Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Thu Aug 13 2020
 
Day 1
Valid 01Z Fri Aug 14 2020 - 12Z Fri Aug 14 2020 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
the UPPER MISSISSIPPI VALLEY AND THE ARKANSAS RIVER VALLEY INTO 
THE ARKLATEX...

...Northern Plains and Upper Mississippi Valley...
Convection has initiated this evening across portions of northwest 
MN. Would expect this activity to exhibit some backbuilding and 
training characteristics over the next several hours, with weak 
Corfidi vectors and deep layer mean flow parallel to the boundary. 
Overnight cells should begin propagating eastward across central 
and northern MN...however by this time things should be 
progressive enough to keep the flash flood threat low. Thus the 
greatest flash flood risk is likely over the next several hours 
from far eastern ND into northwest MN...where some totals over 3" 
appear likely. The Slight risk was confined to this corridor of 
training/backbuilding...as a Marginal should suffice further east 
given the expected forward speed of cells by that time.

Additional convective development is likely tonight over southwest 
ND as a strong wave and upper jet moves into the region. This 
appears to be a good setup for intense convection given the degree 
of instability, wind shear and deep layer forcing present. However 
the flash flood threat should not be all that great given the 
expected quick forward motions of cells. Thus a Marginal risk 
should suffice to cover the localized risk that may result from 
the short duration intense rainfall rates.

...Oklahoma and Arkansas...
A Slight risk was maintained from portions of eastern OK into 
southwest AR. Will likely see convection develop here around 06z 
on the nose of an increasing axis of lower level moisture 
transport, where convergence will be maximized. These cells will 
exhibit some northwest to southeast training parallel to the 
aforementioned convergence axis. This should be enough to get some 
3" amounts later tonight. Although recent HRRR runs do suggest a 
fairly quick southeastward progression of the convective 
complex...so even though we should see some training 
initially...the duration of the event does not appear all that 
long...which should help prevent a more extreme rainfall event. 
The extent/magnitude of the moisture transport axis and the degree 
of upstream instability, both also seem to suggest the duration of 
the event will be somewhat limited. This may keep the flash flood 
risk more isolated in nature...although portions of southeast OK 
and southwest AR saw very heavy rainfall amounts just a couple 
days ago...so these areas should be more susceptible to additional 
flash flooding. Some question remains as to the exact location of 
this threat tonight...but if it does overlap with this earlier 
rainfall axis then a more concentrated flash flood risk could 
evolve...and thus think the Slight risk remains warranted.

...Southeast and Mid Atlantic...
A localized flash flood risk continues across portions of the 
Southeast into the Mid Atlantic. In general the threat should 
continue to gradually decrease through the evening hours...however 
some instances of cell collisions and locally enhanced rainfall 
will persist for at least the next several hours across the 
Marginal risk area.

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