Quantitative Precipitation Forecast
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FOUS30 KWBC 210820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Wed Feb 21 2018

VALID 12Z Wed Feb 21 2018 - 12Z Thu Feb 22 2018

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 S FLP FWC 30 E MIE 20 NW BJJ 20 W BVI 25 S HLG 15 ENE 3I2
10 NW SME 35 SSW MQY 30 NE JAN 25 S HEZ 20 E KXIH 10 N KBBF NQI
35 E COT BMQ 15 E TKI 15 S FLP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SSE GVT 30 NNW LRF 10 NE EVV 15 SE BAK 10 NNE MGY 25 SE ILN
25 ENE LEX 15 SW GLW 30 NNW HKS 10 WNW CWF 20 ESE LBX 25 SSE PKV
15 WNW VCT 20 WNW LHB 15 SSE GVT.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ESE OLV 25 E GLH 30 NNW ESF 25 WSW JAS 10 SSE UTS 40 NW UTS
15 SSW 4F4 25 ESE M89 NQA 25 ESE OLV.


...Eastern portions of the Southern Plains / Lower-mid Mississippi
Valley / Ohio valley...

A synoptic pattern driving a prolonged heavy rain event, with
embedded periods of intense rainfall, continued to evolve, 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 across eastern TX into the
lower-mid MS Valley and OH Valley. PWAT values remain well above
normal, with near record to record values stretching from east TX
all the way into the Great Lakes region. Shortwave energy ejecting
through the upper Midwest had tightened the deep layer gradients
and associated broad upper level jet streak. Increased ascent in
the right entrance region of this jet will sharpen the cold front
over the middle of the country, and help push it eastward with
time. From the mid MS valley northeast into the OH valley and
Great Lakes, the front will be more progressive in nature, helping
cap rainfall magnitudes. Still looks like a broad swath of
moderate rainfall totals though, and some weak instability may try
to move north ahead of the eastward moving front. Given the deep
frost depths and increasingly saturated conditions over this area,
should these higher rates of between 0.5"-1" in an hour
materialize, a flash flood threat is possible on top of the
ongoing areal flooding.

The focus for heaviest rainfall through the day 1 period will be
along an axis stretching SW-NE from northeast TX into much of AR,
northwest LA, northwest MS, western TN, southeast MO, and into
western KY, southern IN, and southern OH. Continued robust
(anomalous) low-level moisture transport into the slow-moving
frontal boundary (accompanied by additional shortwave energy
emanating from the subtropical jet), in tandem with the strong
right-entrance forcing (upper divergence/lower level
frontogenesis) from the broad upper jet streak to the north, will
support and increase convective coverage/intensity through the
day. Convection should be able to tap into enough instability
(MUCAPES averaging 500-1000 j/kg) to allow for a more quasi-linear
look to the activity with time, as some of the stronger segments
may exhibit a more pronounced eastern shift compared to the rest
of the system. WPC continues to address this idea with the latest
QPF as well as the day 1 ERO -- with the eastern periphery bulging
a bit more E-SE than the global model consensus (again owing to
the consensus of high-res CAM guidance, especially the WRF-ARW and
ARW2). Highest 24 hour areal-average totals (3-4+ inches) were
noted from northeast TX into northern LA, southern AR, northwest
MS, and far western TN. Per the CAMs, localized 24hr totals of 7+
inches cannot be ruled out within the MODERATE risk area in the
Day 1 Excessive Rainfall Outlook (ERO).

Hurley
$$




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