Quantitative Precipitation Forecast
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FOUS30 KWBC 241500
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1059 AM EDT Thu May 24 2018

Valid 15Z Thu May 24 2018 - 12Z Fri May 25 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 SSE HXD 20 SSW SSI 30 W VQQ 25 NNW 40J 10 ENE DTS 15 S BFM
30 SW HDC 25 ESE ACP 10 ESE ESF 40 ENE HEZ 25 WNW GZH 25 W LGC
RYY 15 N FQD 10 W EXX 15 NE SOP 10 NNW OAJ 30 SW MRH 65 ESE SUT
70 SSE SUT.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NW SLN 10 S GCK 45 NNW LAA 35 W ITR 30 E MCK 25 N OFK
10 NNW OVL 10 S LUM 15 SW MCW 30 WSW DSM 10 ENE FNB 10 NW SLN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNE CIC 15 NE O54 40 W SIY 40 NNE MFR 60 NNW LKV 20 W BNO
50 SW ONO 55 E REO 20 N WMC 10 NE LOL 55 S NFL 20 SSW BIH
25 NE FAT 30 ENE MOD 15 WSW BLU 20 NNE CIC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 S LZU UZA 10 ESE UDG 15 ESE JYL 15 SSE VDI 25 NNE MGR
15 NW 11J CSG 30 S LZU.


1500 UTC update

No changes made to the previous Excessive Rainfall Outlook.

Oravec



...Southeast States...
A shear axis and embedded circulation centered around 700 mb will
provide some level of organization to diurnal thunderstorms over
Georgia and South Carolina. With slow cell movement but also a
lack of deep layer shear this afternoon, storm cells will be both
heavy-rainers but also short-lived. The greatest duration over a
given spot should occur where CAPE and low level inflow are
maximized. This especially describes central to eastern Georgia
and western South Carolina, where low level inflow will be
slightly enhanced by the presence of the 700 mb circulation over
central GA. Model CAPE fields also would support some propagation
of heavy rain up across the border into southern North Carolina.
Models also signal heavy rain around the southern periphery of the
low in somewhat a repeat of what occurred Wednesday. Moisture
content was well above normal, with PW values greater than 1.75
inches in the pre convective environment over Georgia, and greater
than 1.50 inches all along the Gulf Coast. This should support
2-inch per hour accumulations, and occasionally more prolific
rates wherever cell mergers occur. Given this potential, we
expanded the coverage of both Slight and Marginal Risk areas.

...Central Plains...
A definable northern stream trough will bite into the ridge over
the northern Plains and upper Midwest today, producing
thunderstorm clusters in a near-surface convergence axis /
pre-frontal convergence - from the Dakotas to Kansas and eastward.
There is no clear focus for a heavy rain event. Flash flood
potential will depend more on storm scale outflows and
interactions not very well knowable with much lead time. Certainly
parts of eastern Nebraska
and also parts of Iowa and southern Minnesota are a little more
susceptible right now, per Flash Flood Guidance values. Based on
the best guess deterministic QPF process, we suspect that
thunderstorms forming in upglide this morning around Omaha will be
followed by the more surface-based storms breaking out on the high
plains / western KS and eastern CO this afternoon, and also other
clusters over the northern Plains at that time - closer to the
strongest deep layer ascent. Storms to the north will potentially
cross some saturated soils and low FFG values, while storms to the
south have more potential to train, forming along an axis parallel
to the steering flow in CO/KS/NE.

...Northern California / Western Nevada / Southern Oregon...
Dewpoints remained in the upper 40s and low 50s from northern
California into the northern Great Basin during peak heating
Wednesday. Expect similar values today, and the moisture through
the column - PW - is forecast at around 2.0 standard deviations
above climatology. When combined with the development of an upper
low just off California and difluent upper flow extending inland,
we expect relatively robust deep convection in the late afternoon
and evening hours. Storms may start over the higher terrain of the
Sierras and Shasta Range, and then propagate into lower terrain on
all sides, with a gradual migration northward per the mesoscale
guidance. The degree of lift could support cells regenerating
quickly along outflow boundaries so as to lead to some training
and flash flood concerns in the mountainous terrain. The threat
could become particularly focused on the south and southeast flank
of outflows, facing the southeasterly cloud-base inflow. This
specific risk, however, represents a fairly specific forecast on a
small scale, so on the larger scale we maintained only Marginal
Risk probabilities, but did expand the size of the risk area.

Burke
$$




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