FOUS30 KWBC 140823

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed Apr 14 2021

Day 1
Valid 12Z Wed Apr 14 2021 - 12Z Thu Apr 15 2021


...Lower MS Valley...
Mid-level shortwaves will continue to ride along the fairly zonal
flow atop a surface boundary draped from the Southern Plain into
the Lower MS Valley. The first shortwave has triggered convection
overnight which should be ongoing into portions of Eastern TX and
northern LA at the start of the forecast period.  Given enough
moisture, instability and shear, this convection will become
organized (likely into an MCS) and drop south through the
morning/early afternoon while progressing east. Uncertainty on the
evolution of this precipitation activity continues even within the
near term. Regardless, most models signal heavy rain atop
saturated soils leading to the potential for more widespread
significant flash flooding. 

Precipitable water values will try to surge north again to around
1.75+ inches to just south of the frontal boundary aided by 30+
knot southerly low level flow; this is over 2 standard deviations
above the mean. Instability will climb to around 2000-3000 J/kg
this morning with the best instability across southeast TX and
western LA.  As the shortwave interacts with better instability
and moisture, convection will continue to blossom along the front
across north/central LA this morning.  With enough shear, expect
this convection to organize into an MCS and ride south along the
instability gradient.  While the convection should be fairly
progressive as it moves east, the outflow boundary may be slow to
sink south allowing for more east-west training of convection. 
This could lead to higher hourly rain totals around 2-3
inches/hour.  As a result, areal average precipitation could range
between around 2-5+ inches across central/southern LA into
portions of southern MS.

As previously mentioned there is a bit of uncertainty with respect
to the evolution of this next round of convection.  Much of
southeastern LA is saturated with any additional heavy rain
leading to runoff concerns. One possibility is for the heaviest
rain to sink farther to the southwest where the environment is
less overturned and training may occur along the southwest flank
of the outflow boundary.  Regardless, there seems to be enough
confidence in the potential for heavy rain across portions of
south-central LA where some locations received 3-5+ inches of rain
over the past 24 hours. Therefore, after careful coordination with
the local offices, a small Moderate Risk area was introduced with
the expectation that more widespread and significant flash
flooding may occur within this region.  The Slight and Marginal
Risk areas were slightly modified at the update to account for the
QPE and 00Z model guidance. 

There is another shortwave that will approach the Southern Plains
tonight triggering another round of convection across eastern
TX/OK. This activity will becoming better organized along the
aforementioned front over central LA into MS.  Given recent
rainfall yesterday and what is expected this morning, this region
could see another round of heavy rain and flash flooding by late
tonight/early Thursday.  We will continue to monitor this
potential keeping in mind the ongoing activity this morning, the
surface front position and the latest model guidance.

...Southern Mid-Atlantic...
A deepening mid/upper level low will gradually meander east over
the Great Lakes region with ridging along the east coast shifting
offshore.  Impulses riding the base of the trough axis will
traverse the region and interact with a lee trough and approaching
front.  As a result, a weak surface low will develop by late
afternoon. In response, precipitable water values will climb to
around 1.25 inches (which is over 1.5 standard deviations above
the mean) aided by strengthening low level southwesterly flow.
With some diurnal heating this afternoon, this should help sharpen
the lee trough and create more surface convergence. A lot of
uncertainty surround the amount of instability with MUCAPE values
struggling to get to 1000 J/kg.  Regardless, there should be
enough moisture and boundary layer lift to promote convection that
could drop 1-2 inches of rain per hour.  This activity should be
fairly progressive, but backbuilding/training is possible along
the front as propagation vectors align with the mean wind.  Thus,
areal average precipitation could exceed 2-3+ inches in some
locations within a few hours.  While much of this region has
observed around normal precipitation as of late, there are some
locations that have seen above 200% of normal with recent
convection.  Therefore, a Marginal Risk was introduced with the
potential for FFG being exceeded or heavy rain occurring over an
urban corridor.  Most of the precipitation activity should move
offshore by late Wednesday night allowing the potential for flash
flooding to diminish after that point. 


Day 2
Valid 12Z Thu Apr 15 2021 - 12Z Fri Apr 16 2021


Highlighted a small area, mainly confined to parts of Louisiana,
where there appears to be a marginal risk of excessive rainfall on
Thursday.  In the broad scale picture, deepest
moisture/instability and the potential for heaviest rainfall rates
should be moving out to sea early on Thursday and there should be
a relative lull in activity.  However, the models keep some
rainfall lingering over a region that already had more than 300
percent of normal rainfall in the previous week before localized 2
or 3 inch rainfall amounts fell on Tuesday...with the potential
for more heavy rain in the short-term again today. This area would
be particularly sensitive to flooding from even modest rainfall
amounts on Day 2. As a result, placed a Marginal Risk area that
was roughly co-located with the placement of the Moderate Risk
area on Day 1 despite rainfall amounts that would generally not
prompt a Marginal Risk area. Once there is less uncertainty on how
quickly rain shuts down following the passage of a cold front, it
is possible that the area can be removed in subsequent outlooks.


Day 3
Valid 12Z Fri Apr 16 2021 - 12Z Sat Apr 17 2021


...Louisiana to Southern Mississippi/Alabama...
After a relative lull in convection capable of producing heavy
rainfall on Thursday, the return of a boundary from the Gulf of
Mexico combined with the approach of more shortwave energy from
the west should result in increasing risk of excessive rainfall on
Friday across an area already susceptible to flooding.

Low level flow should begin to back and strengthen across eastern
Texas and western/southern Louisiana on Friday in response to
shortwave energy approaching from the north and west.  As it does
so, the low level flow will be riding up and over a
quasi-stationary boundary across the region. This should foster a
growing precipitation shield in the broad isentropic lift/warm
advection pattern...with convection becoming increasingly able to
produce heavier rainfall rates Friday night and into the pre-dawn
hours on Saturday as precipitable water values increase to between
1.6 and 1.8 inches.  The orientation of the boundary and the
southwesterly low-level flow suggest that cell training is

The numerical guidance is in generally good agreement with this
scenario.  One area of concern was with the 14/00Z GFS Friday
evening over southern Louisiana.  The very large magnitude of
vertical velocities over a large part of southwest Louisiana is a
signature of convectively-induced grid-scale feedback.  The real
concern is not so much that the GFS formed convection where it did
but how far downstream the low level thermal and wind fields
impact the model QPF. Thus, was more inclined to follow a non-GFS
solution for placement of the Slight Risk area at this time.


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