Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Wed Jul 04 2018

Valid 12Z Sun Jul 08 2018 - 12Z Thu Jul 12 2018

Most 00Z-06Z guidance and the 12Z GFS (plus other 12Z runs so far)
have furthered a trend that emerged in some of the 12Z/03 runs,
toward Arctic shortwave energy dropping farther westward than in
prior runs.  This adjustment--which results in stronger downstream
ridging aloft--has led to the GFS/ECMWF/CMC and their means
holding back Bering Sea low pressure originally associated with
T.S. Prapiroon which is currently in the process of becoming
extratropical.  Instead of tracking across the Alaska Peninsula
the majority scenario now has the Bering low gravitating toward
the western coast of the mainland, albeit with a fair amount of
spread for latitude.  The 00Z UKMET was the one holdout reflecting
the more progressive scenario from continuity.  Consensus is
sufficiently strong to favor adjusting to the current majority by
way of a blend among 00Z ECMWF/06Z-12Z GFS/00Z CMC runs early in
the forecast period.  This blend maintains the Arctic front that
drops into the northern mainland but the aforementioned trends
pull the western part of the front well westward.

Detail uncertainties steadily increase by mid-late period. 
Ensemble means are remarkably similar in keeping a mean trough
aloft near the west coast of the mainland through next Wed-Thu. 
However operational model runs show a variety of possibilities for
upstream energy that may dig into the mean trough and interact
with/influence the leading energy, affecting areas from the Bering
Sea/Aleutians into the Gulf of Alaska.  Particularly low in
confidence is the 00Z ECMWF evolution that results in another
strong Bering low.  Recent GFS runs are trending weaker with any
potential surface reflection which is a nod to the ill-defined
means.  A transition from the operational model blend early in the
period to mostly ensemble means late gives enough room for a
plausible weak wave/front to be depicted while awaiting a clearer
signal for anything better defined.  Evolution over the
northwestern Pacific is also a question mark and current spread
yields minimal reflection in the means.

Even with the adjustments at the surface and aloft over the past
day, there is still a good signal for significant precip totals
along the southern coast and Panhandle.  The change in storm track
of the Bering low would bring more moisture into the western
mainland though.  The front over or near the North Slope region
will provide another focus for precip during the period.

Rausch


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