Quantitative Precipitation Forecast
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FOUS30 KWBC 271929
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 PM EDT Sun May 27 2018

Valid 1928Z 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 ENE GXY 25 WSW PUM 15 WSW CTD
45 NNW RKS 45 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
105 ENE HSE 90 ENE FFA 10 NE LFI 30 SE LYH 15 SSW MTV
10 SSW INT 10 WSW TTA 15 WSW RWI 25 S ASJ 25 WSW MQI HSE
80 S HSE.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 ENE OXB 25 SSE OXB 10 SW LKU 25 E HSP 25 N HSP 20 W W99
15 S CBE HGR 20 NW APG 25 ESE APG 10 SSE MIV 25 SSE ACY
40 ESE WWD.

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
30 NE KIPN HRT 15 WSW WRB 10 W CAE MEB 10 NW OAJ 30 S MRH
100 SSE SUT 130 SE CHS 195 ENE DAB.

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.

1500 UTC Update: Given the latest observational trends (satellite
and composite radar) along with the recent CAM guidance, have
opted to drop the small Marginal risk across southwest FL. Earlier
forecast thinking, supported by the guidance at the time, was that
within an overall Slight risk area would be a small region of a
more mitigated excessive rainfall threat. However, with the now
well-established inflow band east of Alberto and off the FL Keys
aligning across this region to include FMY and PGD late this
morning, such is not the case, and as a result have reverted back
to a broad Slight risk over FL (with the exception of the Moderate
over the southeast peninsula). It is over this region (se FL)
where the antecedent conditions (previous heavy rainfall/lower FFG
values this morning), along with the continued multi-model signal
for additional heavy rainfall (3-5+ inches) within a more dominant
eastern inflow band coming ashore, would support a more enhanced
(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.

1920 UTC Update: Expanded the Slight risk a little farther north
(closer to the Mason/Dixon Line) based on the latest convective
trends (renewed/blossoming mid afternoon convection) and recent
heavy rainfall earlier in the day (lower FFG values with the
18Z-based guidance). This thinking was supported by the 12Z HREF
neighborhood probabilities of 3 hourly QPF>3 hourly FFG exceedance
(probs 40-60+ percent between 21-00Z)

...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.

Hurley/Burke
$$




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