Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Mon May 10 2021

Day 1
Valid 16Z Mon May 10 2021 - 12Z Tue May 11 2021

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

Slight Risk is maintained for southern/southeast LA to coastal MS
as the second wave of heavy rain progresses through the area
through midday. The trailing boundary left from what is a fairly
progressive bowing segment just reaching NOLA will persist and may
allow some further heavy rain. Was able to trim a bit of the
Slight area from the north and west.

A Marginal risk continues to surround this Slight risk over
portions of eastern TX into the remainder of LA, into southern AR
and southwestern MS. 12Z guidance is in good agreement for
anafrontal activity currently along the central OK/TX border to
continue to develop as it shifts ESE across TX and along the AR/LA
border into this evening. This should remain north of the
particularly heavy activity along the central TX/LA border
overnight, but get into lower FFGs in AR/LA which have been wetter
over the past week. The Marginal Risk also now extends into the FL
Panhandle based on 12Z progs of the current activity most notably
from the 12Z ARW2 and HRRR.

Maintained the Marginal risk across portions of southwest TX from
approximately Del Rio south towards Zapata. Extreme instability is
forecast to develop over this region, with PWs increasing towards
the climatological 90th percentile as well. Convection should
initiate near the intersection of a stationary front and dry line
this afternoon over TX and also over the terrain of northern
Mexico. Given the degree of instability and southeasterly
inflow...these cells should survive eastward into southwest TX
this evening. Possible we see some upscale growth or at least some
convective clusters evolve. Localized 3-5" rainfall may pose an
isolated flash flood risk across this region.

Chenard/Jackson


Day 2
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...Areas along the Gulf are still on track to see heavy
rainfall amounts associated with the frontal boundary. Latest
guidance has shifted the axis of heaviest amounts a bit further
eastward through portions of central MS/AL with upwards of 1 to 2
inches during this time frame with pockets of 2.5+ inches. These
areas are still sitting at 150 to 600 percent of normal of
rainfall over the past week. With this being said, have extended
the Marginal Risk area further eastward for MS and southward
through southern portions of AL per request of MOB CWA. Have also
extended the Slight Risk area a bit further eastward for these
areas as well as southward through southern TX with latest QPF
guidance showing heavy amounts of 2 to 4 inches for this area.  

Chiari/Campbell

Previous Discussion...

Model agreement has gotten better in terms of the speed and
placement of the front approaching from the north...so only a
minor expansion was made around the western and southern/eastern
periphery of the previously-issued risk area. 

The southward moving front over Texas and the central Gulf states
will slow its progression during this period which results in the
boundary becoming oriented from west-to-east. 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. The signal remains for moderate to heavy rainfall
across parts of the area, although the GFS had backed off the
areal coverage and maximum rainfall amounts somewhat from 09/18Z. 
Even so, am not inclined to make significant changes either to the
QPF or the Excessive Rainfall Outlook given the anomalous amount
of moisture in place (maximum anomalies in excess of 2 standard
deviations above climatology) which is sustained by continued
influx of moisture on low-level southeasterly winds and enough
instability (1000-1500 J/kg MLCAPE) to result in cells being able
to produce downpours.

Bann

Day 3
Valid 12Z Wed May 12 2021 - 12Z Thu May 13 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE GULF
COAST REGION...

21Z Update...Models still in agreement on sending the deepest
moisture/instability out over the Gulf during this time period. No
significant changes made to day 3 QPF, thus no changes made to
previous Marginal Risk area.

Chiari/Campbell

Previous Discussion...

The threat of heavy to excessive rainfall will be lingering into
Wednesday along the Gulf coast given a moist airmass in place and
the presence of a front to help focus storms.  However, the
approach and passage of a broad, low amplitude synoptic-scale
trough should usher the deepest moisture/instability out over the
Gulf ahead from west to east.

Before that happens, a corridor of precipitable water values in
excess on 1.75 inches should extend from coastal regions of the
South Texas to the Upper Texas coast and eastward into portions of
Mississippi/Alabama and Georgia. Given the resulting instability
and moisture convergence along the front, scattered storms capable
of producing heavy rainfall will linger until the front and
associated upper level trough moves through.  With decreasing
precipitable water values and a less uniform low-level flow
pattern, thinking is that rainfall amounts/rates will not be as
high as in previous days.  In due time, any convection should be
moving into areas which have higher FFG.  Thus thinking is that a
Marginal Risk should be sufficient for now...although later shifts
can evaluate the need for a Slight Risk area given a clearer look
at how the pattern evolves.

Bann


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