FOUS30 KWBC 250838
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Aug 25 2024

Day 1
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST ...

...Arizona/New Mexico and Colorado...

The axis of highest precipitable water values start the period
extending from eastern Arizona and western New Mexico into
southwest Colorado...with maximum values at or slightly above an
inch near the international border. This should help fuel
additional late day and evening convection...with at least a low-
end chance for rainfall rates that approach an inch per hour.
Flow increases across the region as the western portion of the
upper high erodes and a closed low over Nevada this morning makes 
its way eastward...which should help convection be progressive 
enough to mitigate at least some of the excessive rainfall threat. 
Even so...even brief heavy rainfall can produce flash flooding 
within complex terrain and especially around any burn scars. 

...Midwest...

With the upper high/warm temperatures firmly in place and
apparently sufficiently strong to suppress buoyancy...and high 
resolution models now struggling to generate more than light 
showers...have removed the Marginal Risk area over portions of Iowa
and Illinois.

Bann


Day 2
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST...

...Southwest US...

The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but anomalously strong steering flow 
should help limit residence time to mitigate some of the flash 
flooding concern. As mentioned earlier...though...even brief heavy 
rainfall rates in a short period of time may still lead to flash 
flooding, and a Marginal Risk was maintained in the region.

...Northern Plains...

An upper trough over the Intermountain region will begin lifting 
northeastward across the High Plains on Monday...helping to 
sharpen a front across parts of the Upper Midwest/Northern Plains.
Precipitable water values are forecast to approach 1.75 inches 
over portions of the eastern Dakotas prior to the arrival of the 
surface front and increasing divergence/difluence associated with 
the upper level system. The expectation is that moisture flux 
convergence along the front will focus convection moving into an 
unstable environment driven by diurnal heating that leads to 
locally heavy downpours. Spread lingers in the guidance with 
respect to how progressive the system will be and how much of 
excessive rainfall threat will be associated with the upper system 
compared to the threat associated with the convection pushing into
the better instability. Kept the westward extension introduced on
Saturday...but there was a consensus that some territory could be 
trimmed out near the international border. 

Bann


Day 3
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE UPPER GREAT LAKES...

...Southwest US...

The risk of excessive rainfall will linger on Tuesday afternoon and
evening across portions of the Southwest US as a moisture plume 
lingers across the area and winds aloft/steering flow weakens in 
response to the a mid level wave moves away over the Northern 
Plains as it begins to fill. Model based ingredients suggest the 
potential for locally heavy downpours may extend a bit west of the 
area shown by the deterministic runs...so made a minor expansion 
that way. 

...Upper Great Lakes...
There is a Marginal risk of excessive rainfall over portions of the
western Great Lakes as the upper trough initially over the Upper
Midwest continues to move northeastward during the day.
Precipitable water values of 1.75 to 2 inches should already be in place
before the flow aloft becomes increasingly difluent along and
north of a quasi-stationary boundary helping enhance rainfall rates
locally. The operational guidance tended to the potential for a 
couple of inches of rain...although there was considerable north-
south spread in the guidance limiting the confidence that excessive
rainfall will occur at any spot. 

Bann
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