AWUS01 KWNH 191344
FFGMPD
FLZ000-ALZ000-MSZ000-LAZ000-191900-

Mesoscale Precipitation Discussion 1158
NWS Weather Prediction Center College Park MD
843 AM EST Tue Nov 19 2024

Areas affected...Eastern LA...Southern MS...Southern AL...Far
Western FL Panhandle...

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

Valid 191345Z - 191900Z

SUMMARY...Highly anomalous deep moisture and strength of flux
allowing for efficient rainfall production with progressive
pre-frontal trough.  Embedded slower moving rotating updrafts will
enhanced localized rainfall totals of 3-5" resulting in possible
flash flooding.

DISCUSSION...GOES-E WV suite depicts a large scale closed low
dominating the northern Plains with negative tilt lower scale wave
moving through the western Great Lakes, this is driving a very
strong mid to upper level jet across the Mississippi Valley which
in turn is spurring an very broad and strong low-level jet out of
the Tropics into the central Gulf of Mexico intersecting the
central Gulf Coast.  CIRA LPW shows nose of 1-1.25" surface to
850mb starting to near the southern TN boarder while as broad as
central LA to western FL Panhandle.  Combined with 40-45kts of
southerly 850mb winds and 60-90 degrees of directional
convergence; brings moisture flux values into the 99th and maximum
percentile rankings over a vast area of the Deep South.

An embedded shortwave/inflection can be analyzed through depth
across S MS at the broad left entrance of the 120kt polar jet
across AR/N MS, but also a weak diffluence region across S MS/AL
in the wake of an exiting sub-tropical jet streak that is rounding
the downstream large scale ridge into the Southern Appalachians. 
So while the height-falls are driving the cold front forward,
there is weak surface to 850mb wave in S MS that is backing
low-level flow and increasing flux convergence in that region, as
well as further upstream in the coldest tops/highest unstable air
across the mouth of the MS River and northern Gulf of Mexico.  The
instability gradient is along the Gulf Coast and as a result
strongest cells/tops to -83C have been measure and with moisture
values of 2.5-2.75", rates of 3"+/hr are possible across SE LA for
the next hour or so.  Near the surface inflection, weaker
instability but solid flux convergence and increased bulk shear
will allow for short-term efficient rainfall production to 2"/hr;
with the vast majority falling in a sub-hourly manner given
forward progress.  This should result in possible flash flooding
for urban and prone areas across E LA into S MS/S AL over the next
few hours as the pre-frontal convergence zone slides east. 

As the morning progresses, bulk shear values increase over 40kts
along and east of the inflection as it slides slower to the east
in the further enhancing right entrance (increasing to 130-140kts)
300mb jet.  This will slow the frontal zone as well, and allow for
some modest/weak instability to build back west to the boundary. 
Embedded rotating updrafts have a higher probability of occurring
and with backed/increased directional moisture flux and reduced
forward speed/propagation... downdrafts with capability to produce
2.5-3"/hr rates may occur.  Recent HRRR and 00z Hi-Res CAMs hint
at this solution across the lower 2-3 rows of counties in
MS/AL...combine this localized increase of 2-4" with the preceding
progressive but intense showers on the pre-frontal trough and
localized 3-5" totals become increasingly possible.   Given the
bulk of unstable air remains offshore, there is some reduction in
confidence that updrafts will be strong enough to overcome the
increased bulk shear to support these increased rates.  However,
the potential remains and would be the most likely driver to
potential flash flooding events even into rural areas where FFG
values of 3-4"/3hrs are more representative of soil conditions.  
As such, flash flooding is considered possible through the morning
into early afternoon.

Gallina

ATTN...WFO...BMX...JAN...LIX...MOB...TAE...

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

LAT...LON   32648645 32208539 30938530 30128581 30188632 
            30098773 29628864 29068883 28878931 29139023 
            30578966 31758890 32518776 

$$
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