Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Wed Sep 15 2021
 
Day 1
Valid 01Z Thu Sep 16 2021 - 12Z Thu Sep 16 2021 

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE 
CENTRAL GULF COAST...


...Central Gulf Coast...
2250 UTC Update -- Dropped the western portion of both the 
Moderate and Slight Risk areas west of Mobile AL, based on the 
current observational/mesoanalysis trends along with the recent 
CAM guidance (including past several HRRR runs and 18Z HREF 
exceedance probabilities) in terms of the additional QPF going 
forward through the remainder of the Day 1 period (12Z Thu).  

Previous Discussion...

Tropical Depression Nicholas will continue to weaken today and 
tonight as it meanders slowly in Louisiana. Despite the weakening 
circulation, high PWs of well over 2" (00Z soundings from LCH and 
LIX measured 2.56" and 2.31" PWs, respectively) will continue to 
advect into the Central Gulf Coast region. Additional heavy 
rainfall of 3-6 inches with locally higher amounts is likely as 
the moisture interacts with confluent low-level flow and interacts 
with a wave over the Ohio Valley in the mid-upper levels. 
Receiving the heaviest rain rates will be dependent on if and 
where instability comes from the Gulf onshore. A Moderate Risk was 
maintained for the area while keeping the eastward shift from the 
previous issuance as Nicholas's current track and model guidance 
supports it.

...Tennessee Valley to Northeast...
2250 UTC Update -- Trimmed quite a bit of the Slight Risk area 
(out of KY and TN) based on the radar and satellite trends and 
latest CAM deterministic as well as probabilistic guidance. As 
such, we have also expanded the Slight Risk area over the 
Susquehanna River Valley and Pocono Mtns in PA, owing largely to 
the aforementioned observational trends along with the favorable 
thermodynamic profile ahead of the front per SPC's mesoanalysis 
page (mixed layer CAPEs 1500-2000 J/kg along with PW values ~1.5 
inches). This as the pre-frontal activity (ahead of the mid-upper 
level trough with transient right-entrance region upper jet streak 
forcing) encounters a region with lingering soil moisture 
saturation now 10+ days following the heavy rainfall from Ida 
(100cm depth soil moisture percentiles between 70-90+, with 14 day 
rainfall still between 200-400% of normal).  

Previous Discussion...

As troughing moves quickly across the Ohio Valley/Great Lakes 
region today, it should push a cold front across the northeastern 
quadrant of the U.S. and into the Ohio Valley. Moisture (PW 
anomalies of +1 to +2 standard deviations) and instability 
streaming into the front, which could be enhanced by Nicholas 
farther south, will lead to convection firing mainly in the 
afternoon ahead of the front. The mean flow should be generally 
parallel to the front, which could lead storms to train from 
southwest to northeast, along with the possibility of high rain 
rates. The Slight Risk was extended southward into the 
Kentucky/north-central Tennessee as several hi-res CAMs showed 
spotty rainfall totals of 3+ inches and HREF probabilities of 
exceeding 3 and 6 hour FFG were approaching 50%. Farther north 
from the northern Mid-Atlantic into the Northeast, rain falling on 
wet antecedent conditions with soil moisture around the 80-90th 
percentiles could lead to flash flooding issues. Meanwhile, the 
Marginal Risk was expanded westward into southeastern Missouri for 
convection ongoing through the morning, while expanded eastward 
into Georgia as hi-res guidance especially shows persistent 
convection there particularly tonight. 

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