Quantitative Precipitation Forecast
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393
FOUS30 KWBC 221511
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1010 AM EST Thu Feb 22 2018

VALID 15Z Thu Feb 22 2018 - 12Z Fri Feb 23 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 N 48I 20 NNE CSV 15 ESE MDQ 10 WSW CBM 10 NW HEZ 10 NW POE
25 SSE 11R AUS 10 W ATT 20 ESE SEP 10 SSE AQR 25 ESE FYV
20 NNE BMG 10 WSW BJJ 10 W YNG DUJ 20 S FIG 25 WNW HGR
35 NNE SHD 15 N 48I.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 N POF 25 N HNB 15 NW CVG 15 NNW FFT 15 N BNA 30 NE MDQ
30 WSW 1M4 35 W GTR 20 NE TVR 40 NNE ESF 20 NNW ESF 30 NNW POE
10 NNW LFK 30 SSW PSN 15 S CRS 10 WSW JWY 20 WNW PRX 15 SSE FSM
35 NNW RUE 35 N POF.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SW SHV 15 W 4F4 10 ENE LBR 20 NNW MWT 40 SSE FLP 20 W ARG
CGI 15 SE CUL 10 E EVV 25 SSW OWB 50 NE MKL 25 SW MSL 30 SSW GWO
30 SE MLU 25 NE IER 30 SSE SHV 30 SW SHV.


15Z Update:

Overall the going forecast looks in good shape. Convection is
hanging on a bit longer than anticipated along the stalled out
front from northern LA into northern MS, and with saturated
conditions, this is resulting in a continued flash flood concern.
Still appears that this activity should gradually weaken through
the morning as the better forcing and low level moisture transport
lifts to the northeast. However the next wave is already moving
across TX as evident by the expanding convection there. This wave
should continue to enhance low level moisture transport into the
area as well as gradually lift the 925-850 mb boundary northward.
Thus would anticipate pockets of training convection to persist
into the afternoon...just shift a bit north into northern LA and
southern/central AR with time. Given this expected evolution and
wet antecedent conditions expanded the moderate risk slightly
south on the southern edge.

As the wave over TX ejects northeast convection will eventually
move back into much of AR and later into western TN/KY. Looks like
a decent setup for training convection behind this wave from
northeast TX across AR as persistent low level moisture transport
runs into the stalled out low level boundary trailing the wave.
Would anticipate most of this action to occur just on the cold
side of the surface front...although still some uncertainty on the
exact axis. Limited instability may cap the rates associated with
this convection...however with very wet antecedent conditions,
even amounts of 1-2" in a three hour period will likely cause
additional flash flood concerns. Thus the broad moderate risk is
warranted. In fact even expanded it a bit northwest on the
northern edge given some potential for this convection later today
into the overnight to spill a bit north of our previous thoughts.
Overall tough to pin down any axis that may see more intense
rainfall rates and flash flooding...however it is safe to say that
a significant flood risk continues over a large area through the
day 1 period.

Chenard


...Previous Discussion...

Northeast TX---lower MS valley---lower TN valley---OH valley

A broad slight and moderate risk areas were maintained for the
upcoming forecast period for what will likely be another round of
widespread heavy to excessive precipitation from northeast
TX---into portions of the lower MS valley---lower TN valley and
into the OH valley.  There is expected to be no big changes to the
broad southwest mid to upper level flow from the southern plains
northeastward into the lower to mid MS valley---Great Lakes and
into the northeast this period.  PW values in this southwest mid
to upper level flow will remain highly anomalous with values 3-4+
standard deviations above the mean.  The current axis of heavy
precip from northeast TX into the lower MS valley---lower TN
valley into the OH valley is expected to weaken after 1200
Thursday.    Precip is then expected to refocus late this
afternoon into Thursday night-early Friday morning in an axis
farther to the northwest from the current early morning activity
as low level flow re-strengthens out of the south and into the
frontal boundary stretching northeast from northeast TX across the
lower MS valley and into the OH valley.  Differences in model QPFs
revolve around the latitude of this next max axis.  Day 1 QPF
leaned toward the solutions that were farther to the south and
east---trending approx 40-60nm farther to the southeast from the
previous forecast.  There is a lot of run to run variability with
the QPF details at the shorter forecast intervals.  While the
second round of precip will likely overrun the above mentioned
frontal boundary fairly far to the north---WPC continued to favor
the max QPF axis solutions that were farther to the southeast
closer to the greater instability near the frontal boundary.
There will likely be another round of potentially training
convection between 0000-1200 utc Friday from northeast
TX---northeastward into AR---northwest MS---western TN---far
southeast MO into the lower OH valley as the low level flow
becomes focused out of the southwest in a region of favorable
right entrance region jet dynamics.  While the next precip event
is expected to be farther to the northwest than the current
activity---there will be potential for overlap of the two heavy
events across portions of northeast TX---northern LA---southern
AR..nw MS---western TN---far southeast MO into western KY.

Oravec

$$





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