Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
822 PM EDT Mon Apr 12 2021
Day 1
Valid 01Z Tue Apr 13 2021 - 12Z Tue Apr 13 2021
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Localized heavy rainfall is likely through 0300-0600Z across
portions of south central TX as convection forms along a
stationary front. Severe weather is the main threat (see SPCs
Slight risk, along with Severe Tstm Watch #93, valid through
0300Z), but some brief training along the front and/or a few
slower moving supercells could drop a quick 1-3" of rain on a
localized basis. Dry antecedent conditions, combined with only
isolated to scattered coverage and some forward propagation of
cells into the instability pool...suggests any flash flood risk
should stay highly localized (40km neighborhood probability less
than 5 percent of exceeding FFG), and focused over any more
susceptible urban/low lying areas. Thus not planning to introduce
a Marginal risk at this time.
Hurley/Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Tue Apr 13 2021
Day 1
Valid 12Z Tue Apr 13 2021 - 12Z Wed Apr 14 2021
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST...
A complex mid/upper level pattern exists with multiple closed lows
anchored within a broad trough across much of the U.S. Ridging
will transition to the East Coast pputting much of the Southern
Plains and Lower MS Valley under weak mid-level zonal flow. With
a cold/stationary front draped within this region, any mid-level
impulses interacting with enough diurnal heating and warm air
advection at the surface could promote convection. And this
appears to be the case by the afternoon across portions of the
Lower MS Valley/Central Gulf Coast and perhaps again tonight in
the Southern Plains/Lower MS Valley. We will break down the
potential convective development and flash flooding impacts below.
...Central Gulf Coast...
Convection developed overnight across portions of Central TX,
along and north a stationary surface boundary. Much of this was
triggered by a weak mid-level shortwave traveling through the
mid-level flow. This impulse will continue to track east and help
to blossom convection across portions of the Lower MS Valley, most
notably central LA. Ahead of this shortwave, low level moisture
will increase with precipitable water values climbing over 1.75
inches aided by fairly weak 850mb southerly flow; this is around 2
standard deviations above the mean. With a bit of diurnal heating
and steep lapse rates in place, especially south of the
aforementioned front draped across northern LA, expect MUCAPE
values to exceed 2500 J/kg. With the shortwave moving atop the
boundary and interacting with above normal deep layer moisture and
sufficient instability, anticipate convective development by early
afternoon. With enough shear, this activity will become linear and
sink southeast along the instability gradient. While the forward
propagation should be progressive, the hourly rain totals will
climb over 2 inches which are supported by the HREF neighborhood
probabilities. Areal average precipitation will range between 2-4
inches with locally higher amounts expected.
Given the wet antecedent conditions in place across this region
over the past week (based on much of southeast LA observing 300+
percent of normal precipitation and the region being within the
95th percentile of 0-40cm soil moisture), an additional bout of
heavy rain, especially across urban locations will likely result
in scattered flash flooding and runoff. Therefore, introduced a
Slight Risk area across southeast LA. While there was a very brief
consideration for a Moderate Risk, the organized convection should
be progressive enough to preclude more significant widespread
flash flooding. However, it should be noted that back building
convection along the outflow boundary across the southwestern
flank across southwestern LA and into eastern LA may be possible
through the afternoon/early evening. Therefore, extended the
Marginal Risk to account for this.
...Southern Plains/Lower MS Valley...
Under weak zonal mid-level flow, another subtle mid-level
shortwave is forecast to approach the Southern Plains tonight,
interacting with deep level moisture and lingering instability
along the stationary boundary draped across the region. There is
a bit more uncertainty with the evolution of this activity and the
overall coverage. However, various high resolution models are
starting to show the signal for convection and thus high rain
rates moving from eastern OK/TX into portions of AR/LA.
Precipitable water values across this region will climb to around
1.5 inches aided by weak low level southerly flow. The
instability gradient will build north through the
afternoon/evening with 2500 J/kg MUCAPE still lingering into the
overnight hours. With enough mid-level forcing for ascent,
convection could blossom with rain rates approaching 1.5
inches/hour. With multiple rounds of heavy rain in any given
location, this could result in around 1-2+ inches of rain within a
6 hour period.
With fairly dry antecedent conditions, feel the potential for
flash flooding is less than 5 percent. However, if QPF increases,
a Marginal Risk may need to be introduced at subsequent forecast
updates.
Pagano
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Tue Apr 13 2021
Day 2
Valid 12Z Wed Apr 14 2021 - 12Z Thu Apr 15 2021
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST...
Maintained the Marginal Risk area with a westward extension that
was introduced on Monday afternoon due to 00Z guidance continuing
to show trends started at 11/12Z. Those trends included some
heavier rainfall amounts late in the Day 1 period that approach
south-central and southeast Louisiana Abundant moisture will be
in place at the beginning of the period with 00Z GFS/ECMWF runs
showing PWATs exceeding 2.5 standard deviations above climatology
mainly over parts of Louisiana on Wednesday. There is a signal in
the guidance that another area of convection could develop near
the Central Gulf Coast very late in the period. Given the overlap
of heavy rainfall at the end of Day 1 and the beginning of Day 2,
some 4 inch amounts in southeast Louisiana have prompted the
inclusion of a Slight Risk area. The question remains with regard
how far east to bring the Slight Risk area. While WPC QPF does
taper off over far southern Mississippi and Alabama, several of
the operational models and their respective ensembles did show
some potential for isolated amounts in excess of 3 inches. Given
rainfall over the past week, the amount of rain forecast on Day 1
and some potential for isolated heavier amounts persisting into
the first half of Wednesday...carried the Slight Risk area towards
Mobile Bay.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Tue Apr 13 2021
Day 3
Valid 12Z Thu Apr 15 2021 - 12Z Fri Apr 16 2021
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE
IMMEDIATE CENTRAL GULF COAST PRIOR TO THE PASSAGE OF A COLD
FRONT...
Locally heavy rainfall will be possible along the immediate coast
of the Central Gulf states early Thursday prior to the passage of
a cold front. There should still be abundant moisture in
place...between 1.5 and 2.5 standardized anomalies greater than
climatology...that could result in briefly heavy rainfall rates
over an area that has had above normal rainfall. But most of the
guidance tended to show generally modest rainfall amounts,
suggesting that any excessive rainfall will be localized in
nature. In addition, it is possible that the front may have
already passed through the area prior to the start of the
forecast. Given the uncertainty, will not issue an area greater
than a Marginal risk and re-evaluate in upcoming outlooks.
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
