Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Tue Apr 24 2018

Valid 12Z Sat Apr 28 2018 - 12Z Wed May 02 2018

The majority clustering of guidance suggests that the mean pattern
will settle into a configuration featuring a northeastern Pacific
ridge extending into Mainland Alaska and the Yukon along with a
Bering Sea/eastern Siberia trough that will likely contain one or
more closed lows.  Progressive flow around the trough and
into/through the downstream ridge should carry along a series of
weak or at least weakening frontal boundaries into the mainland
and Gulf of Alaska from the Aleutians/Bering Sea.  The fast nature
of this flow leads to low predictability for the details of
individual fronts and anchoring low centers though.

At the start of the forecast 00z ECMWF/ECMWF mean trends have led
to better agreement on a shortwave near 160W as of 12z Sat with
associated weakening low pressure/leading front.  Behind this
feature the guidance diverges with GFS and to some degree GEFS
runs being faster/more amplified than most other solutions for the
next bundle of shortwave energy (especially the southern part). 
Historical biases of GFS guidance tending to bring height falls
into downstream ridges too quickly favor some hedge away from the
GFS, but recent faster trends for leading shortwaves recommend
holding onto at least a minority component of its solution.

Spread persists/increases for trailing features with 06z/12z GFS
runs trending south of most other models and decent majority of
ensemble members over the Bering early next week while there is a
loose signal among the GFS/CMC/ECMWF mean for yet another feature
reaching the western/southern Bering by day 8 Wed.  The 06z GEFS
mean brings Bering sea low pressure somewhat farther east than the
ECMWF/CMC means mid-late period.

Across northern areas most of the operational guidance seems to
provide better resolution of upper low energy over/near the
Chukchi Sea compared to the ensemble means.  The 00z ECMWF may be
a little quick to bring in a shortwave ridge by 12z Sun though. 
Guidance consensus shows a stationary front along or a little
south of the northern coast for most of the period.

Based on early period considerations the initial forecast blend
emphasized the 00z ECMWF/06z GFS and to a lesser degree 00z CMC
Sat-Sun.  Then the increased detail uncertainty led to gradually
increasing input of the 00z NAEFS/ECMWF means, reaching 60 percent
total weight by day 8 Wed, while maintaining some operational
weight helped to provide some added definition--especially for the
core of the mean trough aloft.


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

Rausch