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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1906Z Feb 25, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
206 PM EST Sun Feb 25 2018

VALID 21Z Sun Feb 25 2018 - 12Z Mon Feb 26 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NNE GLS 35 N VCT 30 ESE HYI LHB 10 S PSN 15 ESE GGG
10 NNE DTN 30 SSE LLQ GWO 20 NW CBM 15 SSW 3A1 GAD ANB AUO
20 WSW OZR 15 NNE JKA 30 SSE BIX 30 SSE HSA 20 NE 7R3 10 NW 7R5
25 NNE GLS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WSW MCB 25 NNW BTR 25 NE CWF 25 N BPT 45 NW BPT 10 NE CXO
15 NNE UTS 30 W LFK 15 W OCH 20 SSE SHV 20 WNW MLU 20 E BQP
30 N TVR 15 N HKS 25 SSE JAN 15 WSW MCB.


19z Update:

Just made some minor adjustments to the slight and marginal risk
areas based on radar trends...HRRR trends and the new 18z WPC QPF.
Overall the going forecast looks in good shape, with some flash
flood threat continuing from portions of east TX into central and
northern LA. The threat is a bit more uncertain further east in
MS. A decent high res signal for 1-3" of rain here...but unclear
if elevated instability will be high enough to produce rates
intense enough for flash flooding...especially since antecedent
conditions are drier here. Thus while the slight risk extends into
southwest MS...will just keep a marginal for now further south and
east across MS. Will continue to monitor trends. -Chenard


...Previous Discussion...

Water vapor imagery shows a wave moving northeast across TX this
morning, with the area also continuing to see upper level
divergence in the right entrance region of the jet to the north.
850 mb moisture transport has increased ahead of this wave...and
while not extreme in nature...has been strong enough to result in
an uptick in convective activity over southeast TX. This
convection is forming in an area where 850 mb moisture convergence
and roughly 500-1000 j/kg of elevated CAPE intersect. The axis of
850 mb moisture convergence should persist through the day along a
corridor from southeast TX into central and northern LA and
portions of southern and central MS. Recent RAP runs suggest that
this stronger activity over southeast TX should tend to move out
of the better instability as it moves east into far east TX and
LA. This may tend to result in a decrease in intensity and rates
as it moves into this more stable airmass. However a few hundred
j/kg of cape should still exist...which combined with near record
pwats...will still support some locally heavy rates. Soil
conditions also become more saturated across far east TX into
central and northern LA. Thus at least some flash flood threat
exists into the early afternoon hours with this activity over TX/LA

Showers and embedded thunderstorms may tend to persist through the
day and into this evening over portions of far east TX into LA.
However by later today elevated instability should be even
less...resulting in the likelihood of lower rainfall rates than
the morning and early afternoon storms. Still given soil
conditions...an additional flood risk may persist.

With this update we went ahead and shifted the slight risk a bit
northward across southeast TX into LA. This coincides with the
axis depicted by the new 12z high res guidance...while also
agreeing with recent radar trends...and encompassing an area that
has been quite wet over the past week. Given recent radar/model
trends and soil conditions think the slight risk further south
along the Gulf Coast can be made a marginal. The marginal risk
will be maintained across portions of MS/AL and into the southern
Appalachians. Some higher rates are possible in southern MS where
some instability is progged...however given drier antecedent
conditions here...think a marginal should be fine for now. Further
north and east still looking at 1-2" or so of rain...however drier
antecedent conditions and lower rates should keep the threat
marginal at best.

Chenard