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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 2332Z Dec 06, 2022)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
632 PM EST Tue Dec 06 2022

Day 1
Valid 01Z Wed Dec 07 2022 - 12Z Wed Dec 07 2022


Locally heavy rainfall is possible through tonight across portions
of the Lower Ohio Valley and Tennessee Valley and may result in
isolated instances of flash flooding. The latest water vapor
imagery shows an embedded shortwave trough moving through Arkansas
while at the surface, the latest analysis shows a warm front
slowly advancing northward through Kentucky. Showers and
thunderstorms are expected to increase in coverage over the next
several hours as an increase in the low level jet (30-35 kts at
850 mb) surges precipitable water values above 1.5" and elevated
instability of around 500 J/kg is present. With the mean flow out
of the southwest oriented parallel to the expected storm motions,
repeating rounds of convection are likely, initially across
western to central Kentucky then southward across western to
middle Tennessee as a cold front slips southward through the
region. The most recent runs of the HRRR have shown an uptick in
the potential for locally heavy rainfall with 1-1.5" hourly totals
at times and localized 2-2.5" totals possible through 12Z
Wednesday. These types of rates are borderline based on the most
recent flash flood guidance, and NASA SPoRT soil moisture
percentiles show fairly dry soils. The rainfall may end up being
more beneficial but there's a low probability of isolated flash
flooding based on the rates and potential training, so a Marginal
Risk was introduced.


Day 2
Valid 12Z Wed Dec 07 2022 - 12Z Thu Dec 08 2022


...North Texas and Eastern Oklahoma eastward into the western
Tennessee Valley...
Models still suggest the potential for locally heavy to excessive
rainfall from eastern Oklahoma into northern Arkansas and southern
Missouri on Wednesday as a southern stream trough pivots across
the Southwest U.S. on Wednesday and across the High Plains
Wednesday night.  As this occurs, the downstream upper level
ridging underneath the broad/deep low centered over north-central
Canada will result in upper level confluence and an eventual jet
streak across the Central Plains and Upper Midwest on Day 2.
Increasing southern-stream forcing coupled with the frontogenetic,
entrance-region upper jet forcing from the northern stream will
make for a favorable environment for deep layer lift across the
outlook areas. Both the GEFS and SREF showed a relatively quick
bout of robust low-mid layer moisture transport, with 8500-700 mb
moisture flux anomalies topping out between +3-3.5. PW values and
a stripe of 1.5-1.7" PWs traversing the outlook areas Wednesday
and Wednesday night.

The progressive nature of the favorable forcing
(forward-propagating Corfidi or MBE vectors), along with the
relative lack of deep-layer instability (MUCAPEs generally less
than 750 J/Kg) will be the primary inhibitors to the flash flood
risk. In addition, per the latest NASA SPoRT analysis, 0-100cm
soil moisture percentiles are running between 10-30% of normal
across much of the outlook area. Nevertheless, per earlier
collaboration with WFOs in the Plains and given the anomalous
degree of deep-layer forcing and moisture, consistent with the 00Z
and 12Z GEFS-based first-guess fields, maintained the targeted
Slight Risk area and surrounding Marginal Risk areas with few


Day 3
Valid 12Z Thu Dec 08 2022 - 12Z Fri Dec 09 2022

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: