Excessive Rainfall Discussion

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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Tue Feb 18 2020

Day 1
Valid 1734Z Tue Feb 18 2020 - 12Z Wed Feb 19 2020

...There is a Slight Risk of excessive rainfall over the Southeast

The most noteworthy hydrologic condition is the major river
flooding being observed in central Mississippi. A swath of
near-surface based and elevated convection is materializing out
ahead of a frontal zone and along the periphery of a narrow upper
level speed max / ribbon of positive vorticity advection. Deep
layer forcing looks to be maximized near and just downstream of a
CAPE pool over LA and central MS.

Flash flood guidance across the Southeast/Deep South is not
considered too useful as it has recovered far too quickly in the
wake of a historically wet 3-4 week period across the southern
tier. We therefore drew upon a composite approach using 2-week
rainfall anomalies, National Water Model soil moisture analysis,
and observed river flooding to represent antecedent conditions. 
Recent radar trends led to some northward expansion.  This
informed the Slight Risk area, causing us to expand it eastward
from central MS across central Alabama and into western Georgia,
in coordination with local offices overnight.

It looks as though any near-surfaced convection will likely
diminish during the late evening, while deep synoptic ascent
continues to produce rain and embedded thunder in a stripe farther
north, behind the slowly moving cold front. This helps round out
the surrounding Marginal Risk area up into parts of Arkansas,
Tennessee and western South Carolina. The heftiest rain rates,
however, should occur back across parts of LA/MS/AL in the
afternoon/evening, with PW values over 1.50 inches (2.0 to 2.5
standard deviations) and CAPE 500-1000 J/kg supporting isolated
hourly rain rates up to 2" per the high resolution guidance which
fits well with the ingredients in place.


Day 2
Valid 12Z Wed Feb 19 2020 - 12Z Thu Feb 20 2020


...Central Texas across northern Louisiana into central
Short wave energy within the fast mid level flow across the
Southern Plains and the Gulf Coast states interacts with 1.25/1.50
inch precipitable water air (which is between two and three
standard deviations above the mean) to produce banded heavy rain
bands along and north of a elevated boundary extending from
central TX into central MS, mainly after 20/00z. Synoptic scale
ascent in the right entrance region of a 160 knot jet streak
stretching from the Mid MS Valley into the northern Mid Atlantic
states should allow the bands to organize, producing heavy

The surface boundary is expected to remain along the Gulf Coast,
which limits the amount of instability available to the developing
rain bands. NAM/GFS soundings showed less than 100 J/KG of MUCAPE
transported in the upglide to the elevated front, and this could
impact hourly rainfall rates (with the 12z NAM CONUS showing rates
nearing 0.50 inches after 20/00z across northern LA into west
central MS) in the rain bands. However, the 850/300 mb mean wind
becomes better aligned with propagation vectors, suggesting there
could be some training of the bands along the elevated front.

While there is some spread on the latitudinal placement of the
highest rainfall axis, there is good regional/model agreement for
local 1.50/2.00 inch rainfall amounts. Most of the 12z guidance
(with the exception of the 12z NAM) places the axis of highest
rainfall south of areas that are most vulnerable. Portions of
northeast TX across northernmost LA into north central MS received
more than 400 percent of normal rainfall in the past seven days,
with river flooding occurring. Due to the above reasoning, and
after collaborating with WFOs FWD/HGX/SHV/JAN, WPC retained the
Marginal Risk, with no upgrade deemed necessary with this
forecast. However, should the axis of heaviest rainfall move
northward in later model solutions, a Slight Risk could be needed
in later forecasts.


Day 3
Valid 12Z Thu Feb 20 2020 - 12Z Fri Feb 21 2020

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


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

Last Updated: 301 PM EST TUE FEB 18 2020