Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
830 PM EST Fri Feb 22 2019
 
Day 1
Valid 01Z Sat Feb 23 2019 - 12Z Sat Feb 23 2019 

...A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF MS, AR, AL 
AND TN...

0100 UTC Update: Little change in the overall axis and 
meteorological setup with respect to current High/Moderate/Slight 
Risk areas; however, the axis has shifted slightly based on the 
recent observational data/mesoanalysis, along with the latest FFG 
and most recent HRRR and NBM trends with the max QPF axis 
overnight. Still expect the highest risk of QPF>FFG exceedance 
along the lower MS Valley from the Arkansas-northern MS border ene 
into northwest AL and southwest TN -- particularly after 03Z owing 
to the surge of low-level moisture transport/flux convergence out 
ahead of the upper height falls across the Southern Plains 
(increasing left exit region upper forcing) and resultant 
northward push of the surface warm front. The High and Moderate 
risk correspond with the ensemble of most recent high-res CAM 
guidance, again, especially with the past 4 HRRR runs. The High is 
also depicted where the FFG remain quite low (3 hourly values 
generally less than 1.00").

Hurley


---Prior Discussion---
A continued wet pattern across portions of MS/AL/TN will result in 
a significant, and potentially life-threatening, flash flooding 
threat through tonight. A HIGH risk of excessive rainfall remains 
in effect for portions of northern MS into northwest AL and south 
central TN. A MDT risk surrounds this extending across much of TN, 
far southern KY and far southwest VA. Showers and embedded 
thunderstorms are likely across this region almost continuously 
through tonight, although two periods of heavier activity appear 
probable...one this morning into early afternoon...and an 
additional round later tonight into early Saturday.

Convection is increasing in intensity early this Friday morning 
across MS. This is likely driven by the approach of an upper jet 
streak, placing the region in the favorable right entrance region. 
This is resulting in a gradual increase in diffluence aloft, and 
also helping tighten the low level confluent flow. Given the 
nearly stationary convergence axis, would anticipate we see some 
training this morning into early afternoon across portions of 
northern MS into northern AL and far southern TN. Will likely be a 
pretty narrow corridor where the most intense rainfall and thus 
greatest flash flood risk will persist this morning. Some 
uncertainty on this exact axis, however it will likely occur over 
areas that are already saturated. Thus flash flooding, some of 
which could be significant, is likely with this activity through 
the morning and early afternoon hours. 

By this afternoon the upper jet passes by off to the east, which 
should decrease diffluence aloft, with the lower level convergence 
axis also forecast to become less defined. Thus, while showers and 
embedded convection are still likely across the High and MDT risk 
areas, the intensity and organization may take a notch downward. 
Any lull will be rather short lived however, as we see additional 
robust convective activity develop overnight. This activity will 
have much stronger low/mid level moisture transport to work with 
as southerly flow really ramps up. The region will also begin to 
feel the effect of height falls associated with the strong trough 
over the Plains, to go along with the increased isentropic 
lift...thus would expect the coverage of heavy rains to be more 
widespread than what we see earlier today. Given the strong 
southerly flow, this activity may tend to move a but more 
progressively off to the north compared to the earlier 
convection...however would still expect some backbuilding into the 
southerly flow resulting in a training threat. Even without 
training, rainfall rates will likely be high enough to cause flash 
flooding, some of which could be significant in nature, given the 
very wet antecedent conditions over northern MS into central TN 
and southern KY.

Model agreement was pretty good with the overnight guidance. 
Overall a consensus of the 0z HREF members and HRRR seemed to form 
a good middle ground solution for QPF through the period. Rainfall 
of 2-4" appears likely across the High risk area, with localized 
higher totals of 4-6" a possibility. The trend in the guidance was 
to focus the heaviest rains a bit further southwest of our 
previous forecast. For that reason had to expand the High risk 
back to the southwest into more of MS, and was also able to cut 
back a bit on the southeastern extent off the higher risk. Some 
uncertainty with the northern and eastern extent of the higher 
flooding risk. Was a bit liberal with the northern extent of the 
Slight Risk, as think the high res models may end up being too dry 
on the northern edge of the rain shield. Generally confined the 
MDT risk area to those locations that have saturated soil 
conditions and have the best chance of exceeding 1.5" of 
additional rainfall, with the threat tapering to a Slight and 
Marginal Risk outside of these areas.

Chenard

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