Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Tue Jul 03 2018

Valid 12Z Sat Jul 07 2018 - 12Z Wed Jul 11 2018

Over the course of the period the most agreeable cluster of
guidance indicates that the large scale pattern aloft should
evolve toward a mean trough covering a majority of the forecast
domain.  Contributing to this mean trough will be a combination of
Arctic flow forecast to amplify into/near Mainland Alaska and an
upper low/trough progressing eastward from the Bering Sea.  The
Bering system is associated with T.S. Prapiroon which is currently
over the western Pacific.  The overall system/pattern evolution
should lead to highest 5-day precipitation totals between the
Kenai Peninsula and Alaska Panhandle.

Operational model runs display some spread for track/timing of the
Bering system (00Z ECMWF on the faster side, 06Z GFS rather slow
versus the full envelope) but agree upon a deeper surface low than
the ensemble means for at least the first half of the forecast
period.  This favors a compromise among latest GFS/ECMWF/CMC/UKMET
runs through the weekend.  Meanwhile guidance varies somewhat in
how much and when the Arctic troughing aloft drops into the
mainland.  The operational blend provides an intermediate forecast
not far from the ensemble means through the weekend and is similar
to yesterday's forecast of the surface front that pushes into the
mainland.

By the latter half of the period guidance shows greater divergence
with some aspects of the forecast.  Around early next week 00Z/06Z
GFS runs brought in somewhat more ridging over parts of the
eastern Bering and/or mainland.  The 12Z GFS aligns better with
the means for an overall trough near 160W longitude but could get
a little sharp/deep by the middle of next week.  Confidence is not
particularly high in ECMWF/CMC specifics either but they are not
far from the overall mean troughing depicted in the means. 
Teleconnections relative to the negative height anomaly center
expected to the northeast of the mainland by the D+8 time frame
support the idea of broadly cyclonic but not particularly
amplified flow.  Thus the 06Z GEFS/00Z ECMWF means seem to provide
the best overall template for the forecast toward next Tue-Wed. 
However in spite of the 12Z GFS and 00Z ECMWF/CMC possibly being a
little deep with troughing at various times, they are close enough
to the means in principle to merit some lingering minority input
late in 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