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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1457Z May 26, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1057 AM EDT Sat May 26 2018

Valid 15Z Sat May 26 2018 - 12Z Sun May 27 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 N KVQT 10 E KCMB BTR 35 SSE MEI 25 SW MGM 15 NNE OZR
30 W TLH 35 SSW AAF 20 E KIKT.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
135 WSW SRQ 80 W PIE 25 NNW PIE 30 S BOW 35 NE RSW 25 ESE SUA
30 SSE MYGF 30 WSW MYNN 95 SSW MYNN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NE WMC 40 ESE REO 25 NW JER 15 ESE BYI 35 S 77M 40 N ENV
45 S ENV 25 SSE ELY 80 SSW ELY 35 ESE TPH 45 NNW TPH 25 NNE KTMT
25 NE WMC.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW KY19 K20U 45 W JDN 30 ENE CWVN 15 W KD50 20 NNW N60
15 WNW KY19.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 ENE MYL SMN 40 E LLJ 15 N SUN 35 E BOI 40 S MYL 45 ENE MYL.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 ESE WAL 20 WNW JGG 20 NE DAN 10 SSE HBI 15 NE SSC 35 SSW OGB
15 ESE VDI 20 SE EZM 10 WNW EZM 15 SSE MLJ 25 NE MLJ IIY
15 N AHN 10 SSW LZU 10 WNW FTY 20 SSE 4A9 20 N 4A9 20 NNW CHA
45 SE FTK 10 SSW KHLB 25 ENE MIE 25 ENE FDY 15 ENE CAK BVI JST
20 NW CXY 20 NNE RDG 15 NW HPN 45 SSE MTP 135 S ACK.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
95 SW SRQ 60 WSW SRQ 20 W SRQ RSW BCT 40 ESE PMP 80 ESE HST
85 SE HST.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
60 ESE OXB 35 SSE OXB 15 N FYJ 10 WSW LKU 25 N LYH 25 SE BCB
10 NNE MRN 20 NNE AVL 45 NW AVL 45 SE 1A6 40 SE 1A6 20 NW TRI
15 WSW BKW 15 S EKN MRB 15 SSE PHL 35 E ACY 135 ESE ACY.


1500 UTC update

Primary change to the previous Excessive Rainfall Outlook was to
extend the slight risk region over the Mid-Atlantic to Central
Appalachians region farther to the south into far northeast
TN---Southwest VA and northwest NC.  This is for convection that
is currently enhancing across these regions.  The hrrr and hrrr
exp have the best handle on this activity at the moment.  The
remaining hi res models...arw...nmmb...nam conest are depicting
this region poorly.  The simulated radars from the hrrr and hrrr
exp show this activity maintaining itself into this
afternoon---supporting potential for additional heavy to isolated
excessive rainfall amounts across far northeast TN---Southwest VA
and northwest NC.  Please see WPC's mesoscale precipitation
discussion #0200 valid until approximately 2000 UTC that will be
issued shortly.

Oravec


...South Florida...
Sub-tropical storm Alberto is expected to become more organized
and strengthen. Most of the heavy precipitation will affect the
open waters of the Gulf, but also Cuba through Saturday night.
Some heavy rain will fall, however, over the Florida Keys and
south Florida. Alberto is embedded within what is still a weakly
baroclinic, broad scale trough, and the warm conveyor belt /
southerly flow is forecast to migrate out from the center toward
Florida by afternoon/evening. This should enhance convergence
within the very moist and moistening atmosphere that was in place
there. This sets up the increasing potential for tropical
downpours and the focused bands and training of echoes as Alberto
and its parent upper trough take shape. The hi-res numerical
models produce areal average 1.50 inches of rain roughly during
Day 1, but with embedded swaths that may exceed 3 inches. Given
the rich tropical environment, isolated much heavier totals are
possible wherever banding sets up, which is more likely in the
central and lower Keys and southwest Florida - exactly the area
where Flash Flood Guidance has come down a bit owing to rainfall
over the past week.

...Mid Atlantic to Ohio Valley to Northern Georgia...
Focus for more greatly organized thunderstorms is expected to
exist with a shortwave trough over the Ohio Valley and within a
confluent flow regime with strong moisture advection over the
Mid-Atlantic states. Certainly slow-moving heavy rainfall may
cause isolated flash flooding in the Ohio Valley. The Mid Atlantic
is of more concern, however, as the region will see a timely
injection of deep moisture during peak heating, resulting in
strong CAPE values. At the same time, flow fields will strengthen
in association with a confluent flow / jet entrance regime along
the east coast. The upstream shortwave trough approaching through
Ohio could also sustain additional elevated convection overnight
in the wake of the afternoon activity from Virginia through
Maryland and Delaware. Much of the region has been very wet the
past few weeks. In coordination with local offices we introduced a
Slight Risk of excessive rainfall.

...Gulf Coast...
Multiple days of sea breeze and other scattered deep convection
have lowered FFG values here. Cell motions are predicted at a very
slow zero to 5 knots today. The average of the hi-res guidance /
HREF Mean outlines areas of relatively greater convective coverage
expected from the Florida panhandle to southeast Louisiana. Cells
raining at a rate of 2 inches in an hour (very localized), and the
potential for random cell mergers could lead to flash flooding.

...Western U.S...
The upper low over Nevada will move eastward into areas that are
quite a bit drier in the low levels, so that much of the diurnal
thunderstorm activity on Saturday will occur in the post frontal
environment back across Nevada and up through the northern
Rockies. The models also hint strongly at MCS development over
eastern Montana to the north of a developing warm front this
evening. WPC QPF favored the WRF-ARW2 and NAM CONUS Nest. We
outlined a few Marginal Risk areas, mainly following the contours
of available CAPE as predicted in the NAM.