Prognostic Meteorological Discussion
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000
FXAK02 KWNH 192012
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Thu Apr 19 2018

Valid 12Z Mon Apr 23 2018 - 12Z Fri Apr 27 2018

...Guidance/Uncertainty Assessment...

Models/ensembles showed generally good large scale agreement
through much of the extended forecast period, but with some
important differences in the details that increase through time.
The 00Z ECMWF/06Z GFS offered the best consensus during days 4-5
(Mon-Tue), as a deep upper low near the Aleutians drifts east, and
additional shortwave energy (and the associated surface low)
crosses the Gulf and enters Southeast Alaska. By day 6 (Wed),
deterministic model differences begin to increase steadily through
the remainder of the forecast period. While models remain
relatively similar aloft, differences exit as to the exact surface
low track from the Bering Sea, and whether it takes a more
easterly track along the southern AK coast or takes a more
northeasterly track into central AK. Ensemble low scatter plots
show a fairly uniform spread, suggesting that a compromise
approach is best. Thus, began gradually increasing ensemble mean
(00z ECENS/06z GEFS) weighting from day 6 onward. Despite smaller
scale differences, confidence remains perhaps a bit above average
at the large scale even through day 8 as the upper low drifts
slowly east into the Gulf while ridging builds across the Bering
Sea in its wake.


...Weather/Threats Highlights...

A couple low pressure systems south of the deep upper low will
keep rain and snow showers commonplace across the Aleutians and
southern/southeastern Alaska Mon-Tue. Locally heavy precip will be
possible, especially across windward terrain. In the wake of these
systems, the lingering upper low slowly drifting southeastward
will continue precip chances across the same areas through the
duration of the forecast period. Temperatures will be near or
several degrees above average through the period, especially
across central Alaska.


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html

Ryan


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