AWUS01 KWNH 270932
FFGMPD
GAZ000-TNZ000-ALZ000-271530-

Mesoscale Precipitation Discussion 1074
NWS Weather Prediction Center College Park MD
531 AM EDT Fri Sep 27 2024

Areas affected...northwestern GA into northeastern AL and Middle TN

Concerning...Heavy rainfall...Flash flooding likely

Valid 270930Z - 271530Z

SUMMARY...A pivoting axis of heavy rain is expected to focus a
threat for flash flooding from northwestern GA into northeastern
AL and Middle TN this morning. Rainfall rates of 1-2 in/hr,
highest to the south and east, are likely with additional rainfall
of 2-4 inches through 15Z.

DISCUSSION...Regional radar imagery at 09Z showed Tropical Storm
Helene's inner core tracking northward toward the central SC/GA
border. Bands of heavy rain also extended west of the center of
circulation into portions of western GA, northeastern AL and
Middle TN. MRMS and gauge data showed rainfall rates of 0.5 to 1.0
in/hr for most areas west of Helene, but localized rain rates of
1-2 in/hr were estimated via MRMS to the northeast of CSG where
locally higher instability (up to 500 J/kg) was present via 09Z
SPC mesoanalysis data. However, instability values into TN were
quite low at or below 250 J/kg. The bands of heavy rain from
western GA into the tri-state region of GA/AL/TN appeared to be
associated with a low level axis of convergence, centered in the
925-850 mb layer between strong easterly winds to the north of
Helene and northwesterly winds in eastern AL.

Despite the lack of greater instability, strong low level
convergence was present within a very moist airmass characterized
by precipitable water values of 2 inches in Middle TN, increasing
to over 2.5 inches in west-central GA. In addition, water vapor
imagery revealed a diffluent flow pattern aloft across the region.
RAP forecasts show the low level convergence axis slowly pivoting
cyclonically about a point near the tri-state region (GA/AL/TN)
through 15Z with southeasterly steering flow aligned with the
convergence axis at times, which will allow for repeating and
training areas of heavy rain. Due to the limited instability in
place, rainfall rates may struggle to climb above 1 in/hr across
northwestern locations, but a semi-longer duration heavy rainfall
event is expected to produce 2 to 4 inches of rain in a 6 hour
period from portions of Middle TN into northeastern AL and
northwestern GA. Flash flooding is considered likely, especially
in northwestern GA where some areas of flash flooding are ongoing
due to recent heavy rain from Helene.

Otto

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

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

LAT...LON 36528620 36188562 35438513 34358387 33988352
33438330 32758353 32688457 33558577 34798679
36038742 36488722
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