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< Day 2 Outlook
WPC Day 3 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 2017 UTC Sun Sep 20, 2020
Valid: 12 UTC Sep 22, 2020 - 12 UTC Sep 23, 2020
Day 3 Excessive Rainfall Forecast
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Sun Sep 20 2020
Day 3
Valid 12Z Tue Sep 22 2020 - 12Z Wed Sep 23 2020 


..2130 UTC Excessive Rainfall Outlook Discussion...

Much of the 20/12Z numerical guidance still showed Beta weakening 
in response to the system ingesting dry air combined with 
southwesterly shear.  The focus for heavy rain begins to shift to 
the north side of the system as the system evolves into a 
depression and perhaps a remnant low by the end of the period.  
For the moment, the previously-issued Slight Risk and the 
surrounding Marginal Risk areas cover the potential for excessive 
rainfall associated with the storm center early in the period and 
with the area of heavy rain in/near the Arklatex region during the 
latter half of the period.  Placement of the QPF and the 
associated risk of excessive rainfall may shift...especially as 
Beta continues to become better organized and the exact track and 
timing become better defined.  For the moment, the uncertainty 
precludes the confidence to upgrade to a Moderate Risk area but 
remains a possibility for later outlooks.


...0830 UTC Excessive Rainfall Outlook Discussion...

...Southern Plains into the Lower Mississippi Valley...
The main flash flood threat could transition from Beta to inflow 
banded convection across portions of southeast LA into far 
southwest MS during Day 3. The WPC QPF and Excessive Rainfall 
Outlook were based on the most recent NHC, as well as the 00z 

Much of the 00z regional/global guidance showed Beta weakening as 
dry air entraining into the system from the west and increasing 
southwest shear begins to take their toll on the system, slowly 
eroding the deeper moisture associated with it. Low level inflow 
to the east of Beta is expected to focus the highest rainfall 
across far southeast TX (including much of the upper TX coast), 
where 2.25 inch precipitable water in the southeast low level 
inflow could allow bands of convection to produce local totals 
near 4.00 inches. There is some spread in the 00z guidance where 
this occurs, and this may be tied to the positioning of the 
instability gradient near the coast. However, there is enough 
moisture in the column to support a Slight Risk here for Day 3.

The greater flash flood threat could transition from Beta to 
portions of southeast LA and far southwest MS, as there is an 
increasing model signal for banded convection to develop in the 
2.25 inch precipitable water plume in the low to mid level south 
southeast flow. There is some model spread with respect to the 
placement of the band, as the low level convergent flow varies 
longitudinally within the 00z model guidance suite. A blend of the 
00z ECMWF/GFS was used to construct the placement of the inflow 
band across southeast LA, though the 00z ECMWF QPF amounts seem a 
bit high with QPF amounts at this juncture. 

A small Moderate Risk was considered here, but the spread in the 
placement of the axis of highest QPF raises just enough doubt for 
the confidence needed for a Day 3 Moderate (and this decision was 
collaborated with WFO LIX). Should agreement in the placement of 
the rainfall maximum with the banded convection increase with 
later model runs, a Moderate Risk is certainly possible in 
subsequent forecasts. Instead, the Slight Risk from southeast TX 
and southwest LA was extended over this area to cover the threat.

Day 3 threat area:

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