Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Wed Jul 28 2021
 
Day 1
Valid 12Z Wed Jul 28 2021 - 12Z Thu Jul 29 2021 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE UPPER GREAT LAKES INTO THE UPPER MIDWEST...

...Upper Midwest/Great Lakes...
The next shortwave trough makes its way across the area during the 
period with sufficient instability and PW values reaching 1.75 to 
2.0 inches. Soils have been primed through previous days rainfall. 
Soil moisture is already sitting in upwards of the 98th percentile 
and with previous days QPF, the day 1 periods will see much lower 
FFG than what is currently seen, which is already low. Flash 
flooding concerns are on the higher end confidence wise. The 
Slight Risk area has been adjusted for portions of WI into MI to 
better align with latest QPF forecast and model guidance. The 
Marginal Risk area already in play for this region was expanded 
slightly to account for differences among the models and latest 
QPF forecasts.

...Great Basin/Intermountain West...
Monsoonal moisture continues to surge north across the region with 
weak mid-level impulses helping to focus convection farther north. 
This becomes no more evident than by the precipitable water values 
remaining above 1.25 inches in some locations under broad 
southerly low level flow. With plenty of diurnal heating, 
anticipate MUCAPE to climb above 1000 J/kg allowing for 
differential heating across the terrain. Therefore, afternoon 
convection will largely be tied to the mountains and eventually 
shift off into the valleys/deserts through the late 
afternoon/evening. Hourly rain rates could reach between 0.5-1 
inches, though isolated higher amounts are possible, especially 
with slow moving convection. Given the complex terrain, wet 
antecedent conditions across the Southwest/Central-Southern 
Rockies, and burn scars, localized flash flooding could occur and 
the Marginal Risk area has been maintained.

...Southeast...
A boundary should stall near the FL/GA line extending west to the 
central Gulf Coast. Convergence of tropical air along this 
boundary should allow heavy enough rainfall to locally exceed the 
currently high flash flood guidance and combined with previous 
days QPF, the Marginal Risk area was kept in play.

Chiari


 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt