Mesoscale Precipitation Discussion 0258
NWS Weather Prediction Center College Park MD
330 AM EDT Mon May 12 2025

Areas affected...portions of eastern AL

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

Valid 120730Z - 121100Z

Summary...Continued hourly rainfall totals of up to 1-2" are
likely to result in localized 2-3" accumulations over the next
several hours. Isolated to scattered instances of flash flooding
are possible. 

Discussion...Persistent boundary layer moisture flux convergence
just to the north of the triple point of a vertically stacked,
deep layer (sfc-200 mb) cut-off low/cyclone is maintaining a
cluster of thunderstorms across east-central AL (with MRMS
estimating localized hourly totals as high as 1-2" at times). The
convection has become increasingly linearly organized from
south-to-north within the deep layer mean flow, backbuilding
towards the triple point where 20-30 kts of low-level (primarily
near the 925 mb isobaric surface/295K isentropic surface) is
providing locally enhanced convergence/lift. In the mid- to
upper-levels (400-250 mb), broad diffluence (within the left-exit
region of a ~100 kt jet streak) and a distinct potential vorticity
max upstream should continue to promote large scale lift and
convective longevity. The mesoscale environment is characterized
by ML CAPE of 250-1000 J/kg (and increasing by ~200 J/kg over the
past several hours), precipitable water values of 1.3-1.5 inches
(between the 75th and 90th percentile, per BMX sounding
climatology), and effective bulk shear of 20-40 kts.

Given the favorable environment and recent observational trends,
the expectation is that isolated to scattered 1-2" hourly totals
will continue over the next several hours, and the potential
exists for localized training/repeating of these heavy rainfall
rates. Hi-res CAMs (recent hourly runs of the HRRR and RRFS)
insist that this activity will shift abruptly towards the
northeast over the next 2-3 hours (as the low-level flow is
expected to veer over the next couple of hours, largely cutting
off the locally enhanced moisture transport/convergence). Even so,
this will likely result in 2-3" of isolated to scattered rainfall
totals through 10-11z (falling to the north and east of the
Montgomery and Birmingham metro areas where MRMS estimates 1-2" of
rainfall has occurred over the past several hours). Antecedent
soil conditions are quite wet, with accompanying 3-hr Flash Flood
Guidance (FFGs) generally range from 1.5-2.5" (as NASA SPoRT-LIS
0-1m soil moisture remains well above the 90th percentile across
the region). Isolated to scattered instances of flash flooding are
possible.

Churchill

ATTN...WFO...BMX...HUN...

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

LAT...LON   34278653 34188564 33388533 32158541 32018580
            32278604 32718625 33268647 33758657