Quantitative Precipitation Forecast
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527
FOUS30 KWBC 270842
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Sun May 27 2018

Valid 12Z Sun May 27 2018 - 12Z Mon May 28 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 ESE BTM 50 NE BIL 30 NE MLS 25 SSE DIK 10 SSW BIS 10 NE K46D
25 SSE VWU 25 S HIB SAZ 45 NW ABR 30 S PHP 30 NE OGA 30 SE GLD
25 ENE LAA 30 ENE LIC 40 SSW IBM 15 NE VDW 35 NNW ARL 40 SSW LND
40 NNE FIR 30 E PVU 35 SE PVU 50 SW PUC 50 NNE BCE 20 N BCE
30 E MLF 15 SE U24 30 NW U24 30 ENE ELY 10 SSW P68 30 N KTMT
30 N B23 45 WSW TWF 25 WNW JER 10 SW SUN LLJ 35 ESE BTM.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 ENE CYD 10 E PQL 10 NW MOB 30 WNW MXF 10 E 1A6 15 SSW AOO
25 WNW ABE NYC 65 SSE FOK 140 SSE HTO 200 ESE OXB.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SW MLS 30 WSW BHK 25 SE 2WX 10 ENE CDR 25 NNW TOR 30 NNE DRC
30 NW TMH 10 SSE HLD RIW 50 ENE JAC 10 NNE P60 30 SSE LVM
40 SSE 3HT 35 SW MLS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 E OXB 25 S OXB 15 SE CHO 30 ENE HSP 20 SSE W99 GAI
10 NNE WWD 10 NE WWD 60 SE ACY.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
85 NW KEY 40 S APF 30 ESE RSW 45 W FPR 10 SSW BOW 10 NNE SRQ
70 SSW SRQ.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 SE MTH 35 NNE MTH 25 WNW PBI 45 E SUA 15 W MYGF.


...Florida...
...Moderate Risk of Excessive Rainfall over South Florida /
Miami...
The forecast track of the center of Sub-tropical Storm Alberto has
been accelerating, but we will still hold off on Moderate Risk
probabilities over northern Florida until Monday when the system
is more certain to come ashore. Increasingly heavy rain may affect
the Florida panhandle late Sunday night.

Of greater immediate concern is the broader scale moist confluent
region to the east of the center and located within the parent
synoptic trough that helped form the cyclone. While convergence
and heavy rainfall was offshore early this morning, there is some
concern that the convergence pattern will migrate onshore from the
southeast as the cyclone passes off to the northwest. This could
be accompanied also by some boost to instability given the diurnal
cycle, although widespread cloud cover early in the day does
complicate the forecast. Model QPF signal is variable, with some
indicating no focused rainfall, others indicating it offshore, and
others forecasting a sort of middle ground heavy rain event. The
HRRR, however, along with RAP mass fields, have started to
indicate very heavy rainfall, perhaps greater than 5 inches
locally. The HRRR was not verifying too well as of 08Z, but given
the synoptic flow pattern should back up westward over time, and
if the HRRR signal of 7-plus inches were to be anywhere near
realistic over the urbanized portion of south Florida, we chose
the path of least regret and introduced a Moderate Risk of
excessive rainfall.

...Southeast U.S. from Alabama to the Carolinas...
Meanwhile the difluent region farther north and east of the Gulf
coast trough will migrate up through southeast Georgia and into
the Carolinas with time. Expect again heavy areal average rainfall
and embedded totals exceeding 3 inches through Monday morning.
Given potential for tropical / warm rain process - rainfall, along
with embedded convective elements, isolated very heavy totals are
possible, and may overwhelm Flash Flood Guidance.

...Mid Atlantic...
A shortwave trough is expected to ignite a convective event early
this morning from PA to the NJ coast, with other scattered
activity in the moist plume over Virginia to southern Maryland. In
the wake of this wave, convergent 850-700-mb flow is forecast to
set up during peak heating from central Virginia to southern
Maryland. The hi-res models, on average, light up this axis with
organized afternoon thunderstorms and locally heavy rainfall. The
global models, however - ECMWF and GEFS ensemble 6-hour
probabilities - have shown a slight trend to the north which could
place more heavy rain into the Washington D.C. vicinity.
Precipitable water values remain greater than 2 standard
deviations above climatology, supporting locally heavy totals,
especially where any brief training occurs within deep westerly
flow. Soils also remain relatively moist given a very wet month of
May in the region.

...Northern Rockies / Great Basin / Plains...
Simulated reflectivity from the hi-res models suggests storms will
move relatively quickly in Wyoming, but unusually large CAPE there
may drive intense short term rain rates, and when combined with
terrain could lead to flash flooding - supportive of a Slight Risk
of excessive rainfall. Marginal Risk extends again into the Great
Basin where storms will have little organization, but the wet
ground conditions continue to feed back into supporting diurnal
slow moving thunderstorms. Marginal Risk also extends into the
Plains where repeated rounds over the northern Plains and/or cell
mergers on the central High Plains could lead to a few issues.

Burke
$$





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