Day 2 Outlook >
WPC Day 1 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0101 UTC Sat Sep 21, 2019
Valid: 01 UTC Sep 21, 2019 - 12 UTC Sep 21, 2019
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Fri Sep 20 2019
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.
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.
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt