Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
336 PM EDT Sat May 28 2022
 
Day 3
Valid 12Z Mon May 30 2022 - 12Z Tue May 31 2022 

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

A deep mid-level trough centered over the Intermountain West will 
shed a closed low northeastward on Monday, with this feature 
shearing out into an open wave across the Northern Plains by the 
end of D3. At the same time, a secondary shortwave will intensify 
as it digs across the Pacific Northwest, while downstream across 
the eastern CONUS a large mid-level ridge amplifies. Together, 
these will keep the primary trough axis anchored across the High 
Plains during Memorial Day.

The evolution of this trough will allow an upper jet streak to 
round the base and intensify towards 110 kts Monday, placing 
favorable LFQ diffluence for ascent into ND/MN. At the same time, 
a surface low beneath the upper trough will swing a cold front 
eastward through the day. Ahead of this front, impressive 
thermodynamic advection will occur on 850mb flow reaching as high 
as 50kts from the south, driving PWs to 1.5" and MUCape of nearly 
2000 J/kg. The associated theta-e ridging within this warm sector 
will likely pivot north/northwest into a strengthening deformation 
axis north of the surface low, driving locally enhanced ascent, 
and there is likely to be widespread showers and thunderstorms 
across the Northern Plains and Upper Mississippi Valley on Monday.

The guidance continues to feature some longitudinal spread in the 
placement of heaviest rain due to uncertainty in the speed and 
placement of the low and associated forcing. However, this spread 
has decreased, and both the GEFS and ECENS ensemble means indicate 
a threat for more than 3" of rainfall near the ND/MN border and 
arcing into eastern SD. This area seems favorable as storm motions 
may be a bit slower along the deformation axis, producing the 
greatest threat for training. The inherited SLGT risk still looks 
good for the area of heaviest rainfall, with the threat for flash 
flooding also aided by antecedent rainfall expected on D2. Only 
minor adjustments were needed to account for latest guidance.

A secondary area of enhanced rainfall may develop late Monday 
night from eastern KS into western IA as the LLJ ramps up and 
impinges into the eastward moving cold front, and this is 
reflected by an increase in GEFS/ECENS probabilities. At this time 
there is too much uncertainty in amounts and placement for a 
targeted SLGT risk addition, but it is possible one will be needed 
for this area with later updates.

Weiss
 
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt