Quantitative Precipitation Forecast
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870
FOUS30 KWBC 191506
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1005 AM EST Mon Feb 19 2018

VALID 15Z Mon Feb 19 2018 - 12Z Tue Feb 20 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SW OSC 20 NW CWGD 25 N CYXU 25 SSE CWSN 25 WNW HZY
10 NNW AKR 15 SW MFD 15 ESE MIE 15 W FOA 30 ENE UNO 25 NNW RUE
30 WSW RKR 15 S OKM 20 SSE CFV 40 NNW SGF 15 ENE COU 20 SSW UIN
15 NNW UIN 10 ESE FFL 10 NW MXO 20 NE DLL 15 SSE MBL 15 SW OSC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SW PRG 15 E ALN 20 N ALN BMI C09 15 NNE DKB 10 E SFY
20 E DBQ 10 ENE MSN 30 ENE MWC 15 SW GRR 10 ESE TEW DET
10 S CXHA 15 WNW FDY 25 SSE FWA 10 SW PRG.


...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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