potent cold front with much colder air moves in from the Canadian
Prairies. It will run into a potent southerly LLJ with 850 winds as
high as 35 kt advecting some Gulf moisture with PWATs up to 1.25
inches into the region. The combination of the influx of moisture
and strong forcing is expected to cause showers and storms to break
out at the nose of the jet starting this afternoon. The storms will
be fast moving, but the LLJ will supply additional moisture so that
backbuilding and training storms will be possible before the storms
congeal into more of a line, which will limit the potential for
training from then on. The greatest concern for isolated flash
flooding will be in the Twin Cities metro area, where urban
concerns and lower FFGs may be exceeded if the storms train over
the cities. HREF probabilities for exceedance of 3 hour FFGs peak
above 60 percent in the Twin Cities between 21Z/4pm CDT and 00Z/7pm
CDT. Along with reasonably good CAMs guidance agreement on the
development of storms in that general area, this prompted the
Marginal Risk issuance.

Wegman

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...

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...

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