• Heavy Rain/Flood GA/AL/TN

    From Dumas Walker@21:1/175 to All on Fri Sep 27 08:03:00 2024
    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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