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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0826Z Jan 22, 2019)
 
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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
326 AM EST Tue Jan 22 2019

Day 1
Valid 12Z Tue Jan 22 2019 - 12Z Wed Jan 23 2019

...Eastern TX into the Lower MS Valley...
Have decided to remove the Marginal Risk with the new day 1
outlook. Moderate rainfall magnitudes are still likely, however it
appears that flash flooding is unlikely. Will have an overlap of
strong synoptic forcing and low level convergence, which will
favor an expanding area of showers and embedded convective
elements. It is between 0z and 12z that we begin to see strong
right entrance region upper jet dynamics interact with an eastward
moving cold front, supporting an expansion of rain. Thus think the
QPFs from most of the 0z HREF members are underdone, with reality
ending up closer to the heavier ECMWF/UKMET/GFS/GEM Regional.

With that said, will have multiple factors that should reduce the
flash flood potential. First off is the lack of instability, with
MUCAPE only increasing into the 100-200 J/KG range in a narrow
swath ahead of the front. While this may result in some embedded
heavier convective elements, rainfall rates will be limited by
this lack of instability, and the narrow axis of instability
suggests the duration of any heavier rates will be short lived.
Integrated moisture transport and PWs do increase to above average
levels, but in general are not as impressive as what was seen with
the last system. Also, where the heaviest rainfall is forecast
with this system, 1-2" (local 3"), across  east TX into LA and
southern AR...antecedent conditions have been dry...making these
magnitudes unlikely to cause flooding. Where antecedent conditions
have been more moist further north across northeast AR into
western TN, rainfall magnitudes with this system are only expected
to approach 1", with generally light to moderate rates.

Thus, while a broad swath of 1-2" (local 3") of rain is still
forecast across the area, most environmental parameters and
antecedent conditions suggests the flooding threat will remain low.

Chenard



Day 2
Valid 12Z Wed Jan 23 2019 - 12Z Thu Jan 24 2019

...MARGINAL RISK AREA OVER PORTIONS OF THE SOUTHEAST U.S. ON
WEDNESDAY AND WEDNESDAY NIGHT...

...Portions of the Tennessee Valley into the Southern/Gulf Coast
States...

Convection capable of producing moderate to locally heavy rainfall
amounts will be making its way into portions of the Southeast U.S.
and Gulf Coast states on Wednesday in association with the
approach of a deep-layered trough and surface cold front. 
Precipitable water values begin to increase substantially along
the central and eastern Gulf Coast region--from southern
Mississippi/Alabama and into portions of the Florida Panhandle as
low level wind become southerly and accelerate into the 45 kt 5o
55 kt range during day on Wednesday.  Precipitable water values
are forecast to increase from over one inch at the start of the
period at 12Z on Wednesday to nearly two inches by Wednesday
evening and early Thursday morning.  Aloft, the entrance region of
a 135 to 155 kt upper level jet will be sweeping into the area
which should aid in deep layer vertical velocities via
divergence/difluence. Models continue to show a signal for heavy
rainfall along the eastern Gulf Coast with modest values of
CAPE/MUCAPE apparently being offset by the strength of the
dynamics involved.  The model signals remained in the 22/00Z suite
of guidance, although disagreement remains in how the potential
evolves.  The 22/00Z NAM was given the least consideration given
how quickly it became in outlier with respect to QPF in the Day 1
period...while the 22/00Z run of the GFS was given most weighting.
 The resulting QPF remained in the 1.5 to 2.5 inch range from
parts of Alabama and Georgia northward into the terrain of the
Appalachians...so only made some minor adjustments to the
periphery of the Marginal Risk area as the previous Day 3
Excessive Rainfall Outlook becomes the current Day 2 ERO.  Opted
against an increase to Slight Risk given the steady eastward
propagation of the rainfall...thinking this will be enough to
limit any widespread problems.

Bann


Day 3
Valid 12Z Thu Jan 24 2019 - 12Z Fri Jan 25 2019

...MARGINAL RISK AREA OVER PORTIONS OF THE NORTHEAST U.S.
THURSDAY...

...Coastal Mid-Atlantic to Southeast New England...

Moisture will become increasingly deep ahead of a surface cold
front and deep-layered mid- and upper-level trough on Thursday,
setting up the possibility of localized flash flooding concern
prior to the passage of the cold front.  Precipitable water values
of 1.25 inches or greater will be drawn into the area ahead of the
system by southerly winds on the order of 50 kts to 65 kts in the
850 mb to 700 mb layer.  Upper level support in the form of
divergence and difluence will be available as the entrance region
of the upper level jet moves through the area. While the QPF from
the models and WPC are actually pretty modest, the recent wet
conditions and frozen ground my result in localized problems even
if rainfall rates fall short of the Flash Flood Guidance.

Bann



Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt