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497
FXUS02 KWBC 251601
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Fri May 25 2018

Valid 12Z Mon May 28 2018 - 12Z Fri Jun 01 2018

...Subtropical Storm Alberto expected to bring heavy rainfall to
portions of the Gulf Coast and Southeastern states...


...Overview/Synoptic Assessment...

Progressive flow across the northern Pacific Ocean will become a
bit less progressive across the CONUS, with a number of features
showing blocking tendencies. Negative height anomalies are
expected to persist across northeastern Canada/Greenland through
the period, with ridging dominant off the southeast U.S. coast and
across the central U.S. The track of Subtropical Storm Alberto
will be initially follow a weakness between the two ridges, before
coming under increasing influence of northern stream westerlies by
mid to late next week, with a number of model/ensemble solutions
suggesting the potential for the remnant low to move across the
Ohio Valley/Appalachians and perhaps the Mid-Atlantic. Please
refer to products issued by the NHC for the latest information and
forecast for Subtropical Storm Alberto. Elsewhere, upper low
initially in place across the Great Basin/Rockies on Mon is
expected to move northeast into the northern plains by Tue and the
Upper Midwest on Wed as it comes under increasing influence of the
northern stream, and height falls digging into western Canada. The
system is also expected to gradually lose amplitude as it
encounters broad anticyclonic flow across the Great Lakes.
Finally, guidance continues to suggest trough amplification along
the West Coast by Tue-Wed, with heights falling across the western
U.S. as the trough moves inland by Thu-Fri. Additional shortwave
energy may approach the Northwest by next Fri.


...Guidance/Predictability Assessment...

Model consensus continues to improve with respect to the track of
Subtropical Storm Alberto. The ECMWF has been very consistent with
the track of the system for a few runs now, and was also closest
to the official NHC forecast track. Thus, the 00Z ECMWF was
weighted heavily in the WPC forecast, with the 06Z GFS/GEFS and
00Z ECENS also included. The GFS was excluded after day 5 (Wed) as
the model appeared perhaps too quick to weaken the remnant low and
quickly move the mid-level system east. Farther north, model
differences with the energy exiting the Rockies into the northern
plains by mid-week were primarily related to amplitude, with some
timing differences emerging by Thu night-Fri as the wave
deamplifies across the Great Lakes. The aforementioned blend
appeared to represent a good consensus solution here. Finally,
along the West Coast, while all solutions agree on the arrival of
troughing by mid-week, deterministic solutions continue to differ
as to the exact character of the system. The ECMWF showed a more
fragmented trough with two embedded shortwaves, while the GFS
suggests a slightly more phased features and amplified trough.
Weighting of ensemble means was increased a big by the middle to
end of next week to account for these differences, with the blend
showing a trough axis reaching the West Coast Wed night and moving
into the Great Basin by Fri.


...Weather/Threats Highlights...

Subtropical Storm Alberto is expected to pose a significant heavy
rainfall threat across portions of the southern/southeastern
states by early next week, with multi-inch rainfall totals looking
increasingly likely for portions of the Gulf Coast States. Drier
air wrapping around the western side of the system may result in a
sharp gradient/cutoff of precipitation on the western side of
Alberto, although confidence in the specific location of such a
gradient is low at this point. Additional impacts along the Gulf
Coast such as tropical storm conditions and storm surge are
possible, although it is too soon to specify the exact location
and magnitude of these impacts. Additional heavy rains will be
possible across much of Florida and the Southeast as feeder bands
traverse the region east of Alberto. Ensemble guidance continue to
suggest the potential for a rainfall max across the southern
Appalachians and foothills, from portions of north/northeast
Georgia into western portions of the Carolinas, likely due to
upslope enhancement of precipitation. By mid-week, as the remnants
of Alberto move northward, the heavy rainfall threat may extend
into portions of the Ohio Valley/central Appalachians and possibly
the Mid-Atlantic. Farther north, locally heavy rains will be
possible across portions of the central/northern plains and
Midwest as a surface frontal boundary lingers in the region early
next week and upper-level traverses the region.

Above average temperatures are expected early next week across the
central U.S. and Midwest, where highs could reach 10 to 20 deg F
above average. A number of record high temperatures could be in
jeopardy across the central U.S. next week.

Ryan


WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4

$$