Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Wed Apr 24 2024

Day 1
Valid 12Z Wed Apr 24 2024 - 12Z Thu Apr 25 2024

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

...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 by the
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 --
through Thursday night. 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.
Mid-level capping should be minimal based on the forecasted 700 hPa
temperatures, under 9C.

Some degree of convection may be ongoing at the start of the period
over OK/AR but additional 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. Mesocyclones are expected to be embedded within the convective
pattern, 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". There has been considerable convergence in the model guidance
on the location of the heavy rainfall, so introduced a Slight Risk
for portions of KS, OK, AR, and MO for this update. Flash flood
concerns for southern portions of the Slight Risk area account for
both the heavy rainfall potential on Thursday/Thursday night and
saturating soils caused by heavy rainfall Wednesday night into
Thursday morning. The Marginal Risk area was trimmed to account for
the model convergence seen since this time yesterday.

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. Precipitable water
values of 1-1.5" should lie across the region. Instability should
be more than sufficient to help organize convection within an
uncapped atmosphere within the warm conveyor belt circulation of
the cyclone. Both Slight Risk areas -- the one near the ArkLaTex
and the new Slight Risk area near the IL/IA border -- account for
the heavy rainfall expected between now and Friday and recent
heavy rains during the past week, which near the IA/IL border have
depressed the 3 hour flash flood guidance down to 1.5-2". Cell
training and embedded mesocyclones are expected to be the main
causes for flash flooding. Hourly rainfall totals to 2" with local
amounts to 4" are anticipated. Trimmed back the inherited Marginal
Risk area across the northern Plains to locations expected to have
precipitable water values of 1"+ .


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