Quantitative Precipitation Forecast
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000
FOUS30 KWBC 200830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 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.


0100 utc update

No changes made to the previous excessive rainfall potential
outlook.

Oravec

...15Z Update...

In the Upper Midwest and Ohio Valley the axis of moderate and
steady rainfall was slightly north of most of the hi-res model
guidance, although the experimental HRRR and also 00Z NSSL WRF
were not too far off. We took a quick run forward through the QPF
production, and ended up boosting areal average amounts in this
region per the record to near record daily PW values and
persistent forcing mechanisms. This allowed us to connect across
the two small Slight Risk areas to yield an overall expansion of
Slight Risk.

A second area of troublesome convection was developing at 15Z in
eastern Oklahoma and southwest Missouri. No real reason to expect
it to dissipate through at least 18-20z, as confluent low level
flow is sustained beneath energetic southwesterly flow aloft. This
is a decent setup for training echoes, although the event is not
particularly well focused in terms of an anchor point for
near-surface convective initiation. We extended Marginal Risk down
into this region, and will re-evaluate later this afternoon. None
of the model guidance QPFs was particularly useful here.


...Previous Discussion...
...Mid-Upper MS Valley / Great Lakes / Upper Ohio Valley...

Continued amplification of the longwave trough west of the Divide
and resultant downstream ridging across the eastern CONUS will
lead to a highly -PNA toward the end of day 1 (12Z Tuesday), with
500 MB height anomalies around 2.5 standard deviations below/above
normal over the west/east respectively. As the pattern aloft
becomes highly meridional, the surface front draped SW-NE from the
southern-central Plains northeast into the upper Midwest-upper
Great Lakes will exhibit little eastward progression, with the
northern extension of this front (especially over the upper
Midwest-upper Great Lakes) becoming bolstered by the strengthening
upper jet to the north and thus increasing right entrance region
dynamical forcing. Meanwhile, moisture parameters will too become
highly anomalous during the day 1 period, with PW values rising
above 4 standard deviations above normal per the SREF and GEFS and
850 MB moisture flux anomalies peaking over +5 standard deviations
in light of the 50-60 kt moist southwesterly inflow into the
frontal zone. According to the atmospheric river detection from
the ESRL site, a clearly defined Atmospheric River is noted from
the Caribbean and western Gulf, with integrated water vapor
transport values exceeding 750 kg/m/s (IVT anomalies over +3
standard deviations).

From a dynamic and thermodynamic standpoint, precipitation
efficiency is about as high as it gets for mid February,
especially along/north of 40N, in the absence of sufficient deep
layer instability. While the latter will be a limitation to short
term rainfall rates, the robust degree of forcing and moisture
transport into the quasi-stationary front will support a rather
broad Marginal Risk for excessive rainfall during Day 1 across
much of the Midwest into the Great Lakes and upper OH Valley
region, with a focused area of Slight Risk where QPF is higher and
flash flood guidance values lower, from southern Wisconsin across
to lower Michigan and down into the Midwest/Ohio Valley. Over this
region, there remains a multi-model signal for heavy rainfall
along the axis of maximum PW values and 850-700 mb moisture
transport into the frontal zone, so by Monday night more areas
will become susceptible to flash flooding with the additional
rounds of heavy rain.

Hurley/Burke
$$





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