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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 2153Z Feb 23, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
452 PM EST Fri Feb 23 2018

VALID 2152Z Fri Feb 23 2018 - 12Z Sat Feb 24 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ESE SLR JWY 20 WSW GDJ 20 WSW MWL 15 S DUC 15 WSW CQB
20 SW BVO 10 ESE CNU 10 NE UIN 20 NW LAF 30 E MIE 40 S KHLB
25 ESE FTK 40 NW JKL 15 E PKB 10 W CBE 20 NNW HSP 30 WNW TRI
20 WNW CHA 25 SE MEI 15 SE KDLP 15 SSE 9F2 25 WNW MSY 20 ESE HEZ
30 WNW MLU 15 SE TXK 15 ESE SLR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
GCM 45 NE JLN 10 NE JEF 25 E ALN 10 NNE SAR 30 E UNO 30 SSW FLP
25 NW MWT 20 WNW PRX GYI 20 S ADH GCM.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 ENE UTA 10 WSW MKL 20 SSE CKV 40 SSW GLW 20 NW MDQ
30 ENE TUP 30 ESE GWO 10 ENE GLH 10 ENE UTA.


2200z Special Update:

Adjustments to the previous issuance are based on the latest runs
of the HRRR, HREFMEAN along with recent satellite and radar
trends.  The general consensus of the hi-res guidance shows
ongoing convection across the Red River in central Oklahoma
lifting to the northeast this afternoon and evening -- remaining
further to the north and west of areas impacted by the heaviest
rainfall.  Therefore, removed the moderate risk that was
previously across southeast Oklahoma into north-central Arkansas. 
Shifted the previous slight risk further to the northwest across
eastern Oklahoma, western Arkansas and southern Missouri.  Several
hi-res guidance members show locally heavy amounts on the order of
1-2 inches within this region during the evening and overnight
hours.  Again, expect these amounts to fall across areas where
soils are relatively less saturated and pose less of a
flash-flooding threat. 

Included a separate slight risk further to the south across
northern Mississippi into western and Middle Tennessee.  Some of
the guidance show convection developing along an axis of southerly
inflow producing some moderate to locally heavy amounts.  Flash
guidance are relatively lower across this region -- suggesting
flash flooding may become a concern across areas where heavy
amounts do materialize. 

Further east, removed much of the marginal to slight risk that
extended into the upper Ohio valley.  Model consensus shows that
the heavy rainfall threat has momentarily subsided across the
region.

Pereira

15z Update:

Convection is expanding in coverage near the Red River across
north central TX into southern OK. This is ahead of a shortwave
evident on the GOES airmass RGB and on the nose of increasing 850
mb moisture transport. This activity will continue tracking
northeast with time, with some continued backbuilding. Looking at
roughly 1-3" of rain with this activity along an axis from north
central TX into eastern OK, northwest AR and southern MO. While
wet, these areas have not seen as heavy of rain as locations
further southeast in AR. Thus grounds may not be quite as
saturated over these areas...and FFG is indeed higher here. Thus
in general current indications are that there may not be a ton of
overlap between the heaviest rainfall today and the axis of
highest amounts over the past few days. Thus, while significant
ongoing flooding is occurring, there is some hope that at least
for today additional rainfall may not exacerbate the problem
significantly. For that reason opted to shrink the moderate risk
area for day 1. Kept a moderate risk for portions of southeast OK
and west central AR...as here we will see some overlap of the
locally heavy rates and lowest FFG as the convection moves
northeastward today. Further southeast across AR additional
rainfall today should be lighter...thus a slight risk should
suffice for now. Also expanded the western extent of the slight
risk a tad northwest to account for our expected axis of highest
totals through the day...favoring an axis similar to the new 12z
runs of the ARW and ARW2. Do not think a moderate is needed here
at this point, as soils are not quite as saturated this far
northwest...but will continue to monitor.

The other area to watch is along the Ohio River near IL/IN/KY.
Some signal that enhanced moisture transport ahead of the wave
lifting northeast over the plains will result tin renewed locally
heavy convective activity later tonight. These areas are
saturated...and should this occur a flash flood risk would likely
develop. Thus will continue to monitor...and may need to consider
a moderate risk here on later updates depending on trends.

Chenard



...Previous Discussion...


Northeast TX/southeast OK---Arkansas---southeast MO---western TN
into the OH Valley

A very broad slight and moderate risk areas were maintained for
the upcoming day 1 period for yet another round of widespread
heavy to excessive rainfall amounts from northeast TX/southeast
OK---across much of Arkansas---southern MO and into the OH valley.
 The current training precipitation from SW to northeastern AR
into western TN may continue into the beginning of the upcoming
day 1 period.  However---the model consensus is then for
precipitation to begin to focus farther to the
northwest---initially from north central TX into southeast OK as
the low level southerly flow re-focuses across this area Friday
morning ahead of northeastward moving height falls embedded in the
southwest flow across the southern plains.  This developing
activity expected to then shift downstream into central to
northern AR---southern MO and into the OH valley. An overall very
favorable pattern will continue for widespread heavy to excessive
precip totals with an axis of much above average pw values---2 to
3+ standard deviations above the mean along and north of the
surface frontal boundary slowly moving northward day 1 across
these areas.  There are some typical model differences with the
latitude of the heaviest precip totals---with WPC leaning toward
the farther south solutions of the hi res arw--nssl wrf---HREF
mean.  This will support the heaviest rainfall and excessive
rainfall potential from northeast TX/southeast OK---central to
northern AR into the OH/MS river confluence area.  No significant
changes were made to the previous outlook for this period.

Oravec