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Alaska Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1930Z Dec 09, 2018)
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 PM EST Sun Dec 09 2018

Valid 12Z Thu Dec 13 2018 - 12Z Mon Dec 17 2018

At the start of the period early Thu consensus shows a Gulf system
nearing the southeastern coast/Panhandle.  There are still
finer-scale detail uncertainties with guidance generally split
between the 00Z ECMWF/CMC means and to some degree 00Z CMC that
emphasize low pressure just east of the Kenai Peninsula and the
06Z GEFS mean/other operational models that show more
southeastward elongation/farther southeastward low track.  These
differences are well within typical error for day 4 forecasts so
an intermediate solution would be preferable.  Meanwhile the upper
low over/near the western mainland has adjusted a bit east from
yesterday's consensus as of Thu.  After that time there is a fair
degree of spread regarding the path of it or a significant portion
of its energy.  The 00Z ECMWF is somewhat extreme with its
relatively fast northward progression but GFS runs suggest an
eventual northward path as well.  Ensemble means as well as the
00Z CMC show a more gradual movement or even just settling over
the Bering Strait.  Specifics of this type of evolution tend to
have below average predictability so prefer an intermediate
approach with the first priority being to lean away from the 00Z
ECMWF.  By the end of the period next Mon any lingering energy may
end up being incorporated into a larger scale elongated
northwest-southeast upper trough.

During the first half of the period the forecast is still
generally on track for the very strong northern Pacific system
expected to pass just south of the Aleutians late this week.  Most
operational guidance continues to suggest the central pressure may
reach down at least into the 950's mb at the storm's most intense
stage.  Latest GFS/GEFS mean runs provide the one question mark. 
06Z/12Z GFS runs split the upper dynamics late in the short range
time frame, leading to two surface lows with the leading one
ultimately becoming the one that strongly deepens--and tracking
south of the more consolidated system represented by consensus. 
Although the GEFS means are more consolidated, some members may be
exhibiting an evolution like the GFS resulting in latest GEFS mean
runs trending weaker than other means or yesterday's GEFS means
for the surface low.  As a result prefer a starting point
consisting of the 00Z GFS/ECMWF/ECMWF mean for this system from
Thu into early Sat.

Then over the course of the weekend the 00Z ECMWF as well as GFS
runs since 00Z have adjusted toward a stronger ridge extending
from Canada across much of the mainland, hindering the eventual
northward progression of the surface low.  Ensemble means also
show evidence of a slower northward progression into the Gulf of
Alaska but so far do not yet show a pronounced stronger trend with
what ridging aloft extends into the mainland.  The CMC shows more
southerly flow aloft over the mainland.  A further complexity
involves how approaching Bering Sea/Aleutians shortwave energy
interacts with the system by next Sun-Mon with specifics remaining
far from being resolved.  Based on these considerations the
forecast preference by Sun-Mon adjusts toward 70 percent total 00Z
ECMWF mean/06Z GEFS mean input relative to a lingering 00Z
GFS/ECMWF component.  By this time frame the GEFS mean's weakness
is not as extreme so it could be including in the blend, albeit at
a lower weight than the ECMWF mean.

The leading Gulf system will produce some enhanced precipitation
and strong winds early in the period.  The trailing deep northern
Pacific system will generate a broad area of strong winds and high
waves.  Precipitation amounts over the Aleutians will be sensitive
to exact storm track.  Aforementioned uncertainties currently
lower confidence in the timing and magnitude of associated
moisture that could reach areas from the Alaska Peninsula through
the southern coast and Panhandle from about late Fri onward.


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