Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Sun Jul 03 2022
 
Day 1
Valid 01Z Mon Jul 04 2022 - 12Z Mon Jul 04 2022 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
THE NORTHERN HIGH PLAINS AND UPPER MIDWEST...

...Northwest to the Northern Plains and Upper Midwest...
0100 UTC Update -- Based on the latest observational and guidance 
trends, extended the Marginal Risk area a bit farther east across 
northern WI and portions of western Upper MI, while trimming the 
area across southeast MN. Many of the CAMs, and as a result the 
18Z HREF mean, show the leading edge of the MCS's heavy rainfall 
footprint getting into portions of far northern WI and western 
Upper MI toward 12Z. Meanwhile, in terms of the Slight Risk, have 
combined what was 2 separate areas into one, essentially lining up 
with the highest exceedance probabilities from the 18Z HREF (probs 
of 3 hourly QPF exceeding FFG peaking between 50-60%).

Previous discussion below...
Right entrance region of a strengthening ~100kt jet across Lake 
Superior and southern Ontario will promote divergence over the 
northern Plains and Upper Midwest today and tonight as a low-level 
warm front lifts through the region. Moisture flux is poised to be 
maximized in the 850 to 700 mb layer during the evening at over 3 
standardized anomalies thanks to southerly flow of 20 to 30 kts in 
the same layer. Some rainfall this morning will move out before a 
secondary round of rain/convection which rides atop the upper 
ridge moves through later this evening and overnight.  
Precipitable water values between 1.75 inches and 2 inches look to 
be in place across the region by very early Monday morning with 
dew points into the 60s. Therefore, there is at least some 
potential for excessive rainfall in west-east bands as cells push 
through. Maintained the Slight Risk contour though new 12Z 
guidance still shows some north-south disagreement. However, much 
of the area has experienced long-term below-normal precipitation 
which should ease some of the threat of excessive rainfall. Use 
the intersection of the most favorable moisture and moisture 
transport anomalies with the higher QPF/rates probs from 
southeastern ND and northeastern SD eastward across central 
Minnesota. Larger Marginal Risk area continues farther west with 
earlier potential for heavier rainfall and rates across North 
Dakota into Montana. See section below.

For the Northwest U.S. to the Northern Rockies...
Maintained the Marginal Risk area extending from the Washington 
Cascades eastward into the Northern Rockies which connected across 
North Dakota (see above) but added a small Slight Risk are over 
eastern Montana to far western North Dakota. An upper low just off 
the Pacific Northwest coast this morning will slowly wobble 
eastward today and lift northeastward across the Olympic Peninsula 
by Monday morning, which leads to upper level flow becoming highly 
difluent from the Cascades eastward. Given the moisture in place 
(PW values 1-1.25")... precipitation should become increasingly 
widespread with enough mesoscale forcing to enhance rates/amounts. 
In addition...steep lapse rates near the core of the upper low 
could also act to support scattered convection with brief heavy 
rainfall and the attendant risk of flooding. Greater instability 
lies over central to eastern Montana where SPC has an Enhanced 
Risk of severe weather. Just north of that area, 12Z CAM/HREF 
guidance indicates some chance of higher rainfall rates >1"/hr as 
a quickly-moving area of rain/convection moves eastward tonight 
after 00Z. With relatively lower FFG values, localized flooding 
concerns may arise.


...Mid-South and Southeast...
2130 UTC Update -- Removed the Slight Risk area that was in place 
east of the ArkLaTex as per the weakening MCS and for a while the 
airmass stabilization over the last several hours (i.e. with 
convection far less organized). Latest observational trends as 
well as recent CAM guidance suggest more scattered-numerous 
activity percolating farther south of the mesohigh (into southern 
MS-AL), and across much of GA south of the effective front. With 
precipitable water values of 2-2.25", dewpoints in the mid-70s, 
and mixed-layer CAPEs of 1500-2500 J/kg within the untapped 
airmass, spotty rainfall of 2-2.5+ inches within an hour will 
remain a concern throughout the Marginal Risk area for the 
remainder of the afternoon into the early evening. Given the weak 
shear profiles (<20kts between 0-6km), expect pulse convective 
mode and thus lack of sustained/organized activity in limiting the 
flash flooding threat (i.e. isolated).  


...Southeast U.S...
0100 UTC Update -- Pared the Marginal Risk area based on the 
decaying diurnal trends (decreasing mixed layer CAPEs). Scattered 
slow moving convective clusters lingering into the mid-late 
evening hours will maintain a localized flash flood risk.


...Southwest U.S...
Little overall change in the synoptic pattern is expected from the 
previous days...with a plume of deep, monsoonal moisture remaining 
in place from portions of the Southwest U.S. and into the Colorado 
Rockies. This should support another round of mainly late day and 
evening convection that produces locally heavy rainfall and 
isolated risk of excessive rainfall...with the burn scars and the 
complex terrain being most prone for flash flooding.

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