Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
821 PM EDT Sat Aug 18 2018
 
Day 1
Valid 01Z Sun Aug 19 2018 - 12Z Sun Aug 19 2018 

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

...Eastern Colorado...western Kansas...TX/OK panhandle into 
central Oklahoma...
An increasingly difluent upper level pattern will be spreading 
eastward across much of the High Plains southeast of an upper 
level trough.  Convection has developed along a low-level cyclonic 
shear axis, which is currently congealing near the KS/CO border. 
This activity should be progressive in nature given the eastward 
progression of the closed mid/upper low. This quick forward speed 
should overall limit the extent of the flash flood risk. The 
general consensus amongst the 12z HREF members and recent HRRR 
runs is for good coverage of 1-2" amounts...which would not likely 
cause widespread flash flood issues. However this guidance does 
suggest at least localized 3"+ amounts. Given this...and the 
expectation of organized convection within a strongly forced, 
unstable and anomalous PW environment...suggests that some 
clusters of flash flooding are certainly possible. Thus will 
maintain the broad Slight Risk across portions of the central and 
southern Plains. 

May end up with two separate swaths of higher totals...one in the 
existing thunderstorm cluster near the KS/CO border and another 
across the TX Panhandle into southwest OK, where, depending on the 
orientation of the approaching convective line, increasing 
southerly moisture transport may be a bit more supportive of 
backbuilding and training later tonight.  


...Northeast Wyoming...southeast Montana into central South 
Dakota...
Model consensus is for a northern precipitation max closer to the 
primary mid to upper level center associated with the amplifying 
area of height falls moving out of the Northern to Central Rockies 
and into the High Plains, where a Marginal Risk across the 
Northern Plains has been depicted.  Convection has been widely 
scattered so far and will be quite progressive off to the 
east...and any comma head rainfall overnight is expected feature 
rates too low to be considered much of a flash flood threat. The 
12z HREF members and HRRR generally depict 1-2" amounts, which is 
not expected to pose an organized flash flood risk...although 
isolated concerns could still develop.  


...Northeast TX/northern LA...
Thunderstorms are trying to build westward along an old outflow 
boundary across northeast LA due to earlier convection in 
south-central MS while a sea breeze boundary approaches from 
central LA.  Inflow at 850 hPa is convergent in this area though 
light -- random heavy rain issues are possible.  Complicating the 
forecast is an upper disturbance across LA, which is expected to 
weaken/split in the next several hours and become a non-factor.  
Other convection across northeast TX may attempt to move eastward 
into northwest LA with time.  CIN should take hold and continue 
the general downward trend in heavy rain coverage, but since the 
thunderstorms have not completely faded as of yet, maintain a 
marginal risk in this area.


...Southeast...
Ongoing convection is expected to last a few more hours.  There 
was a chance for convection to organize across NC as a disturbance 
aloft moved into the area, but has not come to pass, and chances 
for such are fading.  However, thunderstorms are trying to move 
into the area from northern GA and SC, so there is a chance for 
cell mergers before convection fades for good .


...NJ/Southeast NY...
While afternoon convection has moved stage right into the 
northwest Atlantic, the guidance suggests that there is a chance 
for nocturnal convection within the TROWAL of a surface low that 
is expected to move south of the region.  MU CAPE values are 
expected to fall just below 500 J/kg per RAP forecasts but enough 
effective bulk shear exists in the area to support organized 
convection in the area.  The 12z ECMWF and some of the mesoscale 
guidance were showing this signal, with the ECMWF showing local 
maxima in the 4"+ range.  One learns not to underestimate the 
TROWAL in this area, so have maintained a marginal risk across 
portions of northern NJ and southeast NY as a precaution.


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