Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Wed Apr 24 2024

Day 1
Valid 01Z Thu Apr 25 2024 - 12Z Thu Apr 25 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR EASTERN
OKLAHOMA AND WESTERN ARKANSAS...

01Z UTC update...
Realigned the orientation of the Marginal Risk area to better
match the low-level thetae gradient...with higher values advecting
northward across the western High Plains ahead of an approaching
shortwave trough and cooler/drier air feeding southward from strong
surface high pressure over the Great Lakes region into the
Mississippi Valley region. While there should be increasing
isentropic upglide overnight that leads to the development of
showers and thunderstorms...and a sufficient combination of CAPE on
the warm side of the low level front and increasing moisture over
the outlook area...localized rainfall amounts of 1 to 3 inch
amounts are possible but the latest HREF continued to be
unenthusiastic about the chances for 3 inch totals. Confidence
about the potential for excessive rainfall is lowest in the Kansas
portion of the outlook area given the dewpoint depressions...suggesting
that the threat may not develop until after the end of the Day 1
period at 12Z.

Bann


Previous discussion issued at 0700 UTC...
...OK/AR...
OK/AR will remain an area of diffluent 1000-500 hPa thickness just
east of a migrating mid- level ridge axis translating east from
the Four Corners region. At the surface, a front will lift north
Wednesday night into early Thursday in advance of the approaching
upper level trough from the Southwest. Mid-level capping should be
minimal based on the forecasted 700 hPa temperatures, under 9C.
Precipitable water values 1-2 sigmas above average for late April
-- 1.5-1.75" -- are forecast by the model consensus to be in place
near and just north of the surface front with elevated CAPE of
1000 to 2000 J/kg (higher to the south) may allow for stronger
cells capable of high rainfall rates. Effective bulk shear/low-
level inflow should increase to 35-40 kts. Non- traditional cell
training within a WNW steering flow near and poleward of the front
could allow for hourly totals to 2" and local 4" amounts. While
the 00Z HREF isn't sold on 3"+ amounts in the area, precipitable
water values in the area have trended upwards each day, and now
the 00z Canadian Regional, though at a smaller scale, supports the
wetter ECMWF solution. Went ahead and introduced a Marginal Risk
in this update.

Roth

Day 2
Valid 12Z Thu Apr 25 2024 - 12Z Fri Apr 26 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN AND CENTRAL PLAINS...

A mid-level trough over the southwestern U.S. is forecast to
eject out across the central and southern High Plains by Friday
morning, spurring development of a surface cyclone across the High
Plains. Winds at 850 mb will be south to south-southwest across the
Great Plains but with increased magnitude -- up to 50-60 kt --
and moisture transport will allow precipitable water values to
exceed 1.5" across portions of the Plains and Midwest. A broad area
of instability east of a forming central High Plains surface low
and attendant cold front/dryline, with values of 1000 to 3000 J/kg
from northern TX/southern OK into KS and NE.

Initially, some convection may be ongoing at the start of the
period over OK/AR along a warm front lifting northward, but
additional storm development is anticipated to occur during
Thursday afternoon ahead of the cold front/dryline in KS/OK with
thunderstorm development expanding through the evening and early
overnight across TX and northern locations including NE and IA.
Strong to severe thunderstorms are likely, ramping up the
precipitation potential, despite decently fast cell motions. Deep
layer south- southwest flow supports training of cells with rains
up to 2.5" in an hour and local amounts up to 6". Models continue
to show good agreement for the slight risk across portions of
KS/OK/AR/MO which portions of could overlap with some heavy
rainfall potential tonight into Thursday morning. The surrounding
marginal risk was extended slightly from the overnight issuance
southward along the cold front into central TX, eastward along the
warm front into the Tennessee Valley, and westward more into
Nebraska/South Dakota to the north and west of the surface low.

Santorelli/Roth

Day 3
Valid 12Z Fri Apr 26 2024 - 12Z Sat Apr 27 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS NEAR AND NORTH OF THE
ARKLATEX AND NEAR THE ILLINOIS/IOWA BORDER...

As a surface cyclone pulls northeast across the Plains and Midwest,
moisture and instability will lift northward through the Midwest.
Inflow at 850 hPa will be near or above 50 kts with precipitable
water values as much as 2 standard deviations above normal and
more than sufficient instability to help organize convection.
Within a broad marginal risk from the northern Plains to Upper
Midwest and southward along the cold front into the lower-mid MS
Valley and eastern Plains, continued two slight risk areas - one
near the IA/IL border and another across northeast TX, southeast
OK, and northwest MO. Both of these slight risks remain due to
heavy rainfall expected Wednesday into Thursday, and some recent
rainfall during the past week. Cell training is possible, with some
strong to severe storms as per the latest SPC Outlooks. Hourly
rainfall totals to 2" with local amounts to 4" are possible.

Santorelli/Roth


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