Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
212 PM EDT Thu May 24 2018

Valid 12Z Mon May 28 2018 - 12Z Fri Jun 01 2018

...Guidance/Predictability Assessment...

Guidance clustering in the day 4/5 (Monday/Tuesday) timeframe over
Alaska is about average. A composite of the latest
GFS/ECMWF/UKMET/Canadian models blended with the latest GEFS/ECMWF
ensemble means seems to provide a good starting point for the
forecast. Model variance still offers some timing/emphasis
challenges over time, especially with Pacific systems entering the
Bering Sea and downstream undercutting energy into the Gulf of
Alaska later period. Accordingly, transitioned to better clustered
GEFS/ECMWF ensembles days 6-8 (Wednesday-Next Friday) amid growing
forecast spread to best maintain WPC continuity. Leaned weighting
toward ECMWF ensembles days 6-8 to maintain better WPC continuity
and then applied manual modifications to insure sufficiently deep
offshore lows consistent with expected support aloft and
continuity.

...Weather/Threats Highlights...

It still seems reasonably likely that two main deep lows will
highlight the weather forecast in the vicinity of Alaska leading
into and through the days 4-8 medium range forecast period. A
early period lead low roams over the Gulf of Alaska this extended
holiday weekend, to weaken by early next week. Wrapping moisture
will fuel high winds/waves and widespread precipitation over the
north-central and eastern Gulf of Alaska that should increasingly
spread onshore into southern and southeastern Alaska with gradual
low approach into early next week. Less defined vort centers
lingering overtop the interior offer less predictability but
overall suggest some lingering periods of precipitation underneath
an arctic ridge bridging just to the north of the state.
Meanwhile, a second main low has mixed guidance support, but whose
potential approach into the Aleutians and southern Bering Sea from
the northern Pacific early to mid next week would threaten the
region with high winds/waves and sweeping precipitation. Energy
propagating downstream may lead to triple point low development in
about a week from the Alaskan penisula to the northern Gulf of
Alaska, but details remain uncertain at these long lead time
frames. Development would lead to renewed system weather focus for
the region and maritime interests.

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

Schichtel