Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Thu Mar 30 2023

Day 1
Valid 01Z Fri Mar 31 2023 - 12Z Fri Mar 31 2023

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

Continued to trim some territory from the Marginal Risk area in
the Upper Midwest given the proximity of wintery precipitation
especially in northern Wisconsin. 00Z surface observations still
show dewpoints high enough to support rain remained well south of
an east-west warm front...but close enough to support several run
of CAM guidance for some enhanced rainfall amounts immediately
north of the warm front in a region of frontogenesis and warm
advection in the low levels.  The latest HRRR runs show at least
some hint at some precipitation amounts over 0.5 inches late
evening or overnight where precipitation type is expected to be in
the form of rain.  More issues could arise if the warm front ends
up being farther north and brings an added component of snow melt.

At 00Z...a complex area of low pressure was beginning to organize
over the Western High Plains as a vigorous deep-layer trough makes
its way out of the Great Basin and crosses the Rockies, 35 to 50
kt low level winds over Nebraska and Iowa will continue to advance
northward and ride over the surface warm front...helping transport
enough instability and boost precipitable water values where
rainfall rates could support isolated excessive runoff.

Bann



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 MID-SOUTH AND LOWER MISSISSIPPI/OHIO/TENNESSEE VALLEYS...

...2030Z Update...

The overall meteorological pattern described below still looks on
track for this update cycle, with the potential for quick-hitting
storms but with high rain rates, as initially possibly more
discrete convection is likely to organize by evening/night into a
cold frontal squall line. This squall line should be mainly
progressive farther north but with the most potential for the
front to "lay down" a bit, becoming more southwest to northeast or
even west-east oriented rather than south to north, across the
Mid-South (southern Tennessee into northern Mississippi and
Alabama). This could lead to more training of convection, and
indeed the highest probabilities for elements like QPF to exceed
thresholds of an inch, QPF to exceed FFG, etc. in the first run of
the HREF to go through this full period are also maximized there.
Thus the Slight Risk was maintained across these regions, though
it was trimmed back on the northwestern side per the newer model
guidance and WPC QPF. Additionally, the northwestern side of the
Marginal Risk was shaved off due to QPF being predominately
snow/ice. See the previous discussion for more details on the
pattern.

Tate


...Previous Discussion...

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...

...2030Z Update...

No notable changes were needed to the day 3 ERO. The tail end of
the cold front and associated convection along/ahead of it could
cause isolated flash flooding issues for parts of the Southeast,
parts of which have above average soil moisture in place. See the
below discussion for more details.

Tate


...Previous Discussion...

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