Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Tue Jul 27 2021
 
Day 1
Valid 12Z Tue Jul 27 2021 - 12Z Wed Jul 28 2021 

...THERE A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE SOUTHEAST...

...Southeast...
As a surface low tracks along the surface boundary through the 
region, expect to see periods of heavy rainfall. PW values sit 
near 2.25 inches sitting near 2.5 std deviations above normal for 
this area. Weak steering flow and possible training storms, could 
lead to flash flooding. Latest model guidance places signals of 2 
to 4 inches for portions of central to southern GA up along the 
coast of of SC. The Slight Risk was left in play and adjusted 
slightly to account for latest model guidance. Much of the 
Southeast will see periods of heavy rain with this system, with 
much of this area seeing 150-400% of normal precipitation over the 
past two weeks, making flash flooding more probable for this 
portion of the Country. The Marginal Risk area was also adjusted 
slightly to account for latest Hi-Res guidance and forecast QPF.

...Great Basin through the Southern Rockies...
Monsoonal moisture will bring the next round of showers and 
thunderstorms across the Intermountain West. Areas west of the 
ridge axis will continue to see well above normal moisture, 
particularly over the Great Basin and over far north CA into 
central and eastern OR where precipitable water values of 1" to 
1.25" are 2 to 3 standard deviations above normal. The Marginal 
Risk area was maintained over terrain from central OR to southern 
AZ by way of UT with only minor adjustments made to account for 
latest guidance. 

...Northeast...
A northern stream trough will pivot through the region Tuesday 
night with an associated cold front becoming oriented northeast to 
southwest opening the window for training convection. Ahead of the 
trough axis, southerly flow will increase ushering in higher 
precipitable water values which will exceed 1.75 inches aided by 
35+ knot 850mb west-southwesterly winds. This low level flow will 
become aligned with the frontal zone allowing for multiple rounds 
and/or training convection over any given location. With 
instability climbing above 1000 J/kg and enough mid/upper level 
lift, convective coverage should be widespread. With the available 
moisture, instability, and large scale lift, expect hourly rain 
rates to exceed 1.5 inches in some locations. Due the progressive 
nature of the activity, only areas of repeating convection should 
have excessive rain, so the flash flood risk should be isolated. 
The Marginal Risk area was maintained and expanded a bit to 
include all of the eastern Great Lakes.  

Chiari/Jackson


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