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FXAK02 KWNH 171914

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
313 PM EDT Sun Jun 17 2018

Valid 12Z Thu Jun 21 2018 - 12Z Mon Jun 25 2018

The most consistent and confident part of the forecast is for
western Canada mean ridging that may extend into Mainland Alaska
to some degree.  Other aspects of the forecast either display some
continuity adjustments from yesterday or at least maintain enough
spread/detail uncertainty to temper confidence.

Within mean troughing aloft initially over the western coast of
the mainland and portions of the Bering Sea, today`s guidance has
gravitated toward emphasizing upper low energy over/near the
western mainland more than a separate farther westward feature
dropping southward into the Bering.  Thus the forecast represents
an adjustment to this scenario relative to the intermediate
longitude of the overall trough axis (and a little more of a
Bering surface reflection) when greater spread existed yesterday.

Meanwhile there is still a decent model/ensemble signal for low
pressure tracking from the mid-latitude Pacific into the
northeastern Pacific/Gulf of Alaska during the period.  Individual
model runs continue to suggest that low pressure track and
intensity will be sensitive to the relative strength/possible
interaction among a leading shortwave aloft and trailing energy.
GFS runs are strongest with the leading feature aloft, leading to
a farther northward track.  On the other hand the 00Z CMC
ultimately places more emphasis on trailing energy which results
in a more southern track.  The 12Z GFS has backed away from the
northern edge of the envelope a bit but ensemble means do favor a
solution in the middle to northern half of the full solution
envelope.  Overall an average among the GFS/ECMWF and their
ensemble means offered a reasonable depiction of the most likely
scenario.  That said, new 12Z CMC/ECMWF runs maintain lingering
potential for a much more southern track so forecast confidence is
not particularly high.

Perhaps not surprising given recent spread/variability among
individual solutions, the Arctic still shows some disagreement on
specifics aloft.  The ensemble means still agree on development of
some high latitude troughing northwest of the mainland though with
an axis a bit eastward versus yesterday--especially in the ECMWF
mean.  A general model/ensemble mean blend provided a reasonable
starting point for the forecast.

Guidance spread/day to day changes lead to low confidence in
details across the western-central Pacific and Aleutians.  Over
the past day through the 00Z-06Z cycles there has been a trend
among the ensemble means and ECMWF toward a more
suppressed/disjointed appearance to low pressure (as hinted at by
06Z-12Z/16 GFS runs), with a wave tracking well south of the
Aleutians and another area of low pressure hanging back over the
western Pacific.  However some individual runs like the 00Z CMC
and 06Z parallel GFS as well as one or more members from each of
the GEFS/ECMWF/CMC ensemble systems still show potential for one
or more waves to reach at least as far north as the Aleutians.
The 12Z GFS has also brought low pressure farther northward.
Today`s forecast adjusted toward the majority solution that is
more suppressed but partial inclusion of the 12Z GFS at least
helped to lower surface pressures a bit near the Aleutians late in
the period relative to other guidance input.

The model/ensemble mean blend was weighted 80 percent in favor of
the operational 06Z-12Z GFS/00Z ECMWF days 4-5 Thu-Fri and then
gradually increased the 06Z GEFS/00Z ECMWF means so that the
forecast reached about even model/mean weight by day 8 Mon.


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

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