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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2017Z Aug 10, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Fri Aug 10 2018

Valid 12Z Tue Aug 14 2018 - 12Z Sat Aug 18 2018

Model solutions present a complex and difficult forecast for
Alaska during the extended range. At least early in the forecast
period, model consensus was sufficient on day 4 (Tue) to justify
use of a slight majority of deterministic guidance (ECMWF/GFS) as
a basis for the forecast. This consensus appears to be a
reasonable depiction of the upper wave and associated surface
front crossing southern mainland Alaska and the Gulf. At the large
scale, ensembles show some support for breaking down the
persistent upper ridging across eastern Asia, but this looks to be
a rather slow process, and it is unclear whether this would be
enough to precipitate a large scale flow regime shift. This is the
conflict that emerges in the guidance by later in the extended,
with deterministic solutions/ensemble members split between
building ridging across the Bering Sea (in response to the ridge
breakdown farther west), or keeping a trough/upper low in place.
Additional complexities are added by the potential
arrival/interaction of Arctic shortwave energy with additional
energy crossing the North Pacific. As a result of these factors,
forecast confidence rapidly decreases after day 5. Ensemble means
(ECENS/NAEFS) were used as a majority of the forecast starting
point starting on day 6, with deterministic guidance eliminated
entirely by the end of the forecast period. The consensus of the
ECENS/NAEFS solutions was for a somewhat slower pattern shift
relative to what is shown by the GEFS, keeping negative height
anomalies in place across the Bering Sea through late next week.

Precipitation is expected to initially be widespread, and locally
heavy from the Interior to southern/southeastern Alaska in
association with the passing upper wave/frontal system on Tue. In
the wake of this system, precipitation should become more isolated
to scattered in nature, some some drying possible across the
Interior by the middle of next week beneath short wave ridging.
The next low pressure system should also begin to spread
precipitation across the Aleutians by mid to late next week,
although confidence is very low in this aspect of the forecast.
Additionally, some potential remains for Arctic low pressure to
bring precipitation to the Northern slope and northern interior by
late next week, although confidence is also low in this aspect of
the forecast.


Ryan


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