Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 PM EDT Fri Apr 18 2025

Day 1
Valid 16Z Fri Apr 18 2025 - 12Z Sat Apr 19 2025

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

16Z Excessive Rainfall Discussion Update...
Few changes needed with respect to the Slight Risk area across
portions of the Southern Plains. The area covers the corridor
highlighted by the 12Z run of the HREF and the associated HREF
neighborhood probabilities. This also aligns where highest
equivalent potential temperature was located at the surface and
low levels was already in place this morning. Given the persistent
low level jet pulling moisture northward from the Gulf pretty much
along the same trajectory...felt that no major changes were needed.
Did expand the Marginal risk area a bit southward where the HREF
started to show potential for one or more periods of rainfall rates
above 1 inch in an hour. Any flooding concerns should be isolated
there but enough to pull the Marginal southward.

Bann

Previous Excessive Rainfall Discussion...

A persistent southerly low-level jet transporting deep tropical
moisture into the Texas Gulf Coast and along a very slow moving
frontal zone draped across the Plains to the upper Great Lakes will
lead to the development of convection during peak heating. However,
the strongest of the storms will likely be during the late evening
and overnight hours. 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 setup will be favorable for
training of thunderstorms and some of the individual cells may
advance quickly to the northeast as the low-level jet intensifies
to 50 kt over northwestern Arkansas Friday night.

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 2 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. A Marginal Risk area spans from the
vicinity of the Oklahoma/Texas border northeastward to
Illinois/Indiana. The Slight Risk was adjusted a small amount to
the west than the previous forecast.

Campbell

Day 2
Valid 12Z Sat Apr 19 2025 - 12Z Sun Apr 20 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM NORTH-
CENTRAL TEXAS THROUGH MUCH OF SOUTHERN MISSOURI...

2030Z Excessive Rainfall Discussion...
The 12Z suite of numerical guidance highlights the continuation of
rainfall in a corridor from central Texas northeastward into 
portions of the Mid- South and neighboring portions of the Mid- 
Mississippi Valley on Saturday. A consistent run to run westward 
nudge in the placement of the heaviest rainfall was noted yet again
in both the model runs and the deterministic WPC QPF...and the 
current outlook was expanded just a bit to the west and south 
compared to the previous outlook. The overall synoptic picture 
largely remained consistent from earlier outlooks.

Previous Excessive Rainfall Discussion...
A stalled boundary will lead to a stark gradient between two 
airmasses, on with some adjustments on the day shift cycle to pull 
it a little more south into the Lower Mississippi Valley along the 
front where the better instability should be located. e abnormally 
cool for mid-April and the rich in moisture. On the north side the 
cold, dry northeasterly flow will be weakening while in the warm 
sector there will be 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 westward trend of QPF persisted into this time period, placing
the highest amounts over far north-central Texas, southwest and central
Oklahoma. A narrow swath of amounts 3 inches or greater are
expected in this part of the region. 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. 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.

The Moderate Risk area was expanded west/southwest farther into 
north-central Texas. The Slight Risk was shifted into central Texas
and the Marginal Risk had minor expansions to the west. With the 
overall western shift of all of the Risk thresholds, the eastern 
bounds of each were trimmed back to the west as well.

Campbell/Wegman


Day 3
Valid 12Z Sun Apr 20 2025 - 12Z Mon Apr 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL
PLAINS AND PORTIONS OF THE MIDDLE AND LOWER MISSISSIPPI VALLEY...

2030Z Excessive Rainfall Discussion...
The system responsible for the potential of heavy/excessive
rainfall on Saturday will begin to accelerate to the northeast as
the upper trough begins to fill. Rainfall amounts should not rival
amounts that are expected on Saturday...but additional rainfall on
top recently flooded areas remains a potential concern.

Bann


Previous Excessive Rainfall Discussion...
The upper trough and surface low pressure system will continue to
spread showers and thunderstorms across the central U.S. Sunday as
it lifts northeastward. The amount of moisture and instability may
be reduced when compared to Saturday, however it will likely be
above normal. The nature of the low track and associated storms are
expected to be a little more progressive which should limit
rainfall accumulations. Additionally, the bulk of the higher
rainfall amounts is forecast to largely fall north of the Saturday
footprint. A Marginal Risk is in place for the Middle and Lower
Mississippi Valley and eastern portions of the Central Plains.
These areas are north and west of the most sensitive areas of the
Mississippi/Ohio River Valleys where streamflows remain high after
recent flooding. However, some rain atop recently flooded areas,
and runoff from other areas into these rivers, could exacerbate
ongoing flooding issues in some areas, so continue to monitor.

Campbell

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