Quantitative Precipitation Forecast
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FOUS30 KWBC 120819

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Mon Apr 12 2021

Day 1
Valid 12Z Mon Apr 12 2021 - 12Z Tue Apr 13 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.


Day 2
Valid 12Z Tue Apr 13 2021 - 12Z Wed Apr 14 2021


Maintained the Marginal Risk area over parts of the Central Gulf
Coast from Tuesday into early Wednesday morning.  The models still
show a low- to mid-level ridge over the southern Gulf of Mexico
during the period with a southern stream impulse approaching the
northern periphery of the ridge over central Gulf Coast early in
the period. A well defined boundary (oriented northwest to
southeast) will separate 1.5+ inch PWATs to its south and west
from less than 0.75 inch PWATs to its north and east. This will
set up a warm advection pattern along the coastal states as south
to southwesterly flow interacts with the boundary.  That will lead
to increasing instability and deeper moisture.  Cell movement
should generally be from southwest to northeast, but any organized
clusters should tend to follow along the axis of the low-level
boundary, potentially setting up a situation for training heavy
rain as inflow will be from the south-southwest. The thermodynamic
environment would be supportive of rainfall rates of at least 1-2

After the initial round of expected convection early in the day on
Tuesday, a second round of storms is possible late Tuesday
night/early Wednesday morning as increasing low level flow from
the Gulf meets with advancing convection moving eastward out of
eastern Texas.

Multiple rounds of convection could lead to 2-4 inches of rain
(locally higher) over the 24 hour period ending 12Z Wednesday in
areas soaked just a couple of days ago...with many areas in this
part of the U.S. receiving more than 300 percent of normal
rainfall over the past week.  Those areas would be most prone to
flash flooding. With the numerical guidance still depicting a
range of solutions as to placement of the heavy rainfall, will
maintain a Marginal Risk for now.


Day 3
Valid 12Z Wed Apr 14 2021 - 12Z Thu Apr 15 2021


The risk of flash flooding will continue on Wednesday over parts
of the Central Gulf Coast as the area remains immediately north of
a low level boundary over the Gulf of Mexico and one or more
shortwaves move eastward across the southern tier of states.

The numerical guidance is split on the evolution of the convection
from Wednesday into early Thursday morning...but the signal for
additional heavy rainfall was becoming stronger.  The GFS develops
convection over parts of Louisiana...southern Mississippi/Alabama
and then moves the convection southeast over the Gulf of Mexico by
Wednesday evening.  The NAM solution moves the convection and the
associated moderate to heavy rainfall into the Southeast U.S.
ahead of an approaching cold front...while the ECMWF tended to
offer a considerably drier solution.

With precipitable water values forecast to approach 2 inches near
the southeast Louisiana coast and the lingering low level boundary
from Wednesday night...am inclined to favor the wetter non-ECMWF
solutions.  This potentially puts 2 to 4 inches of rainfall over
parts of far southern Mississippi and Alabama that received
similar amounts earlier in the weekend.

A Marginal Risk was placed here given the potential for more rain
over an area that was recently soaked.  An upgrade to a Slight
Risk area may be needed in subsequent outlooks if confidence
builds in the placement of heaviest rainfall...especially if there
is overlap with the area of heavy rainfall on Wednesday.


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


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