Quantitative Precipitation Forecast
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550
FOUS30 KWBC 200845
QPFERD

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

VALID 12Z 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 25 E GTU 20 N BMQ
15 E 7F9 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 E PSN 25 SE ACT 25 NNE 05F 15 W FWS 15 N AQR 25 ENE GMJ
35 SW AIZ 25 ESE VIH 10 NNE FAM 20 NW CGI 30 NW BYH 15 E LRF
35 NW DTN 10 E PSN.

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.


...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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