Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
324 PM EDT Thu May 10 2018

Valid 12Z Mon May 14 2018 - 12Z Fri May 18 2018

At the start of the period there are timing differences for a
system reaching the Gulf of Alaska in the latter part of the short
range time frame.  By the start of day 4 Mon recent GFS runs are
faster than most other solutions including the 06z GEFS mean.  The
00z/12z runs are not quite as fast as the 06z version.  Based on
consensus through arrival of the 12z GFS, preference was to lean
slower than the GFS with more surface low pressure lingering over
the Gulf as of early Mon.  Looking at remaining new guidance, the
ECMWF has adjusted faster but the UKMET/CMC hold onto the slower
timing.

Behind this system a majority percentage of guidance indicates
maintenance of a mean trough aloft over the eastern Bering and
western coast of the mainland.  Operational 00z/06z models show a
trend toward a stronger bundle of energy (along with surface low
pressure/frontal system) dropping southeast from Siberia into the
eastern Bering followed by interaction with Pacific low pressure. 
This interaction would lead to the Pacific system tracking toward
the Alaska Peninsula/southeastern Bering by the latter part of
next week with leading low level flow bringing a significant
increase of moisture to at least some areas between the Alaska
Peninsula and Kenai Peninsula.  Overall an operational model blend
early in the week and trending toward 30-50 percent total
inclusion of the 06z GEFS/00z ECMWF means late week provides the
best account for system definition early-mid period and increasing
detail uncertainty thereafter.  While this represents the more
probable forecast based on recent data, other significantly
different evolutions are possible.  Earlier operational runs have
varied considerably and as a whole the new 12z cycle shows less
stream interaction--keeping Pacific low pressure farther south. 
The 12z GFS is a bit on the extreme side in keeping Siberia-Bering
energy farther northeast though.  Perhaps a slightly more reliable
trend is a stronger tendency for the downstream upper ridge that
develops from western Canada into portions of the mainland.

Farther upstream, by late in the period the 12z GFS shows a
farther north track versus most other guidance for low pressure
that emerges from the western Pacific.  A fairly robust consensus
of models/means would keep the system well to the south of the
Aleutians.

Albeit with some question marks for the details, there is decent
guidance agreement toward a mean ridge at the surface and aloft
over the Arctic north-northeast of the mainland.  Even with some
differences in ridge location aloft/possible weakness(es), the
surface pattern looks similar.



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