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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0101Z May 10, 2021)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Sun May 09 2021

Day 1
Valid 01Z Mon May 10 2021 - 12Z Mon May 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

0100 UTC Update...
For the 0100 UTC update, removed the mid Mississippi and Ohio
Valley from the Marginal Risk area where the cold front has passed
and the heavy rainfall threat has ended.  For eastern Texas and
the lower Mississippi Valley, reoriented the risk areas a little
farther south and west - reflecting current observations and
recent hi-res model guidance.  Recent runs of the HRRR show
backbuilding/training convection shifting farther south into
southeastern Texas and southern Louisiana overnight.  With support
from from the ARW runs and the parallel 18Z HREF as well, extended
the both the Marginal and Slight Risk areas a little farther south
and west across this area.

Refer to MPD #163 for further details concerning the potential for
heavy rainfall and flash flooding across portions of the the lower
Mississippi valley over the next few hours.

Pereira

1600 UTC Update...

Made slight modifications to the Day 1 ERO, based on the
convective/mesoanalysis trends as well as the 12Z high-res
guidance (including the HREF exceedance probabilities). Extended
the Slight Risk a little farther south across more of east TX and
central LA, based on the latest ARW/ARW2 progs (including parallel
runs), while also encompassing the enhanced 2+ inch/hr
probabilities per the 12Z HREF.

Hurley

The Slight risk of excessive rainfall for later today into tonight
stretches from northeast TX into central and northern LA, far
southern AR and central MS. Convection will break out this
afternoon along/ahead of a southeastward moving cold front. Plenty
of instability will be in place to support deep/strong
convection...and PWs are forecast to increase over the
climatological 90th percentile. Thus likely to have some efficient
rainfall producers from cells that develop. The stronger forcing
and core of 850mb moisture transport will generally be focused
northeast of this region. However, often on the tail of these
features we do end up with an enhanced corridor of low level
confluence along with a slowing of the front. This stronger
sfc-925mb convergence combined with a 300mb jet streak overhead
should be enough for organized convective development...with the
slowing off the front allowing for some training/backbuilding. The
general consensus within the Slight risk area is for a swath of
2-3" areal averaged rainfall...with localized totals of 4-6". FFG
across this region is pretty high...although soil saturation is
running above average over the area. This suggests that we may see
flooding impacts develop pretty quickly once FFG is
approached/exceeded. Thus scattered flash flooding is anticipated
over the Slight risk area. This Slight risk was shifted a bit
southeast from our previous issuance...taking into account 00z
global and HREFv3 model trends.

Convection is expected to eventually forward propagate or
redevelop towards the Gulf Coast overnight. Thus while some
training of cells over the Slight risk is expected...the duration
of this training will be somewhat minimized by this eventual
forward propagation. This will eventually bring the heavy rain
potential further south into southern MS and southern LA. There is
a bit more uncertainty with regards to the intensity/organization
of activity by this time...but at least an isolated flash flood
threat may evolve...warranting a southward expansion of the
Marginal risk into these areas.

Convection over northern MS, AL, TN and southern KY will also pose
a heavy rainfall threat today. Activity here should generally stay
progressive...and the ingredients for flash flooding here are not
as good as what is forecast further southwest over the
aforementioned Slight risk area. Nonetheless, there has been an
uptick in forecast QPF over these areas, and isolated pockets of
flash flooding are certainly possible. Probably not enough
coverage for a Slight risk, but enough of a threat to warrant a
northward and eastward expansion of the Marginal risk to encompass
this entire area.

We did opt to remove the Marginal risk over IN, OH and PA. While
1-2" of rain is forecast here...this will be primarily stratiform
in nature...with rates not expected to get high enough for a flash
flood threat. Localized areas of ponding water within urban and/or
low lying areas are possible...but not enough to warrant a
Marginal risk.

Chenard


Day 2
Valid 12Z Mon May 10 2021 - 12Z Tue May 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS THE
LOWER MISSISSIPPI VALLEY...

21Z update... The Marginal Risk area was extended to southeast,
close to New Orleans, Louisiana and Mobile, Alabama. While there
are likely still two concentrated areas of 1+ inch maximums, there
is an increasing signal for the area near the Mississippi Delta to
have higher QPF. This adjustment was made with the QPF and is now
reflected in the revised Marginal Risk area.

Campbell

...Northeast Texas to Central or Southern Mississippi/Alabama...
Made relatively few changes to the Excessive Rainfall Outlook. 
Convection may be on-going at the start of the period on Monday
morning as a front approaching the Gulf Coast states from the
north.  Thinking is that most of that activity will shift from
Louisiana into Mississippi or Alabama early in the day...followed
by another round of showers and thunderstorms which form later in
the afternoon across parts of eastern Texas as low level moisture
begins to move back northward ahead of mid-level shortwave energy.
Operational deterministic model runs still showed two QPF
maxima...with heaviest amounts just either side of 1 inch in each
area.  Spaghetti plots of QPF from the ensembles still showed
enough of a signal for 1 to 2 inch amounts anywhere between the
areas to warrant keeping one risk area considering antecedent
conditions across the area.  With precipitable water values
remaining above 1.5 inches...some briefly enhanced rainfall rates
seem possible even with the modest model QPF values.

Bann





Day 3
Valid 12Z Tue May 11 2021 - 12Z Wed May 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE GULF COAST REGION TO EASTERN AND SOUTH TEXAS...

21Z update... The Marginal Risk was extended further west over the
Texas Hill Country and into South Texas- areas southwest of Corpus
Christi, Texas. The Slight Risk area was extended more to the
west/southwest across east-central Texas. The southward moving
front over Texas and the central Gulf states will slow its
progression during this period, in a west-to-east orientated
boundary. With very moist air streaming in from the east/southeast
over boundary, convection will build westward and support training
cells along and north of this boundary. There is an increasing
signal for moderate to heavy rainfall to initialize over the Hill
Country and surrounding locations, with some guidance exceeding 2
inches. QPF was increased over this region, as well as, eastern
Texas to western Mississippi. There is an elevated threat for
flash flooding across a large portion of Texas during this period.

Campbell

Previous discussion... The front which moved into the Gulf coast
region on Day 2 will advance deeper into Texas on Tuesday...with
east to southeast low level flow becoming established over South
Texas ahead of the front.  Aloft, mid- and upper-level height
falls associated with a low-amplitude trough coming out of the
southern Rockies will start to draw deeper moisture
northward...with the NCEP models showing precipitable water values
over 1.75 inches spreading northeastward covering much of the
Marginal Risk area while 2 inch precipitable water values develop
closer to the Gulf coast.  The result should be increasing
coverage of cells capable of producing downpours whether driven by
instability south of the front or by elevated convection farther
north.

Opted for a Slight Risk from parts of eastern Texas eastward into
Mississippi based on the anomalously wet period there combined
with anticipated rainfall amount in the Day 1 and 2 periods. 
Thinking that a Marginal Risk area will be sufficient for areas
south of the front given drier conditions.

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