Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 10 2019

Day 1
Valid 12Z Tue Dec 10 2019 - 12Z Wed Dec 11 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE DEEP SOUTH...

...Louisiana / Mississippi...
WPC has introduced a small Marginal Risk area running roughly from
Lafayette, LA, to Meridian, MS, and not quite including the city
of New Orleans. Although the recent weather has been dry, and the
streams can take a good amount of water, it is an intriguing setup
in which banded convective storms could train for several hours to
produce isolated rapid runoff from 2-3 inches of rain. Ahead of a
large scale weather system, deep layer ascent will be maximized
farther north, but a period of height falls this morning will
combine with frontal passage to produce several hours of well
defined synoptic lift, even as cooler and more stable air begins
to filter into the area at low levels. During this several hour
period, from about 10-16Z, heavy swaths of QPF show remarkable
similarity between the WRF-ARW2 and multiple recent runs of the
HRRR. The QPF signal has other support from the NAM, UKMET, and
ECMWF, giving us confidence in the location and at least the
potential for locally heavy amounts.

Low level inflow / 850 mb winds/ of 25 knots are not especially
strong, but should be sufficient to sustain convection, and the
predicted cell motions are ideal for training as the Corfidi
forward-propagation vectors show zero deviation from the mean 0-6
km wind. While the threat should be short lived and somewhat
low-end, the convergence of several pieces of evidence prompted us
to introduce the Marginal Risk area.

Burke


Day 2
Valid 12Z Wed Dec 11 2019 - 12Z Thu Dec 12 2019

The probability of rainfall exceeding flash flood guidance is less
than 5 percent. 

Pagano

Day 3
Valid 12Z Thu Dec 12 2019 - 12Z Fri Dec 13 2019

The probability of rainfall exceeding flash flood guidance is less
than 5 percent. 

Pagano


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt