Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sun Sep 20 2020
 
Day 1
Valid 16Z Sun Sep 20 2020 - 12Z Mon Sep 21 2020 

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS OF 
THE MIDDLE/UPPER TEXAS COAST AND THE SOUTHEASTERN LOUISIANA 
COAST...

1600 UTC update

No changes made to the previous Excessive Rainfall Outlook.

Oravec

0900 UTC discussion

...Middle/Upper Texas Coast...
Tropical Storm Beta will continue to approach the middle Texas 
coast through Sunday night, following the 03Z NHC advisory, 
bringing what could be a tight gradient in rainfall very close to 
the shore. As of 07Z this morning, infrared satellite imagery 
shows bursts of convection, displaced northeast of the center (due 
to southwesterly shear), as well as deeper convection in a 
northwest to southeast arc, off of the southeastern Louisiana 
coast. As the center of Beta gets closer to the Gulf Coast, 
precipitable water values will surge upward across much of the 
middle and upper Texas coasts with values reaching into the 2.25 
to 2.5 inch range by 12Z/21. 850 mb winds are expected to veer 
from northeasterly to southeasterly during the period with speeds 
of at least 30 kt by 12Z Monday along the middle/upper Texas 
coastline. With low level winds in excess of deeper layer mean 
winds, the potential for training axes of heavy rain with rates of 
at least 1-2 in/hr increases along with the potential for flash 
flooding. Forecasts of instability are relatively low as one moves 
inland from the coast which should tend to limit rates across 
inland locations until during the day on Monday.

...Southeast Louisiana...
As Beta nears the Texas coast, the model consensus shows dry air 
wrapping in to the south of the center, and then east of the 
center by 12Z Monday. East of the dry intrusion, a low level 
confluent axis sets up with deep moisture (precipitable water 
values peaking in the 2.25 to 2.5 inch range). Infrared satellite 
imagery already supports a secondary area of deeper convection 
south of the Louisiana coast and heavy rain will likely continue 
to edge closer to the coast during the day today with the 
convective axis shifting westward Sunday night and possibly 
re-orienting as Beta continues to move to the west.

...Eastern Florida Peninsula...
East-northeasterly low level flow will remain across much of the 
eastern Florida peninsula during the day today and tonight. The 
nose of stronger 850 mb winds (30 kt at MLB as of early Sunday 
morning) and axis of high precipitable water values (2.25 to 2.5 
inches) is expected to translate southward during the day as high 
pressure continues to build south down the East Coast. 850 mb wind 
speeds are forecast to weaken and broaden near and beyond 00Z 
along the eastern Peninsula and deep-layer mean winds (storm 
motion proxy) are expected to be slightly stronger than what was 
seen during the day on Saturday. Therefore, while the threat for 
locally heavy rain will continue today, the focus does not appear 
as clear as Saturday's event. Heavy rain, in excess of 5 inches, 
may be limited to one or two rather small areas. The rainfall rate 
probabilities for 3+ in/hr and 5 inches in 24 hours (ending 12Z 
Monday) from the 00Z HREF are less than 30 percent (compared to 
the 80-90 percent forecast from Friday night for the 24 hour 
period ending 12Z Sunday). 

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