Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Mon Aug 20 2018
 
Day 2
Valid 12Z Tue Aug 21 2018 - 12Z Wed Aug 22 2018 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL NORTH FROM THE 
CENTRAL APPALACHIANS AND IN ARIZONA...

...Eastern Great Lakes to Mid-Atlantic Region..
A shortwave trough will be drawn northeast across the Great Lakes 
to the base of a larger-scale trough extending from Hudson Bay 
Tuesday. Southerly flow, deep moisture and the formation of a 
mid/upper level jet streak will set the stage for afternoon and 
night time storms capable of producing heavy rainfall south from 
the eastern Great Lakes. A Slight Risk was maintained over 
portions of the Eastern Great Lakes to the central Appalachians 
given the Precipitable Water Values of 1.75 to 2.00 inches (2 to 
2.5 standardized anomalies above climatology) expected. Eastern 
portions of this area have had above normal rainfall in the past 
month. 

Farther south over the southern Appalachians and VA/NC Tidewater 
forcing and instability remain limited, so a Marginal Risk was 
maintained despite well above normal moisture.

...Desert Southwest....
A plume of deep moisture continues to advance north on return flow 
west of the high centered over NM/TX Tuesday. Precipitable water 
values of 1.75 inches drawn from southwest Arizona will cross the 
higher terrain of central AZ. Precipitable Water values of an inch 
or more (generally two standard deviations above normal in this 
high terrain area) will reach the southern Wasatch by late Tuesday 
afternoon. This will support scattered late day and evening 
convection that will be capable of producing locally heavy 
rainfall across the four corners region. Maintained a Slight Risk 
over portions of Arizona where the moisture will be deepest and 
where shortwave energy is expected to rotate around the periphery 
of a mid/upper level high to the east. Will have to continue to 
monitor smaller scale features and timing for the most at risk 
areas. 

...South-Central High Plains...
A shortwave riding zonal flow south of the northwestern CONUS 
trough will encounter the nocturnal low level jet over the Plains 
Tuesday night. 1.5 inch PW (1 to 1.5 standard deviations above 
normal) will allow thunderstorms to organize (most 12Z guidance 
suggest two tracks across KS and the Panhandles). A broad Marginal 
Risk continues to cover both of these areas.

Jackson

 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt