Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Fri Sep 20 2019
 
Day 1
Valid 16Z Fri Sep 20 2019 - 12Z Sat Sep 21 2019 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE SOUTHERN AND NORTHERN PLAINS...

...Oklahoma and Texas...
Thunderstorms making use of tropical moisture at the tail end of a 
mid level trough associated with remnants of Imelda... have 
produced impressive rain rates this afternoon along the Red River. 
A threat of flash flooding will continue here for a few hours 
during the evening, described by mesoscale discussion 0875. The 
synoptic pattern otherwise features deep layer southwesterly flow 
and a healthy and broad low level jet overnight throughout the 
southern Plains. This could support additional clusters of 
thunderstorms over west Texas and western to central Oklahoma, as 
models forecast deep layer ascent and convergence to back up to 
the north and west with time. A scouring of instability within the 
inflow region, however, will likely limit the flash flood risk 
outside of the areas of ongoing concern along the Red River at 01Z.

...Northern Plains...
A strongly synoptically forced event marked also by unusually rich 
moisture was unfolding generally as anticipated, with several 
training line segments and embedded mesocyclones as of mid 
evening. We adjusted the back edge of the Marginal and Slight Risk 
areas using primarily the NAM CONUS Nest and recent HRRR runs. 
Generally anticipate an increase to convective coverage over South 
Dakota overnight as the second shortwave in a pair of coupled 
shortwaves ejects from the Rockies. Outflow and propagation should 
keep rainfall from getting too far out of hand, but intense short 
term rates of 2 inches or more per hour will pose a substantial 
flash flood threat over the Slight Risk area in both the Dakotas 
and western Minnesota.

Previous reasoning follows...
Strong synoptic forcing will overspread the northern Plains today 
into tonight. Plentiful instability and moisture will advect north 
into the region as well...with PWs near climatological record 
values. Thus all the ingredients are in place for widespread 
organized convective development. Cell motions will be quick off 
to the north...however the impressive magnitude of the southerly 
low level moisture transport will favor a southward 
backbuilding/propagation of activity. The combination of 
progressive mid/upper forcing and an eventual cold pool 
propagation...should eventually allow for a rather quick eastward 
progression of activity. This should end up capping the upper 
magnitude of rainfall totals. However, before this happens there 
should be an opportunity for some training of cells...as they 
backbuild into the low level flow counter to the deep layer mean 
flow. Given the PW airmass in place...cells will be efficient...so 
this short term training should be enough to result in at least 
scattered flash flood concerns. We should end up with an axis of 
2-5" with this setup.

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