Prognostic Meteorological Discussion
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FXUS02 KWBC 260554

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
153 AM EDT Sat May 26 2018

Valid 12Z Tue May 29 2018 - 12Z Sat Jun 02 2018

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

...Overview and Guidance Assessment...

The models continue to forecast Subtropical Storm Alberto to move
onshore into the Gulf coast and move north and gradually recurve
under increasing influence of northern stream westerlies by mid to
late next week. A number of model/ensemble solutions suggests
potential for the remnant low to move across the Ohio
Valley/Appalachians and reform as an extratropical low over the
Mid-Atlantic. Please refer to products issued by the NHC for the
latest information and forecast for Subtropical Storm Alberto.
The 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
and was also closest to the official NHC forecast track. Thus, the
12z ECMWF and ecmwf Ensemble Mean was weighted heavily in the WPC
forecast. The 18z GFS was not included as its timing was faster
than the NHC track.

Elsewhere, upper low initially in place in Wyoming 12z Tue 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
crosses the Great Lakes. The trough also may phase with the
remnants of Alberto as its upper trough moves across the Ohio
Valley, maintaining timing and phasing differences that lower
confidence in the forecast and lead to more weighting on a
multi-model and ensemble consensus until better agreement develops.

Guidance continues to suggest trough amplification along the West
Coast Tue night-Wed, with heights falling across the western U.S.
as the trough moves inland by Thu-Fri.
Model agreement breaks down as there are timing differences
regarding the next upper trough to eject east from the Aleutians
across the north Pacific towards the Pacific northwest.  The 12z
ECMWF was on the amplified and fast side of the suites of ECMWF
model/ensemble solutions, as well as other operational models.
This faster solution in turn ejected the Great Basin trough faster
east Late Thu through Fri. Weighting of ensemble means was
increased Thu-Sat 02 Jun to account for these differences, with a
more persistent western trough than the operational 12z ECMWF the
result of the preference.

...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.
See the latest NHC advisories for the complete list of hazards.

Heavy rains will be possible across much of the Southeast and
lower MS Valley as Alberto tracks inland. 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
rainfall is expected to expand into portions of the Ohio
Valley/central Appalachians and possibly the Mid-Atlantic.

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 trough
traverses the region.

Above average temperatures are expected next week across the mid
MS Valley to the  southern Plains and lower MS Valley, where highs
are expected to reach 10 to 20 deg F above average. A number of
record high temperatures could be in jeopardy next week in these
areas. Highs from 100 to 110 degrees are expected the latter half
of next week in west to central portions of Texas.


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

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