Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Sat Aug 04 2018

Valid 12Z Wed Aug 08 2018 - 12Z Sun Aug 12 2018

A powerful upper low during the short range period will evacuate
the Gulf of Alaska as it drifts down into the east-central
Pacific. This gives way to a longwave trough which slowly tracks
from the Aleutians to southeastern Alaska during the Day 4-6,
August 8-10, window. While this is a relatively certain aspect of
the forecast, what looms upstream is much more chaotic and
unpredictable. The combination of what is currently Typhoon
Shanshan and other tropical invests create a very nebulous
forecast during the extratropical transition process. Some lead
feature pushes out ahead of this mentioned typhoon with lowering
pressures reaching just south of the western Aleutians by early
Thursday. With a well established synoptic-scale pattern in place
through Day 5/August 9, was able to utilize a modest amount of
operational guidance. More specifically, a combination of the 12Z
GFS/00Z ECMWF combined with the 00Z ECMWF/NAEFS means. Thereafter,
poor run-to-run continuity with the handling of Shanshan decreases
certainty in the forecast. The previous few GFS runs have been
more aggressive bringing sub-970 mb pressures south of Aleutians
by the conclusion of next weekend. The 00Z ECMWF was largely
subdued with its evolution although the latest 12Z ECMWF trended
considerably stronger. Ultimately, the choice was to utilize
ensemble means from Day 6/August 10 onward, albeit with some
consequences. Most notably, the ensembles were averaging the lead
system with the typhoon which allowed for a more centered
position. Unfortunately this is unrealistic in nature but could
not trust the operational models by late next week. With recent
GEFS mean solutions being the deepest, continued with the 00Z
ECMWF/NAEFS ensemble approach which is a little more conservative.

Ultimately the period will be quite wet with the series of upper
troughs impacting the northeastern Pacific and locations upstream.
If the surface pressures are as low as advertised by some of the
models, it could prove to be rather windy by late in the period.
The ensemble approach does not really reflect this but operational
runs have been easily 20-30 mb stronger during this time of
interest.


Rubin-Oster



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