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

Day 1
Valid 16Z Sat May 18 2024 - 12Z Sun May 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/CENTRAL GULF COAST...

...16Z Update...

Maintained continuity through a majority of the previously covered
areas within the SLGT and MRGL risk areas. The prospects for flash
flooding remain highest over the Southeast U.S with convergence
along the cold front slowly progressing eastward through the
period. The primary period will be through the 00z hour before the
threat wanes 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 into northern MS/AL by this afternoon
providing a focal point for convective initiation during peak
instability. The PWAT anomalies are fairly tame for a higher end
threat, but some isolated spots could see 2-3" over the course of
the afternoon with a risk of flash flooding while cells move
overhead. Given the probabilities for 1"/hr running between 40-60%
and 2"/hr around 10-15% at peak, the threat is certainly capped for
any prospects for widespread flooding, so the MRGl risk in place
will suffice for the setup. 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 backdoor front will progress to the south during the
afternoon and evening with 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 likely to form within the next few hours
across the terrain west of the Triad. Some isolated totals will
likely hit the 2-3" marker with perhaps a max around 4" based on
the latest HREFpmm and higher-end deterministic, so the threat is
certainly within reason, but the isolated nature of the potential
leans this more on the cusp of a true SLGT risk and a MRGL.

Considering the downward trend of some of the probability fields
from the previous iteration of the ensemble, as well as
collaboration with the affected WFO's, decided to maintain
continuity to allow for consistent messaging and the threat being
truly isolated. There is a potential for a short term upgrade, but
not at this time.

Kleebauer


...Previous Discussion...

...Southern Alabama/Georgia into the Florida Panhandle...

Mid-level trough/weak closed low over the Lower MS Valley/Mid-
South this morning will continue eastward across southern TN today
and into the southern Appalachians by early Sunday. On its
southeast side, a stream of mid- level impulses will move across
the FL Panhandle and southern AL/GA today ahead of a cold front
that will finally clear through the area by tomorrow. SW flow out
of the Gulf will maintain relatively high precipitable water values
(~1.75" or +1.5-2 sigma) into the region ahead of the trough and
surface front, with more impressive 850-700mb moisture flux
anomalies into the region (+3 to +4 sigma). With ongoing convection
over southern LA moving eastward, expect this to continue past 12Z
along the I-10 corridor from southern MS eastward through early
afternoon. Thereafter, models suggest a loss of mid-level vorticity
dynamics to sustain heavier rainfall but this may be underdone
given the approach of the cold front with still sufficient moisture
available. Slight Risk outline was generally constrained to the
higher probabilities of >2" QPF in the 12-18Z period this morning.
Broader Marginal Risk covers much of the rest of the Southeast into
the Carolinas where isolated heavier rain rates are possible.
Trimmed away the northern/northeastern side of the Marginal Risk
where instability is low (or non-existent) on easterly flow.

Fracasso


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