Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
657 PM EDT Sat May 18 2024

Day 1
Valid 01Z Sun May 19 2024 - 12Z Sun May 19 2024

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

The prospects for flash flooding remain highest over the Southeast
U.S with convergence along the cold front slowly progressing
eastward through the period and a slow-moving outflow boundary
across north FL. The general threat is waning as the best ascent
moves away from the region and we see more isolated to scattered
rainfall coverage with less prominence in the hourly rates. Further
north over AL, a remnant MCV will move east- Southeast,
interacting with the front. Some isolated spots could see 2-3" over
the course of the evening and overnight with a risk of flash
flooding while cells move overhead. Across NC, antecedent
conditions favor a better opportunity for flash flooding concerns
with streamflow anomalies relatively high after the recent active
pattern across the region. A stationary front will cause a
convergent low-level ascent pattern developing from western NC
through east-central portions of the state. Instability and PWAT
anomalies are generally modest with scattered convection the
expectation. Some isolated totals will likely hit the 2-3" marker
with perhaps a max around 4", so the threat is certainly within
reason, but the isolated nature of the potential lends more to a
MRGL than a SLGT.

Roth/Kleebauer


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


Day 3
Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT...

...20Z Update...

The MRGL risk was maintained, but the orientation was changed
from the previous forecast issuance considering the latest trends
within the QPF fields in the bias corrected ensemble and ML
guidance. The premise for the heaviest rainfall over the Sandhills
of NE remains the same with that area mostly void of the risk area
due to very high FFGs. Further to the south, models are beginning
to focus an initiation for heavy rainfall across northeast CO into
southwest NE where the FFG indices are more favorable, albeit not
significantly to favor a flash flood potential with the current
precip forecast. We still reside outside of the CAMs window, but
the pattern is ripe for elevated convective concerns and locally
enhanced rates within the diffluent side of the sharpening
shortwave trough entering the high plains. The MRGL was maneuvered
towards the eastern fringes of the Sandhill's down towards the
KS/CO/NE border where the latest ML guidance continues to hint a
QPF maximum where the frontal boundary will be focused. This
extends back into northeast CO where the convective initiation is
being depicted, as well as north of the Sandhill's in southern SD
where the best QPF footprint outside the higher FFG areas is
located.

Kleebauer


...Previous Discussion...

Upper pattern remains fairly similar to D2 across the Plains, with
broad SW flow over the region. One wave will exit through the
Upper Midwest as troughing over the West deepens via northern and
incoming southern stream vorticity. Upper diffluence east of the
Rockies will support widespread showers/storms over the central
Plains Monday into Monday night along the stalled surface boundary,
with sufficient moisture in place and sfc CAPE 500-1500J/kg.
Heaviest rainfall per the ensembles lie over western Nebraska (Sand
Hills) where FFG values are highest, so focused the area to the
southeast and east into Iowa where rainfall on D2 may lower FFG
values a bit (from already somewhat lower numbers).

Fracasso


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