Quantitative Precipitation Forecast
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FOUS30 KWBC 211555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1055 AM EST Wed Feb 21 2018

VALID 15Z 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 SE 3I2
30 SSE JKL 25 N CHA 25 SW 3A1 15 NE JAN 30 W MCB 10 NW LFT
20 E KXIH KBBF 30 NW KOPM 10 NNW BKS 30 NNW HBV 25 WSW UVA
35 W ECU 30 S E29 10 E E29 30 NE JCT 35 NNE 6R9 15 E TKI
15 S FLP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW KBQX 10 N VCT 25 NE LHB 15 NW 3T1 10 NE MWT 15 SSW BVX
30 ENE M30 25 SSW CVG 45 WNW HTS 25 SW HTS 20 SSW LOZ 35 W CSV
30 NNW HKS 45 SE ESF 15 N KVBS 15 WNW KGVX 15 WNW KBQX.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ESE OLV 20 ESE GLH 20 WNW TVR ESF DRI 30 NW BPT 30 E UTS
20 E JSO 30 NW SHV 25 ESE M89 NQA 25 ESE OLV.


15z Update:

Only minor adjustments made to the going risk areas. Will continue
to see a broad area of moderate to locally heavy rainfall from
east TX northeast in portions of the TN and OH valley. Across the
northern portion of this area a lack of instability will keep
rates relatively low...however increasingly saturated ground will
result in a continued flooding threat. The area of greatest
concern of heavy rainfall rates continues to be from east TX into
central and northern LA into southern AR. It is here where the
frontal boundary is expected to stall out...with continued strong
850 mb moisture transport into this boundary supporting a
convective training threat into this evening. Just got a look at
the new 12z high res guidance...and while not perfect...the
general evolution depicted by the 12z ARW and ARW2 both seem
reasonable...along with the 12z experimental HRRRv3. These all
depict a swath of 3-5" of rain within the moderate risk area (most
likely from east central TX into central/northern LA) with
isolated 5-7" amounts possible through this evening.


Chenard


...Previous Discussion...


...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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