Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
657 PM EDT Sat May 18 2024
Day 2
Valid 12Z Sun May 19 2024 - 12Z Mon May 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS, UPPER MIDWEST, AND COASTAL SOUTH
CAROLINA/GEORGIA...

...20Z Update...

The previous forecast necessitated very little change across the
central plains and corn belt with the synoptic pattern very much on
track from prior NWP iterations. The only change was the expansion
of the MRGL risk further north into MN and now the western portion
WI, mainly the region north of LaCrosse. Recent trends in the hi-
res convective evolution downstream are the reason for the addition
with much of the CAMs showing fairly robust coverage within an
upscale growth regime that originates over SD into western MN. A
tongue of elevated theta-E's is forecast to be advected into
southern MN with the multi-cell cluster out of the Dakotas to pivot
eastward along the leading edge of the instability gradient. 12z
HREF signal for rates >1"/hr have risen substantially from previous
output with a swath of 40-60% probabilities and some bullseye's
eclipsing the 70% marker, a signal indicative of greater potential
overall. The rates are likely to be capped, however as the best
instability will lie south of the MN border by the end of the
period when the convection makes its way to the MN/WI border.
Regardless, shortwave propagation within the broad southwest flow
regime will be enough to maintain the organized nature of the
convection and provide some locally heavy precip across the
northern Midwest. The ECMWF is still holding on to the primary
threat residing within IA, but there seems to be a split with one
batch in KS and another in MN. Considering the bit of uncertainty
and with the environment favorable for heavy rainfall from KS and
points northeast, kept the MRGL risk with an opportunity for a SLGT
risk upgrade if CAMs latch on to a particular area with greater
impact (ie: Kansas or MN/WI).

An additional MRGL risk was added portions of coastal SC/GA. More
on that in the sub-section below...

...Coastal South Carolina and Georgia...

Backdoor front will be pressing south through the Carolinas the
beginning of the period with a surface wave riding up along a cold
front present across southern GA. A combination of focused low-
level forcing from the north and onshore development near the SC/GA
border will create a time frame for prime convective development in
the overlap of the onshore regime and frontal convergence over
land. The instability fields depict this setup very well with a
sharp inflection in the MUCAPE fields with little to no CAPE north
and west of the coastal plain, but a skinny area of 1500-2500 J/kg
located from Charleston down to just south of Savannah. This is
where the latest CAMs are pinpointing the best threat of convection
and rates bordering 1-2"/hr with very high neighborhood
probabilities showing up within the latest HREF for at least 1"/hr
(60-80%) and reasonable up to 2"/hr (20-30%). The biggest factor
will be the speed of the front to the north and where does the
intersection point occur. Some are further into GA and others
closer to Charleston, which is a major factor in the flash flood
potential as the urban corridors will undoubtedly be the most
susceptible. After some collaboration with the local WFO, the
threat was great enough to warrant a MRGL, but likely on the lower-
end of the threshold necessary with a focus on the population
centers.

Kleebauer


...Previous Discussion...

...Central Plains and Upper Midwest...

Broad SW flow over the central Plains across the Corn Belt will
bring in increasing moisture to the region on Sunday with a surface
frontal boundary hung up over Iowa. Mid-level vorticity will exit
the central Rockies and moves across the central Plains as
precipitable water values rise to around 1.50" (+1.5-2 sigma) with
some instability in the afternoon/early evening (500-2000J/kg) that
could yield some 1"/hr rates (per 00Z HREF thru 00Z Mon) which are
near FFG values over Iowa and nearby areas. The region has been
fairly dry the past week (most of the region has had about 50% of
the normal rainfall) so much of the rain will be beneficial, but
given 1-hr FFG values near 1"/hr (between MCI- OMA- DSM),
maintained the Marginal Risk outline for the region. Higher
probabilities exist farther southwest into central Kansas (to where
the Marginal was extended), but FFG values are higher as well
(2"/hr). 12Z CAM guidance through 12Z Mon may shed a little more
light on the higher rainfall potential over the entire 24 hours.


Fracasso