Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Thu May 24 2018

Valid 01Z Fri May 25 2018 - 12Z Fri May 25 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SW HDE 30 NE OFK 20 E FSD 10 NNE ILL 15 NNE RZN 25 SSW RCX
20 W ONA 10 NW EBS 15 NNW RDK STJ 25 SSW SZL 20 NNW UNO
20 NNE BVX SRC LIT 25 SSW RUE 15 ENE TQH 15 WNW RVS 10 SSE END
PTT 30 NNE GCK 20 ESE GLD 10 SW HDE.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 WSW 5J0 35 E BNO 10 N REO 60 W WMC 25 NE RNO 45 SE TVL
20 SW BIH 40 ENE FAT 30 SSW TVL 30 ENE RBL 15 SE MFR 70 SSW RDM
55 WSW 5J0.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 E SSI 25 NNE NRB 15 SSW VQQ 15 W GNV 20 NNW CTY 15 NW 40J
TLH 15 NW ECP 15 WSW HRT 30 SE PQL 25 S GPT 25 SSE HSA 10 W 9F2
25 N KEIR 10 SSE 7R4 15 E LFT 15 NE HEZ 15 NNW GWO 40 W TUP
20 ENE TUP 25 NNW TCL 20 SE BHM 20 WSW VPC 25 WNW AVL 15 SE SVH
10 WNW MEB 15 NW CRE 45 S MYR.


0100 UTC update...

Based on the latest convective trends, have removed the Slight
risk area across the Southeast.

Hurley


2100 UTC update...

Adjustments made to the day 1 ERO included expanding the Marginal
Risk area across the central U.S. and over the Deep South -- owing
to the latest observational/convective trends along with more
recent CAM output. Mesoscale-convectively aided shortwave over the
central high plains will be the impetus for new convective
initiation across NE-KS, activity that may struggle to grow
upscale later this evening as it pushes southeast into a more
difluent thickness pattern across western MO, northeast OK, and
northern AR as 0-6 km bulk shear values remain under 20 kts.
Plenty of deep-layer instability (MUCAPES 2000-3000 j/kg) along
with the relatively light flow/slow cell motions would favor the
potential for localized flash flooding underneath the isolated,
stronger storms. However, at the same time there is enough
directional shear in play to support forward or downwind
propagation, as depicted by the northwesterly Corfidi vectors
between 5-10 kts.

Elsewhere...have also broadened the Marginal risk across central
MS-AL, as well as northern GA, also in light of the late-day
convective trends. Again, relatively light deep-layer flow
(850-300 mb wind 10 kts or less) and attendant weak 0-6km bulk
shear would favor more isolated/pulse activity which, given the
available instability (2000-3000 j/kg) and PWATs (1.75 to just
under 2.00" per the GPS and satellite blended TPW), localized
hourly rainfall rates exceeding FFGs (which are generally
2-2.5"/hour) will be possible before these cells collapse from the
cold pools/outflows.

Hurley


...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