Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM EDT Tue May 30 2023
 
Day 1
Valid 16Z Tue May 30 2023 - 12Z Wed May 31 2023 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF 
THE WEST, MIDWEST, AND PLAINS...

...Northern CA northeast across ID and southwestern 
MT/northwestern WY...
A Mid-upper low off the Central CA Coast this morning will slowly 
push inland and wobble through SoCal this evening. Favorable 
upper-level diffluence north of the compact mid-upper low, south 
of the digging northern stream upper trough/height falls across 
the Pacific NW, will generate favorable broad-scale forcing across 
the outlook area this afternoon. PWATs are expected to climb to 
between 0.75-1.00", which is ~1.5 standard deviations above normal 
for the end of May (and above the 90th percentile), which is 
sufficient for heavy rain concerns at elevation. Combination of 
upper forcing, moisture, weak flow aloft, along with increased 
daytime instability (mixed layer CAPEs peaking between 500-1000 
J/Kg) will allow for scattered to more numerous showers/storms 
within the outlook area -- a few of which will be capable of 
hourly or sub-hourly rainfall rates of 0.50-1.00" which is around 
FFG values. 12Z HREF neighborhood probs for exceeding 1"/hr rates 
and/or 1/3-hr FFG values max out around 40%, supporting at least a 
Marginal Risk outline. Lack of focus for convection (aside from 
terrain) and dependencies on subsequent convection-induced 
mesoscale boundaries precludes a focused Slight Risk contour. 
Generally, a localized flash flood risk, especially over 
urban/poor drainage areas and any recent burn scars into northeast 
CA and southern OR, have a locally higher risk. 


...Northern and Central Plains into Midwest...
A pair of mid/upper level shortwave troughs will pivot across 
eastern portions of the Northern Plains and across the far 
northern MS Valley today. Weak/modest upper divergence combined 
with surface lift near and in advance of a frontal boundary will 
support a heavy rainfall threat with afternoon and early evening 
convection. This largely covers a similar area to the SPC area as 
well. PWATs look to peak around 1.3" (near the 75th percentile, 
per TOP sounding climatology), with SB CAPE rising to as high as 
3000 J/Kg into the outlook area. Low-level inflow and effective 
bulk shear appear sufficient to organize convection that forms, 
especially initially along the CO/KS border and moving east. 
Southern extent has potential for a more appreciable excessive 
rainfall threat (OK/TX Panhandles, where FFG values are a bit 
lower) depending on the forward speed and development of the 
southern end of convection. 12Z HREF neighborhood exceedance 
probabilities continue to suggest the potential for locally 
extreme amounts (3-5"+), but it is too difficult to pinpoint a 
specific area within this broad region (as indicated by a lack of 
consistent HREF EAS neighborhood probabilities for 1" exceedance). 
Did adjust the area slightly over southwestern MO and the TX 
Panhandle to account for some 12Z CAM guidance that showed 
potential for convection farther south later this 
evening/overnight. 


...Southeast OK into Northeast TX and Ark-La-Tex...
Precipitable water values of 1.5"+ and SB CAPE expected to 
increase to 1000-3000 J/kg support the Marginal Risk area near the 
Ark-La-Tex this afternoon. Low-level inflow appears weak, only on 
the order of 10-15 kts, but the mean 850-300 hPa winds are 
virtually null (which suggests minimal if not chaotic storm motion 
and better rainfall efficiency than average from storms that do 
form and maintain). Effective bulk shear may be as high as 25 kts 
on the western edge of the MRGL risk area, which may help to 
organize storms (and would potentially organize a complex, should 
upscale growth be sufficient). Even so, FFG values are quite high 
in this area so any excessive rainfall concerns should remain 
isolated. 


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