Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Sat Jun 10 2023
 
Day 1
Valid 12Z Sat Jun 10 2023 - 12Z Sun Jun 11 2023 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
THE PLAINS, ARKLATEX, AND THE MIDWEST...

Northern High Plains...   
Anomalous moisture continues early on before dropping off and 
southward as an upper level shortwave potentially splits, with 
some energy moving north into Canada and some undercutting the 
ridge and dropping southward.  Instability lingers early on, so 
the existing Slight Risk still makes sense, but it was refined 
using the latest guidance.   
   
   
Midwest...   
A shortwave drops south around a forming closed low across 
southeast Ontario, with the overall system on a strengthening 
trend.  A low- to mid-level low drops southeast and lures PW 
values up to 1.5-1.75" into portions of KS, MO, NE, and IA.  
Inflow at 850 hPa and effective bulk shear should be increasing, 
particularly late in the period when the low-level jet tends to 
ramp up Saturday night into Sunday morning which portends 
organized convection.  CAPE values upstream are forecast as high 
as 4000 J/kg on the 21z SREF guidance during daytime heating, 
falling towards 1000 J/kg overnight.  The guidance signal is more 
scattered than on previous nights in and near the complex border 
junction of NE/MO/KS/IA, near where mid-level heights/thickness 
pattern are diffluent (eastern NE, western IA, and western MO), 
with the guidance (particularly the 00z NAM and 00z Canadian 
Regional) still showing a signal for local 3-5" amounts, with the 
00z HREF guidance showing a non-zero chance for 5"+ amounts.  
Flash flood guidance values, particularly outside MO, are modest, 
but the Kansas City urban area would naturally be susceptible to 
heavy rainfall.  The ingredients available could support hourly 
rain totals to 2.5", which would be a problem in urban areas.  The 
Slight Risk area was re-expanded due to the above, but care was 
taken not to go too far into IA per their lesser sensitivity 
mentioned in prior coordination.  A broad Marginal Risk area 
continues to surround the Slight due to some lingering spread as 
the guidance is no longer coherent if a convective complex is 
going to form, even if all the ingredients appear to be there for 
the formation of such a complex.  The re-expansion was coordinated 
with the DMX/Des Moines IA and OAX/Valley NE forecast offices.
   
   
Portions of CA/NV...   
An anomalously strong closed low drops into CA Saturday into 
Sunday.  Precipitable water values remain elevated, with moisture 
increasing in the Central Valley as the cold low develops and 
moves in.  Instability shouldn't be a problem as temperatures 
aloft decrease.  Believe a Marginal Risk of excessive  
rainfall/flash flood should continue Saturday into Sunday.  Hourly 
rain totals up to 1.5" should be possible where cells backbuild, 
train, and/or merge.   
   
   
ArkLaTex to the Central Gulf Coast...   
As a southern stream shortwave phases with a deepening positively 
tilted upper trough, PWs rise towards 2" along a section of the 
polar front making some progress eastward.  Low-level 
inflow/effective bulk shear appear to be sufficient for convection 
with some level of organization, which would support hourly rain 
totals to 2.5" somewhere between eastern TX, LA, and MS.  The 
NAM/GFS still advertises local 4-6" amounts in eastern TX 
especially, so the Slight Risk area remains in place in that 
region.   
   
Roth
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt