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FOUS30 KWBC 031549
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Wed Jul 3 2024

Day 1
Valid 16Z Wed Jul 03 2024 - 12Z Thu Jul 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS EASTWARD TO THE OHIO VALLEY|...

...16Z Update...

Previous SLGT risk forecast remains on target with an elevated
flash flood risk positioned across southern KS through southern MO
with an emphasis over south-central and southeastern MO. Remnant
outflow from nocturnal convection is currently analyzed across
southern MO and will be the focal point for afternoon and evening
convection with heavy rainfall likely given the antecedent
environment. PWATs over 2" are aligned along and ahead of the
quasi-stationary front across KS into MO, as well as within the
confines of the aforementioned lingering outflow. 12z HREF
neighborhood probabilities are favorable for at least 3" of
rainfall across much of south-central MO to the KS/MO state line
with values settling between 45-70% within that corridor. This
correlates well with the elevated probabilities for at least 2"/hr
rain rates suggested within the latest hi-res ensemble suite
(25-40%). The above area will run on the higher end of the SLGT
risk threshold with a non-zero chance at a targeted upgrade later
pending how convection evolves. For now, the signals are strong
enough for the current forecast, but a bit under what we would want
to see for a higher forecast risk area.

The SLGT risk was extended through portions of the Ohio Valley as
the quasi-stationary front will be a target for convective
development later this evening within a secondary tongue of
elevated theta-E`s being advected northeast into southern OH. CAMs
are leaning towards the possibility of scattered heavy rain
signatures within areal convective development. The signal for at
least 2" of precip within that area of OH has risen since the last
update, indicative of the evolving pattern with favorable
thermodynamics overlaid across that portion of the Ohio Valley.
Rates between 1-2"/hr are plausible within this corridor, enough to
be a signal for 1hr FFG exceedance within the latest hi-res
ensemble. This was enough to necessitate the extension through
portions of southern OH and adjacent KY.

There are no changes to the MRGL risk area across the Southwest as
a monsoonal setup is still forecasted to evolve later this
afternoon with isolated flash flooding possible, especially in
complex terrain and burn scars present.

Kleebauer

...Previous Discussion...

...Mid Mississippi Valley and Great Plains...

A cold front from a passing trough over the northern tier of states
will stall in the vicinity of Kansas and Missouri later today,
providing focus for renewed thunderstorm development from eastern
Kansas and southern and central Missouri eastward into the Lower
Ohio Valley. Except for a few adjustments based on the latest suite
of global and ensemble guidance from the 03/00Z model runs...there
Slight Risk area was largely unchanged from the previous issuance.
The guidance still depicts strong instability (CAPE in excess of
2000 j/kg) and PWs in excess of 2 inches (above the 95th
percentile) combining to support organized convection with rain
rates exceeding 2 inches per hour at times combining with the
potential for cell training or backbuilding.

Elsewhere the Marginal Risk areas were generally maintained in the
Upper Ohio Valley and Northern Plains where low chances of heavy
rain rates and flash flooding persist.

...Southwest...

Continued to trim the Marginal risk area as mid- and upper-level
heights continue to build into the region....helping to suppress
late day and evening convection even further.

Bann


Day 2
Valid 12Z Thu Jul 04 2024 - 12Z Fri Jul 05 2024

...Upper Midwest to Mid Mississippi Valley...

The environment should become increasing supportive of showers and
thunderstorms as mid- and upper-level height falls spread across
the Upper Midwest into a region with deeper moisture, The rain
threat will be organized by a developing surface low pressure and
an inverted trough axis extending to the north. There has continued
to be multi- run shifts over the past 24-48 hours to the south
with the cyclone, and QPF guidance has shifted south as well. Given
a lack of convergence on the specific location of heavy rainfall
thus far, the Slight Risk has been maintained with yet another
shift to the south but not as much as before. Another key
uncertainty is the level of instability. Models are only showing
modest instability, generally with CAPE near or below 1000 j/kg,
and PWs are not that anomalous, with the primary plume of high PW
air pushed south by a cold front in previous days. Therefore, the
threat of flash flooding in this case would likely come from two
key factors: (1) sustaining moderate to occasionally high rain
rates for longer durations (3-6 hours) and (2) the unusually wet
ground conditions across the region increasing vulnerability.
Therefore, pinpointing confidence in placement of the low-level
cyclone and associated deformation axis will be important for any
future shifts in the Slight Risk.

...Southern Plains and Mid-Mississippi Valley into the Ohio Valley
and Mid Atlantic States...

The plume of deeper and more anomalous moisture will be in place
farther to the south, in the vicinity of the stalled front. PWs
are likely to generally exceed the 90th percentile along the entire
front from OK and KS eastward to the Mid-Atlantic coast. The
Marginal Risk was generally maintained in this area, and expanded
slightly in the Plains and Mid-Atlantic. No major changes were made
at this time, as corridors of greater chances of flash flooding
would be very dependent on convective evolution from the previous
day (current Day 1 period). However, where sufficient instability
exists, moisture levels will be high enough to support rain rates
approaching or exceeding 2 inches per hour in the most organized
thunderstorms and this may lead to flash flooding.

Bann


Day 3
Valid 12Z Fri Jul 05 2024 - 12Z Sat Jul 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN AND EASTERN UNITED STATES...

Corridors of higher chances of heavy rainfall will continue to be
aligned with a quasi-stationary front extending from the southern
Plains into the Southeast US on Friday where higher values of
precipitable water should be pooling in proximity to sufficient
instability. With weaker mesoscale forcing in the Southeast US...exactly
where convection develops is less certain so followed the better
clustering shown by spaghetti plots of QPF from the GEFS/SREF as
well as NCEP global runs and the ECMWF. Over the portions of the
Southwest...a renewed risk of locally heavy rainfall develops late
in the period as low level winds draw Gulf moisture upslope and is
aided by some isentropic lift as the low rides up and over the
tail end of the stalled front.

Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt