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FXAK02 KWNH 202020
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Fri Apr 20 2018

Valid 12Z Tue Apr 24 2018 - 12Z Sat Apr 28 2018

...Guidance/Uncertainty Assessment...

Models/ensembles show above average consensus with respect to
large upper low/deep surface low in the southern Bering Sea at the
beginning of day 4 (12Z Tue). Ensemble surface low scatter plots
revealed a tightly clustered consensus generally surrounding the
00Z ECMWF/CMC/GFS solutions, and these deterministic solutions
comprised a majority of the forecast blend during days 4-5. A
secondary low developing farther east along the occluded front is
forecast to move north across the Bering, just off the west coast
of Alaska. The 12Z GFS seemed too quick to develop and deepen this
low, and thus was discarded in favor or the 00Z run, which was
much closer to the aforementioned consensus. The associated
surface front is forecast to move east across the Gulf of Alaska
Tue-Wed, with a period of decent onshore flow across
southern/southeastern Alaska. While the upper low is forecast by
all guidance to linger across the southern Bering, and slowly
drift southeastward toward the Gulf by late next week, guidance
shows some degree of shortwave ridging building across the Bering
between the upper low and the next system. As the next relatively
high amplitude upper shortwave approaches the Bering late next
week, models show some significant timing differences, and the GFS
has continued to be a fast outlier, bringing another deep low into
the Bering at least a day sooner than the ECMWF/CMC, which were
much closer to the consensus. Due to the increasing spread,
ensemble mean (00Z ECENS/06Z GEFS) weighting was increased to a
majority of the forecast blend during days 6-8 (Thu-Sat), and the
GFS was removed from the forecast entirely in this time frame
given the large difference described above.


...Weather/Threats Highlights...

A period of strong onshore flow will be possible across
southern/southeastern Alaska Tue-Wed ahead of the strong Bering
Sea low pressure system. The onshore flow will result in numerous
rain/snow showers, with locally heavy precipitation possible in
favored windward terrain areas. Farther north, the upper low will
also produce scattered rain/snow showers across the Yukon Valley.
Even after the surface front passes by, the lingering upper low
slowly drifting southeastward will keep scattered precip chances
across the same areas through the rest of the week. The deep
Bering Sea low early in the period may also produce a period of
gusty winds especially for coastal areas of southern Alaska as
well as the Aleutians. By late in the week, rain/snow and
potentially gusty winds may begin spreading across the Aleutian
Chain as the next significant storm system approaches.
Temperatures will be near or several degrees above average through
the period across most of the state. An Arctic air mass will
linger north of the Brooks Range, however, with cold temperatures
early next week, and some slow warming occurring through the week.


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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