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WPC Met Watch

 
Mesoscale Precipitation Discussion: #0793 (2020)
(Issued at 708 AM EDT Thu Sep 24 2020 )

MPD Selection
 


Graphic for MPD #0793

Mesoscale Precipitation Discussion 0793
NWS Weather Prediction Center College Park MD
708 AM EDT Thu Sep 24 2020

Areas affected...MIssissippi, southern Tennessee, northwest
Alabama

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

Valid 241108Z - 241700Z

Summary...Heavy rainfall associated with Post Tropical Cyclone
Beta will continue across the Southeast. Periods of moderate to
heavy rain, with rates at times exceeding 1"/hr will move slowly
northeastward through the morning. Additional rainfall of 1-3"
with locally higher amounts is likely. This may lead to flash
flooding as some areas have already received 1-3" of rain which
have preconditioned the soils.

Discussion...Post Tropical Cyclone Beta is analyzed by WPC at 09Z
to be near the MS/LA border, with a well defined warm front and
cold front extending to the east, and south, respectively. Within
the warm sector, a tongue of SBCape exceeding 500 J/kg was lifting
into MS out of the Gulf of Mexico, while elevated MUCape  greater
than 250 J/kg was as far north as the MS/TN border. 850mb winds of
30-35 kts were converging across the Gulf, driving robust
warm/moist advection northward. As this impinges upon the warm
front it is being forced to lift isentropically into MS. These
mechanisms are clearly evident on morning regional radar, with
confluent bands of convection spiraling northward around Beta,
congealing into a more widespread shield of moderate to heavy rain
just north of the warm front. Lift was further being aided by the
RRQ of an elongated 100kt jet streak across TN/KY, with PWs across
the region measured by GPS to be 1.75-2.0", above the 90th
percentile according to NAEFS ensemble tables. This overlap of
forcing and thermodynamics had produced morning rainfall of 1-3"
already, with rain rates sampled by KDGX WSR-88D exceeding 1"/hr
in the strongest convective cores.

As Beta continues to lift northeast, the warm front will be drawn
northward, allowing for instability to surge into MS/AL. Although
Beta will begin to degenerate, causing a reduction in the highest
PWs, RAP forecast fields indicate PWs will remain near 1.75"
across the area. These favorable thermodynamics combined with
persistent deep layer ascent due to isentropic lift atop the warm
front, and continued upper ventilation within the RRQ of the
aforementioned jet streak, will be more than sufficient to
maintain torrential rainfall. Recent HREF probabilities indicate a
20-30% chance for 1"/hr rates continuing into the aftn and
spreading northeastward. The heaviest rates are likely within the
warm sector where destabilization will be greatest to create more
robust updrafts, creating efficient rain makers in the deeply
saturated column with warm cloud depths of 14,000-15,000 ft and
tall skinny CAPE profiles. However, in the primary precip shield
just north of the front, more continuous rainfall is likely, which
may also at times reach 1"/hr.

The CAMs are in agreement in amounts through the early aftn,
suggesting 1-3" with locally higher amounts, but feature a wide
variation in latitudinal placement. It is likely that the heaviest
rain will occur along and just north of the warm front where
locally enhanced ascent will persist. So while training of the
excessive rain rates is likely in the warm sector, the temporal
duration of heavy rainfall is expected to be less than along and
north of the warm front, which is where the maximum rainfall
should occur. This region has compromised FFG as low as 1.5"/3hrs,
so soils are saturated and rainfall may quickly turn to runoff as
this FFG becomes exceeded. Flash flooding is possible, but is most
likely across the primed soils of central and northern MS and into
northwest AL.

Weiss

ATTN...WFO...BMX...HUN...JAN...MEG...OHX...

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

LAT...LON   35618751 35558696 35428666 35248653 34878652
            34358686 34018742 33778783 33308832 32868856
            32558873 32288894 32138924 32088949 32118971
            32248991 32319020 32439040 32699063 32859069
            33569068 34099054 34399040 34709020 34968996
            35268932 35468871 35588807


Last Updated: 708 AM EDT Thu Sep 24 2020
 

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