Quantitative Precipitation Forecast
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000
FOUS30 KWBC 201508
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1007 AM EST Tue Feb 20 2018

VALID 15Z Tue Feb 20 2018 - 12Z Wed Feb 21 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
HNB 10 ENE DYR 15 SSW PBF 15 W LFK 10 SSE 3T5 15 SE RND
10 SSE ERV 35 W 6R9 15 NW 7F9 15 NNW 7F9 20 WNW 1F0 15 NE GOK
30 E WLD 25 ESE UKL 20 NNE IRK JVL 15 ESE HTL 30 SSW CWWX
30 NNE CXPC 20 WSW CYWA 40 SE CWRK 10 S CXDI 10 SW FDY HNB.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
IND 15 ENE M30 15 ESE JBR 35 N ELD 35 S PSN 40 ENE LHB ATT
15 SW 6R9 35 NNW 6R9 15 S 7F9 15 WSW SEP 10 NW GLE 10 WSW ADH
30 ENE SWO 25 ENE CNU 35 S IRK 15 E SQI 20 SSW RQB 30 N CWLS
10 SSW CYKF 10 SSW TTF IND.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 ESE PNT 10 SW BIV 25 ENE GRR 10 SW RNP 10 SE JXN 20 N GUS
MTO 20 S DEC 15 N DEC 10 ESE PNT.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NE GGG 10 ENE PSN 40 S CRS HLR 35 WNW GRK 20 SE SEP 15 W FWS
15 N AQR 15 W SLG 15 SW SGF 30 S TBN 45 ENE UNO 20 SSW POF
10 NNW JBR 15 E LRF 30 NE GGG.


15Z UPDATE:

Convection continues to expand in coverage this morning over a
large portion of the the southern plains northeast into the the
mid MS Valley and portions of the OH Valley. The overall evolution
of the convection seems to be coming a bit clearer. The heaviest
magnitude of rainfall through day 1 appears to be setting up from
northeast TX into southeast OK and portions of western AR. Note
two areas of 850 MB moisture transport maximums this morning...one
over central OK (responsible for the developing elevated
convection there) and the other over TX (responsible for the
growing convection there). These two areas should merge over
southeastern OK and northeast TX over the next several
hours...which should expand convective coverage and intensity over
those areas. Anticipate this setup will result in an increasing
southwest to northeast training threat over this
corridor...although some southeastward progression is still likely
through the afternoon. The strengthening upper jet to the north is
enhancing right entrance jet dynamics...which will help more well
define the cold front and progress it off to the east. However,
overnight as the forcing pulls away to the north...the
southwestern edge of the front will try to hang up over northeast
TX, far southeast OK and western AR...continuing the potential
training threat there.

Thus to summarize...the heaviest magnitude of amounts and greatest
flash flood risk is probably setting up from northeast TX into
southeast OK and western AR...with 3-6" of rain in that axis
through 12z. Amounts should be lighter across the northern extent
of the moderate risk over northern AR and southern MO...where
instability is less and the front should be more progressive.
Still looking at 1-4" here though, and with FFG lower than further
south, a flash flood risk still exists. The northern moderate risk
looks good as well across portions of IL/IN/MI. This area has
received as much as 2-4" of rain already, with a multi model
signal for an additional 1-3". Also, looks like some convective
elements may make it this far north late this afternoon and
evening ahead of the cold front, which could result in some
locally higher rates...possibly resulting in a  flash flood risk
on top of the ongoing areal flooding. Overall we think the new 12z
high res ARW/ARW2 along with both the HRRR and EXP HRRRv3 show
reasonable evolutions over the day 1 period...although with the
normal minor axis differences still in place...will have to
continue to closely monitor trends.

Only minimal changes made to the moderate risk areas. Extended the
moderate a bit southwest in TX and cut it back a tad over southern
MO.

Chenard


...Previous Discussion...


...Eastern Plains / Mid-Upper MS Valley / Great Lakes / Upper Ohio
Valley...

A synoptic pattern that should spark great concern over heavy
rainfall continues to come into place, with large scale
amplification of the western U.S. trough, blocked by a 594 DM
ridge in the Atlantic, allowing for prolonged, elongated moisture
trajectories emanating from the Caribbean before encountering a
slow moving frontal zone in the middle of the nation. Moisture
quality looks more like March or April. Precipitable water
increasing to at least 1.50 inches southern Great Plains and
tapering only to around 1.30 inches to the southern Great Lakes
would represent max values for late February per GEFS M-climate
data. Expect repeated waves of moderate rainfall, and embedded
increasingly heavy rainfall through Wed/Thu in some areas,
resulting in a wide swath that will induce local flash flooding
and may overwhelm some main stem rivers. Especially concerned in
the southern Plains / lower Mississippi Valley with the greatest
available instability, and the southern Great Lakes where deep
frost depth/frozen ground and some snow melt water will exacerbate
the runoff/short-term flooding potential.

Although the QPF was not overly complicated on the large scale
with the synoptic pattern driving the situation, of growing
concern per the 00z guidance is the increased W-E spread with the
axis of heaviest rainfall, particularly over the southern
periphery where the CAMs (including the high-res means) are
trending a bit faster with the eastward progression in light of
(most likely) some convective feedback. For day 1, WPC utilized a
blend of the ECMWF, in-house ensemble QPF, and the previous
forecast, which continues to keep the max QPF axis a little
farther west than the ensemble of high-res guidance. This idea
represents a relatively smooth transition from continuity
(previous forecast), as given the highly-amplified pattern
(deep-layer s-sw flow) along with limited instability, feel more
confident in a slower eastward progression. The result is a
continual broad swath of 1.0 to 2+ inch areal average 24-hour
rainfall totals from Texas to Michigan, with the heaviest amounts
(areal-average 3+ inches) focused from northeast TX-eastern OK
into northwest AR and southern MO, where modest deep-layer
instability will be present to enhance the rainfall rates. Over
these areas, per the majority of high-res CAMs, localized 24 hour
totals exceeding 5 inches are likely -- and as such WPC maintained
a broad MODERATE risk area of excessive rainfall in the day 1
Excessive Rainfall Outlook, largely similar to the area advertised
in yesterday`s day 2 ERO.

The other MODERATE risk area was noted over portions of the upper
Midwest (eastern IL, northwest IN, and southwest lower MI) based
on the antecedent heavier rainfall/lowering FFG values through
early Tue and thus higher probabilities of 3-6 hourly QPF>FFG
exceedance.

Hurley
$$





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