Quantitative Precipitation Forecast
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FOUS30 KWBC 151531
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1131 AM EDT Sun Apr 15 2018

VALID 15Z Sun Apr 15 2018 - 12Z Mon Apr 16 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WSW GWW 25 WNW CHS 35 ESE VAD 30 SW 40J 30 S TLH 20 SE ABY
35 SSW WDR 30 NNE 47A 20 NW RHP 45 SW TRI 15 NE LNP 20 WNW CRW
20 S ZZV BJJ 15 N LPR 25 NNW CGF 10 SSW HZY 15 E BTP 20 SW FIG
20 S ELM 15 NE SEG 30 SSW AVP 15 SSW MSV 10 SW DXR 10 SE FRG
15 WNW ACY 25 W MFV 10 WSW GWW.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NE GRD 10 ENE CEU 25 SSE RHP 10 NE RHP 20 NNW AVL TRI
15 S 6V3 15 SSE I16 20 SW 48I 15 ESE PKB 35 S HLG 15 WNW MGW
20 SW 2G4 10 SSE CBE 20 N HGR 15 WSW CXY 15 S LNS MTN ADW RMN
15 WSW LKU 10 NW LYH 10 N MTV 10 NNW EXX RUQ 20 S UZA 20 NE GRD.


1500 UTC Update...

Minor changes were made to the day 1 ERO, most notably trimming
the western periphery of the MARGINAL area across western GA.

Otherwise, an MCV or convectively-aided vort max lifting northward
across GA, along with an expanding area of elevated instability
(MUCAPES 200-500 j/kg), will spread widespread convection and
heavy 1-3 hour rainfall rates into the elevated terrain across
Upstate SC and western NC this afternoon. The 14Z HRRR picks up on
the latest mesoanalysis reasonably well -- noting a swath of 3-5"
amounts just east of Asheville. Have maintained a SLIGHT risk over
this area given the antecedent soil moisture conditions and latest
FFG; however, will continue to monitor trends for a potential
upgrade to a MODERATE risk this afternoon.

Hurley

Previous discussion below..


...Southeast into the Mid Atlantic...

Convection is ongoing as of 08z across eastern MS into the FL
Panhandle. This convection should maintain as it moves across the
southeast today. As we head through the day, instability should
increase out ahead of the line, with the trough to the west also
taking on more of a negative tilt with time. The combination of
this increasing instability and increasing upper level divergence
ahead of the trough, should allow for an intensification of the
line by mid to late morning into the evening hours. In general the
convective line will remain progressive in nature, likely limiting
the potential for any extreme rainfall amounts. However, deep
layer mean winds parallel to the line supports at least some south
to north training of cells embedded within the line. Also, current
convection is actually forming ahead of the true cold front, thus
the potential exists for additional storms to fire on the true
cold front as well, which would result in multiple rounds of
locally heavy rainfall in any one location. Overall, this looks
like a widespread 1-3" rainfall from the Southeast into the Mid
Atlantic region...with the aforementioned training and repeat cell
potential allowing for localized 3-5" amounts. Impressive 850 mb
moisture transport and PWAT vales exceeding the climatological
90th percentile certainly support these totals and the potential
for high rates. Thus at least some flash flood risk exists where
any cell training or repeat convection is able to occur.

The orientation of the line will likely be more favorable for
short term training and repeat cells from northwest NC into
portions of WV/VA/MD closer to the negatively tilting trough and
better mid/upper level forcing. Strong southerly flow ahead of the
low should help enhance rainfall totals some into the southern and
eastern slopes of the Blue Ridge and Appalachians as well. Thus
will carry carry a Slight risk from northwest NC into eastern WV,
western and northern VA into MD. It is across these areas where
the best chance for localized 3-5" totals exists to go along with
a bit lower FFG. For this issuance opted to include the
DC/Baltimore urban corridor as well. Dry conditions of late should
help, but this corridor is still more susceptible to flash
flooding...and note several HREF members and the 0z extended
experimental HRRR showing localized 3-5" totals in the
vicinity...and most of this would fall over a short period. Thus
while not a sure bet, think the conditional threat for flash
flooding is high enough to warrant the slight risk inclusion at
this time.

Instability will be greater across SC/NC...so certainly some
chance the convective line really gets organized there this
afternoon...which could block moisture transport and limit amounts
a bit further northwest over southwest VA and far northwest NC.
Tough to nail down these details though with much lead time, so
will need to monitor trends. Given this potential, also opted to
include the Charlotte metro in the slight risk, given their
susceptibility, and to account for the possibility of a further
south focus.

Will extend a marginal risk into the Piedmont of NC/VA. Still some
threat for flash flooding here, however the threat for localized
3" plus appears lower, and FFG is higher. Further south the line
should generally be more progressive, keeping the flood potential
lower. Although did note some of the CAMs depicting some locally
heavy totals, and with the moisture profiles in place, thought a
Marginal would be warranted.

Chenard


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