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

Day 1
Valid 16Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023

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

...16Z Update...

The main change to the previous ERO issuance was to trim and
narrow the Marginal Risk particularly on the northeastern side
given the likelihood for wintry precipitation especially for
northern Wisconsin into Michigan. Additionally, the west-east
oriented warm front where sufficiently high dewpoints will be and
rain will fall to its south looks to also stay south of areas with
notable snow cover, so snowmelt is not too large of a threat. More
issues could arise if the warm front ends up being farther north
but that appears unlikely at this point. See the previous
discussion for more details on the overall pattern and
meteorological ingredients.

Farther south, there may be a nonzero threat for isolated flash
flooding mainly this evening in the general vicinity of the
Oklahoma/Arkansas border and eastward where precipitable water
anomalies rise above the 90th percentile. Storms that form
generally appear to move quickly eastward in the hi-res guidance
though, which would limit any flash flooding threat. Interestingly
the probabilities for exceeding an inch of QPF for example from
the EC ensemble is much higher than the HREF and GEFS
probabilities--perhaps indicative of QPF likely right around an
inch. And QPF in those amounts should not cause too many issues,
hence no Marginal Risk looks needed at this point.

Tate


...Previous Discussion...

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 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

The Day 3 outlook will be updated by 2030Z.


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