FOUS30 KWBC 030827
QPFERD

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

Day 1
Valid 12Z 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|...

...Midwest to 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

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