071
AWUS01 KWNH 261853
FFGMPD
FLZ000-GAZ000-ALZ000-270000-

Mesoscale Precipitation Discussion 1068
NWS Weather Prediction Center College Park MD
253 PM EDT Thu Sep 26 2024

Areas affected...Central FL Panhandle into central GA

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

Valid 261852Z - 270000Z

Summary...Nearly continuous rainfall with rates of 1-3"/hr will
persist into this evening ahead of Hurricane Helene. Training of
these rates will result in widespread additional rainfall of 2-3"
by this evening, with local maxima around 5" possible. This will
expand and enhance ongoing flash flooding, with significant
impacts likely in the central FL Panhandle.

Discussion...The GOES-E visible imagery this afternoon shows a
well defined and intensifying Hurricane Helene across the Gulf of
Mexico blifting north towards the Florida Panhandle. Downstream of
Helene, impressive moisture advection is persisting on
strengthening SE flow around the hurricane, driving PWs to as high
as 2.5" as measured by GPS, which is above the daily record for
the central FL Panhandle and into southern GA. This exceptionally
moist air is pivoting northward into an area of intense deep layer
ascent driven via strong mid-level divergence overlapping the
diffluent RRQ of a northward arcing jet streak. Additionally, a
cold front draped from northern GA into the Gulf of Mexico is
helping to produce focused moisture convergence along which heavy
rain rates will train. 24-hr MRMS rainfall across this region has
been generally 4-8" with locally as much as 12", and ongoing
rainfall rates of 1-2"/hr are producing MRMS FLASH unit streamflow
responses as high as 300-500 cfs/smi. Flash flooding is currently
ongoing.

During the next several hours, moisture north of Helene will
continue to surge northward and impinge into a region of focused
ascent from the central FL Panhandle to as far north as Atlanta,
GA. This will likely result in both an expansion and
intensification of rainfall, with rates potentially peaking around
3"/hr in stronger convection, although will more commonly be
1-2"/hr. The strengthening 850mb inflow will not only enhance
moisture transport, resulting in stronger moisture convergence
onshore and into the front, but also cause Corfidi vectors to
become anti-parallel to the flow near the Gulf Coast, merging
towards parallel along the front. This suggests increased
backbuilding and training of echoes within spiraling and
convergent convective clusters, and the HREF neighborhood
probabilities indicate a 30-60% chance for 3" from the Forgotten
Coast of FL northward along the front into central GA, while the
WoFS confines the highest probabilities to the FL Panhandle where
locally 4-5" is possible.

This rain will be occurring atop soils that are fully saturated
from heavy 24-hr rainfall,. This is reflected by NASA SPoRT 0-10cm
RSM that is above 85%, and resultant 3-hr FFG as low as 0.75-1.5".
This will likely be exceeded in many areas, and where the heaviest
rates train across the most vulnerable soils, especially in the
vicinity of Apalachicola, resultant impacts will become
significant even before Helene`s landfall later this evening.


Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...BMX...FFC...MOB...TAE...

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

LAT...LON   34078422 33438374 32898324 32088278 31858354
            31248398 30698390 30068403 29838433 29528491
            29678553 30278618 31508624 33388549 34058495