Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
843 PM EDT Fri May 25 2018

Valid 01Z Sat May 26 2018 - 12Z Sat May 26 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 NW LOL 30 NW NFL 55 SSE NFL 45 N MMH 15 E TVL 55 S TVL
35 SE AUN 25 ENE OVE 20 ENE RBL 10 SSW RBL 30 ENE UKI 20 NNE UKI
35 E O87 35 ENE ACV 35 ENE CEC 70 ENE RBG 45 ENE RDM 15 SW ALW
10 NNE PUW 25 SSE MLP 35 SSW MSO 50 E MYL 30 SSW BOI 45 SE REO
20 ESE WMC 30 S WMC 40 WNW WMC 50 NW LOL.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 SW AAF PFN 20 ESE CEW 25 WNW NSE 20 N ASD 20 NNW ASD
20 E ARA 25 ENE CWF 20 E POE 15 ESE BQP LLQ 20 N HOT 40 ESE FYV
25 ESE SGF 30 ESE VIH SET 25 NNW SPI 20 S PNT 10 W DNV
10 ENE CUL 25 E CKV MDQ ANB 15 N CCO 10 SW GVL 25 ENE 1A5
10 S MRN 10 S JQF 20 E SSC 35 S OGB 15 NW LHW 30 NW VQQ
20 WSW OCF LAL 45 WSW SUA 45 ESE PMP 85 ESE HST 95 NW MUCC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NW MUVR 50 S KEY 15 SSW KEY 20 N NQX 45 WSW TMB 10 W MIA
25 E HST 65 ESE MTH.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 ENE OPN AND 25 ENE GRD 25 SW CAE 15 NW TBR 25 N DQH ABY
25 SSW LSF 15 E AUO 20 ENE OPN.


2200 UTC update...

...Ozarks into the lower-mid Mississippi Valley...

Updated the Marginal risk area based from the latest
observational/mesoanalysis trends -- which included paring the
western periphery given the airmass modification (developing CIN)
following earlier convection. Have also broadened the Marginal
risk up into the mid MS Valley and western portions of the OH-TN
Valleys, based in large part to (again) the convective
trends/mesoscale environment, but also from the more recent CAM
guidance. The deep layer shear (0-6 km bulk shear 20 kts or less)
would again favor more pulse-type convection, with the
organization and persistence of stronger clusters inhibited by the
subsequent cold pools/outflows. Still, with mixed layer CAPES of
1000-2000 j/kg along within such a moist environment (PW values
1.75-2.00 inches), hourly rainfall rates that approach or exceed
FFG (1.5-2.0" in a large portion of the outlook area) will be
possible per the latest HREF.

Hurley

Previous discussion below...


...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.

...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