Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Thu Mar 30 2023

Day 1
Valid 12Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE UPPER MIDWEST...

A vigorous deep-layer trough traversing the Great Basin and
Southwest today will lift across the central-southern Rockies and
High Plains tonight and early Friday. Increasingly difluent
upper-levels across the Upper Midwest will make for a more
favorable environment for deep-layer ascent, particularly later
tonight as the the models depict an enhanced right-entrance region
of a departing northern stream jet streak (of 130+ kts). A rapid
increase in low- to mid-layer moisture transport from the western
Gulf of Mexico is expected, as 850-700 mb moisture flux anomalies
climb quickly to between 4 and 5 standard deviations above normal
tonight into early Friday (on the heels of robust SSW 850 mb flow
of 50-60 kts). On the nose of that strong flow, enough instability
(500-1000 J/kg of MUCAPE) and convergence will materialize to
support deep convection that may become surface-based along the
warm front across northeastern Nebraska and vicinity this evening
before migrating east-northeastward into a slightly more stable
airmass. The orientation of convection could allow for spotty
areas of convective training, with FFG thresholds generally around
1"/hr supporting isolated potential for excessive runoff. The main
limitation to a higher excessive rainfall threat is the transient
upper flow pattern, with the progressive closed 500 mb low and
associated surface frontal features (along with relatively dry
antecedent conditions, per NASA SPoRT-LIS 0-40 cm soil moisture
anomalies at or below the 10th percentile for much of the region).
Such a pattern, one without the benefit of more prolonged low-mid
level frontogenesis, would limit the risk of cell training (though
that starts to become more of a factor going into Day 2).

Churchill/Hurley/Cook


Day 2
Valid 12Z Fri Mar 31 2023 - 12Z Sat Apr 01 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE ARK-LA-TEX, MID-SOUTH, AND LOWER MISSISSIPPI/OHIO/TENNESSEE
VALLEYS...

The guidance remains in fairly good agreement with the timing of
the compact mid-upper level trough traversing the Central Plains
into the Upper Midwest from Friday afternoon/evening into early
Saturday. The relatively swift progression of this feature will
limit the excessive rainfall potential over the northern OH Valley
and western Great Lakes; however, farther south, under a more
confluent, zonal mid-upper flow, there will be a better chance of
cell training along outflow-generated effective fronts oriented
quasi-parallel to the deep-layer westerly flow. Where instability
is plentiful and the dynamics are most impressive, deep convection
is expected to rapidly develop with quasi-discrete supercells
initially expected to the preferred storm mode. Some of the
strongest activity is anticipated to be across the newly
introduced Slight Risk area, where hourly rates of 1-2"+/hr are
possible (with these hourly rates largely driving the flash flood
threat, as the bulk of the forecast precipitation is expected over
a period of 3-6 hours. Low-level moisture transport is expected to
be quite impressive, as a large low-level jet (50-70 kts at 850
mb) ushers in precipitable water values of 1.2-1.8 inches (above
the 90th percentile for the bulk of the MS/OH/TN Valleys). Farther
to the north in the Mid-West and Great Lakes region, rainfall
rates will be less impressive (perhaps as high as 1.5"/hr at
times), but the prolonged nature of the rainfall may lead to some
higher areal average totals (closer to ~2", though localized
totals will likely be higher to the south). The Slight may need to
be expanded northward in future cycles, which may be partially
dependent on how rainfall totals evolve late on Day 1. Farther
south of the new Slight Risk (into portions of the Deep South),
flash flood guidance is generally higher and the dynamics of the
system are less impressive (so an expansion of the Slight Risk
southward is less likely).

Churchill/Hurley

Day 3
Valid 12Z Sat Apr 01 2023 - 12Z Sun Apr 02 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST...

Heading into Saturday morning, deep convective activity is
expected to be most robust along the southern edge of a departing
robust mid-level short wave (moving from the Great Lakes into New
England, only lifting slightly in the process). The best overlap
of available moisture and instability looks to be from
south-central AL into GA/SC and the northern FL Panhandle. Some
1-2"+ totals may occur over a relatively short period, with the FL
Panhandle and surroundings most likely to experience localized
training along the tail end of a cold front. The rapid movement of
the convection will likely limit the potential for flash flooding
to isolated/localized areas (though relatively wet antecedent
conditions are noted, per NASA SPoRT-LIS 0-100 cm moisture
anomalies as high as the 90th percentile across a portions of the
region, which may necessitate a future targeted Slight Risk
upgrade).

Churchill


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