Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Sat Sep 14 2019
 
Day 1
Valid 01Z Sun Sep 15 2019 - 12Z Sun Sep 15 2019 

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF 
THE UPPER MIDWEST...

...Upper Midwest...
Still on track for an expansion of convective development over 
eastern IA this evening, spreading into northern IL overnight. 
Southwest to westerly 850mb flow will ramp up in response to a low 
level wave to the north and the typical nocturnal increase in the 
low level jet. This will increase moisture transport and advect 
instability northeastward. All indications are that convection 
will expand near the nose of the 850mb moisture transport axis. 
Low level winds of similar magnitude, and exhibiting a strong 
parallel component, to the deep layer mean flow supports some 
backbuilding of cells into the overnight hours. The wave and 
moisture transport corridor is generally progressive, so the 
aforementioned backbuilding should not last all that long at any 
one location. With that said, still looks like a good opportunity 
for a corridor of 2-4" of rain through the overnight hours, which 
given wet antecedent conditions, supports scattered flash 
flooding. Some uncertainty on exactly where this axis sets 
up...but the past several HRRR runs have been very consistent in 
favoring areas from eastern IA into northern IL and far southwest 
WI. An extrapolation of recent radar trends also seems to support 
this general region. Thus the Slight risk was adjusted a bit 
southeastward from our previous issuance, to better match recent 
HRRR and radar trends.

...Southwest...
A well defined wave is moving slowly north over southeast AZ this 
evening. Thick cloud cover has generally suppressed more robust 
convective activity near this wave, however a region of steady 
light to moderate rainfall exists. We have been able to get some 
stronger convection on the periphery of the cloud shield where 
instability was able to develop. One area over central NM recently 
flared up, however this activity will continue to quickly erode 
available instability which should lead to a weakening. The other 
area of recent intensification is over central AZ, where locally 
have rain is likely over the next few hours. These cells are 
propagating along outflows, and should also help erode instability 
with time. An isolated flash flood risk likely exists both over 
central AZ and NM over the next few hours with this activity. Near 
the wave a steady rainfall will continue into the overnight, with 
additional amounts around 1" expected through 12z in the vicinity 
of the AZ/NM border.

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