Quantitative Precipitation Forecast
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FOUS30 KWBC 280827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon May 28 2018

Valid 12Z Mon May 28 2018 - 12Z Tue May 29 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NNW RAM 30 SE MEI 20 NW 1M4 40 SW MQY 20 SSW CSV 25 SE 1A6
15 S I16 15 SW 48I W99 2W6 45 ESE OXB 105 SE OXB 140 ENE FFA.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 N 20V 20 S BIT 40 N RKS 35 ESE JAC P60 45 N BIL 15 SSE OLF
30 WNW K5H4 15 E ROS 30 NW CMY 20 ESE LVN ETH 25 ESE MBG
30 W PIR 15 N VTN 20 NNW GRI 10 W SLN 10 N AVK 40 ESE HHF
35 S PPA 10 W BGD 25 NNE EHA 35 SSE ITR 15 N LIC GXY 50 N 20V.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 NNW KEY 30 SSE APF 15 E BOW 30 N DAB 40 E HXD 40 N CHS
10 SW OGB 25 WNW TBR 20 N DQH 20 W CTY 105 W PIE 140 S AAF
180 WSW SRQ.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 NW MLS 25 WNW GDV 20 NW K20U 45 SSW BHK 20 NNE GCC 10 S IDV
15 NNW WRL 15 SSW BIL 40 NW MLS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
GBD 45 NNW WWR 20 ESE LBL 30 NW GCK GLD 30 SSE AKO 35 ENE GXY
VDW 25 E ARL SIB 35 NNE BFF 30 S AIA LBF 40 E HLC GBD.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 S HRT 10 WSW GZH 15 WNW GZH 45 SSE TCL 20 NW BHM 20 SW 8A0
30 E GAD 10 SSE FFC 30 NNW ABY 20 WSW MGR 10 W 40J 55 ESE AAF
85 S AAF 110 SSW AAF.

HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 S DTS VPS 30 WSW TOI MGM 15 SSE ALX 30 NNW 11J 20 SW BGE
10 SSW TLH 20 E AAF 50 SW AAF.


...High Risk of Excessive Rainfall...
...Alberto / Florida panhandle / Alabama / Georgia...
There is a strong model consensus as to QPF along the track of
Alberto. Between the combination of shield precipitation of a
tropical nature and inflow bands - enhanced by frictional
convergence at the coast - the deterministic QPF paints 4 to 6
inch amounts over the Florida panhandle into eastern Alabama. The
deterministic QPF is typically conservative, being an areal
average. Isolated totals of 6 to 12 inches are expected, based on
experience with land-falling tropical and sub-tropical cyclones as
well as available hi-res model guidance. During internal
collaboration WFO Tallahassee made mention of hourly rain rates as
estimated by radar reaching 3 to 4 inches offshore this morning.
Diurnal heating may further boost instability this afternoon,
contributing to such rain rates in convective bands, including
within both the High and Moderate Risk areas which would extend
into Georgia and farther east in Florida. Then expect that the
core of the system may breathe in overnight, with heavy tropical
rain becoming focused up into central and eastern Alabama Monday
night.

Based on the pattern and the model QPF consensus, we upgraded to a
High Risk of excessive rainfall from just west of Tallahassee to
just east of Pensacola and extending northward to include
Montgomery, Alabama. We expanded Moderate Risk northward a bit,
and it now includes Birmingham.

...Southeast U.S. from Central Georgia to Southeast Virginia and
down through the Florida Peninsula...
Tropical moisture, strong May sun angle, and areas of forcing
associated with Alberto and its parent upper trough - support a
Slight Risk of excessive rainfall over much of the southeast
United States. The exception is a relative dry slot anticipated
over central and northeast Florida. In looking for focused,
mesoscale areas of rainfall, expectations are increasing somewhat
in the upslope regions of the Blue Ridge / southern Appalachians
over the western Carolinas. Models indicate some build of up
instability just upwind, and low level flow steadily increases
into the mountains later Monday. The flash flood threat will
likely increase in this area from Day 1 going into Day 2. For now
the region remains in Slight Risk probabilities on Day 1, with an
escalation to Moderate Risk on Day 2, in coordination with the
local offices.

Farther east along the coast, broad upper difluence and warm
advection will contribute to widespread generous rainfall. The
event over the coastal plain lacks focus, however, and there is
some varying model signal as to where the heaviest rain might
fall. This may also be the day that convergence in the trailing
moist axis sets up along the east coast of Florida, including
Miami - whereas this failed to set up on Sunday. WPC QPF leaned
toward the WRF-ARW2 for some of the details, and the National
Blend of Models also looked reasonable, especially with respect to
placement.

...Rockies / High Plains...
The long-lived upper low over the Great Basin will finally eject
toward the Plains during this period as the northern stream
becomes more active over the Pacific Northwest, providing a
kicker. Instability and upslope flow will continue to favor
northern Wyoming and southeast Montana for on and off rain and
thunderstorms. With the synoptic system on the move, inverted low
level troughing actually becomes quite focused in this area this
afternoon, and may support a sharply defined MCS, one that
potentially will be slow moving as it forms, feeding off deep
easterly low level flow. Down at the tail end of the upper forcing
there is also some concern for a slow moving MCS to form over
western Kansas.

The GFS has had some grid-scale feedback type 10-plus inch
bulls-eyes in this area, but it has done so consistently from run
to run. Very warm boundary layer will promote cold outflows, but
the enhanced inflow above the surface owing to the large scale
system ejecting - could support a slow moving system producing
very heavy rain. Much of the hi-res guidance, while not as
outlandish as the GFS, does paint heavy rain over western Kansas.
Everywhere in between, from northwest Kansas to southeast Wyoming
and western South Dakota, expect another active convective cycle.
Parts of this area - specifically the tri-state border region of
CO/WY/NE have experienced rainfall 3 to 5 inches above normal for
the past two-week period. We introduced a Slight Risk of excessive
rainfall here, from eastern Wyoming to western Kansas, and
separately for the potential in northern Wyoming / southeast
Montana.

Burke
$$




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