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Day 2 Outlook >
 
WPC Day 1 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
 
Updated: 1246 UTC Sun Jul 12, 2020
Valid: 1245 UTC Jul 12, 2020 - 12 UTC Jul 13, 2020
 
Day 1 Excessive Rainfall Forecast
 
Forecast Discussion
 
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
845 AM EDT Sun Jul 12 2020
 
Day 1
Valid 1245Z Sun Jul 12 2020 - 12Z Mon Jul 13 2020 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER WEST-CENTRAL 
TENNESSEE AND NORTHWEST ALABAMA...

...Lower Mississippi Valley and Deep South into the Ohio Valley 
and Central Appalachians...
1230Z UPDATE: A Slight Risk was introduced across portions of 
west-central Tennessee and into adjacent parts of northwest 
Alabama. An MCS across the area was oriented roughly parallel to 
the mean flow, and although the leading portion was beginning to 
show more forward propagation, and a drift into the instability 
axis (to the southwest), there does seem to be enough ascent from 
westerly 925-850mb flow to promote some training and backbuilding, 
particularly with the instability maximum positioned toward the 
upstream portion of the MCS. Therefore, a concentrated area of 
heavy rainfall seems likely, and should pose a more focused threat 
of flash flooding, warranting the Slight Risk upgrade.

PREVIOUS DISCUSSION: An upper level trough with a mid-level 
impulse will pivot across the OH Valley and into the central 
Appalachians through today. The large-scale lift and enhanced 
low-level frontogenesis ahead of the upper shortwave, with 
prolonged (albeit at most modest) low-level moisture flux 
convergence ahead of the slowly-approaching cold front, will 
support organized, more widespread convection during the peak 
heating hours this afternoon into evening across the OH Valley 
into the central Appalachians. PWs of 1.5-1.75" along with 
mixed-layer CAPEs between 1000-2000 j/kg will support hourly 
rainfall rates of 1 to 1.5+ inches underneath the heaviest cores 
(supported by the latest HREF 40km neighborhood probabilities), 
which considering the current FFG, will lead to localized runoff 
issues. The latest CAMs continue to show isolated maximum totals 
of 3+ inches. 

Farther south toward the Deep South and Lower MS Valley, 
deep-layer instability will be more robust (MUCAPEs 2500-4000+ 
j/kg along the tail end of the upper shortwave and separate 
surface front. This along with PWs between 1.75-2.00+ inches will 
create a favorable thermodynamic environment for heavy rainfall 
rates, i.e. 2+ inches/hr. However, deep shear within the NW 
steering flow along with the strong DCAPE potential (progressive 
outflow boundaries) will lead to increasingly forward or downwind 
propagation, thus mitigating the excessive rainfall risk somewhat 
(isolated/marginal). 

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

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