Quantitative Precipitation Forecast
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000
FOUS30 KWBC 250807
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Fri May 25 2018

Valid 12Z Fri May 25 2018 - 12Z Sat May 26 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.


...South Florida...

With a shortwave trough dropping in to strengthen the broad upper
trough over this region, the models develop an inverted trough
axis over south Florida along with upwind convergence in the
strengthening and very moist southeasterly flow coming off the
Caribbean. There is an increased model signal for widespread 1 to
2 inch areal average rainfall, and the potential for locally more
intense amounts as the pattern remains locked in, allowing
repeated tropical convection into the Keys and parts of south
Florida. Precipitable water values will increase to 2.00 to 2.25
inches, representing 2.5 to 3.0 standard deviations from
climatology, which is very impressive for this latitude. The
WRF-ARW and NAM CONUS Nest, in particular, paint several inches of
rain, perhaps more than 5 inches, over parts of the Florida Keys
during the Day 1 period. It is not a sure thing because there is
no clear low level focus, but many of the ingredients for
excessive rainfall are moving into place, and support a Slight
Risk. Farther up the west coast of the peninsula, Marginal Risk
extends through Tampa Bay, with southeasterly flow favoring west
coast convergence.

...Mississippi to South Carolina...
Diurnal convection should again become fairly widespread in
coverage over the southern tier. The small vort max that has aided
in rainfall production over the southeast will lift a bit and
begin to shear toward the Carolinas. The pattern
overall still lacks much focus, however, with very weak winds in
much of the column over the eastern and southern states. Cells
will be slow moving, and mergers could lead to multi-inch rainfall
on very small scales, but predictability on those scales is
lacking. The risk of excessive rainfall is driven more by wet
antecedent conditions, especially in parts of GA, SC, and
especially as the mean QPF signal from the hi-res models is again
somewhat maximized in that neighborhood this afternoon. We
maintained a Slight Risk area here, surrounded by Marginal Risk
extending toward Mississippi where convective coverage may be
enough to warrant probabilities just a touch above climatology.

...Ozarks...
Early day warm advection followed by strong heating will again
allow for a few clusters of pseudo-organized convection in the
lower Missouri Valley and southern Plains. WPC favored many of the
details in the NAM CONUS Nest. The resulting WPC QPF places a
local maximum of areal average QPF over southwest Missouri and
northern Arkansas, as well as northeast Oklahoma. This area has
been in a wet pattern, warranting a small Marginal Risk area for
excessive rainfall.

...The West...
The event north of an upper low should transition to longer
duration stratiform rainfall behind the surface boundary as it
pushes out of northern California and south-central Oregon. An
environment more supportive of deep convection and/or more
repetitive convection should exist farther east, from southeast
Oregon into parts of northern Nevada and western Idaho.
Instability drops off with eastward extent through NV/UT given
drier air in that region, but thunderstorms should be able to make
some eastward progress before drying out. WPC favored the NAM
CONUS NEST for its more expansive coverage, but perhaps the HREF
mean for more toned down amounts. The odds of training cells
appears lower today with the low now closed. But hourly rain rates
will likely exceed a half an inch locally during the stronger
individual thunderstorms. Hi-res model hourly rain rates and
simulated reflectivity fields helped to delineate the more
convective environment and determine the shape of a Marginal Risk
area.

Burke
$$





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