Quantitative Precipitation Forecast
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359
FOUS30 KWBC 221926
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Thu Feb 22 2018

VALID 1925Z Thu Feb 22 2018 - 12Z Fri Feb 23 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SW FAM 15 N RSV GEZ 10 ENE MGY 40 ESE LUK 60 ENE MKL
25 ENE GLH POE 15 WNW UTS 10 NNW CNW FWS 10 SSE AQR 25 ESE FYV
10 SW FAM.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 N POF 10 WNW CUL 30 SSW BMG 30 N LOU 30 SSW FTK 15 NE MKL
35 N GLH 25 NNW IER 15 E OCH 10 W CRS 10 WNW GVT 20 WNW PRX
45 SSE MLC 10 SSW RKR 15 SSE FSM 35 NNW RUE 35 N POF.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 N ARG 10 SE CGI 30 SE HSB 35 SSW OWB 15 ESE DYR 30 E SGT
35 ENE ELD BAD GGG OSA 20 N LBR 30 N DEQ RUE 40 SSE FLP 20 W ARG
15 N ARG.


19z Update:

The below thinking still looks good. Just cut back on the eastern
edge of our risk areas with convection now focusing further west
from northeast TX into central AR. Used a 12z high res model
consensus and the new 18z WPC QPF to outline the areas. -Chenard

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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