Mesoscale Precipitation Discussion 0177
NWS Weather Prediction Center College Park MD
310 PM EDT Fri Apr 25 2025

Areas affected...portions of northern AL and southern Middle TN

Concerning...Heavy rainfall...Flash flooding possible

Valid 251910Z - 252300Z

Summary...Thunderstorms initiating along a gust front with the
potential for training with associated flash flood threat of 1-2"
localized totals.

Discussion...A gust front from earlier cells this morning has
begun to initiate new convection, along a line from just west of
Birmingham to Huntsville. The mesoscale environment along and out
ahead of this line is characterized by ML CAPE 500-1000 J/kg,
PWATs of 1.3-1.6 inches (near or above the 90th percentile, per
BMX sounding climatology), and effective bulk shear of 15-20 kts.
Hourly rainfall estimates (per MRMS) are already approaching an
inch, and the 850-300 mb mean flow is nearly parallel to the line
of developing storms. In addition, upwind propagation vectors
suggest only limited forward propagation towards the east, which
may increase residence time of training storms.

Latest HRRR runs seem to be initializing the position of the gust
front fairly well, and indicate 1-2" localized totals (through
23z) along an apparent training axis. The 12z HREF suggests
somewhat higher totals are possible, indicating 20-30% chances for
localized 2" exceedance (per 40-km neighborhood exceedance
probabilities). Lastly, the experimental RRFS and REFS depict
similar solutions as well (with 2" exceedance probabilities from
the 06z REFS also being 20-30%).

Rainfall totals over the past 3 days across the region generally
range from 1-3", and NASA SPoRT-LIS 0-1 m soil moisture indicates
values above the 90th percentile for much of the area. These wet 
antecedent soil conditions suggest that localized instances of
flash flooding are possible along this axis of storms.

Churchill/Shieh

ATTN...WFO...BMX...FFC...HUN...MRX...OHX...

ATTN...RFC...LMRFC...OHRFC...SERFC...NWC...

LAT...LON   35578579 35078490 33808564 33118663 33118778
            33328794 33608767 34328716 34718697 35168678
            35458648