Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Wed May 16 2018

Valid 12Z Sun May 20 2018 - 12Z Thu May 24 2018

Models continue to indicate a transition from a complex and blocky
flow pattern this weekend to a more progressive pattern by early
to middle next week.  An upper ridge axis is expected to initially
extend from western Canada into eastern Alaska at the start of the
medium range period on Sunday, with a broad upper-level low
centered just south of the Aleutians along with multiple shortwave
perturbations within the flow.  Models and ensembles have come
into better agreement regarding the associated surface low over
the Gulf of Alaska through Tuesday, and the 12Z GFS/00Z ECMWF fit
well within this consensus.  Therefore, a majority deterministic
blend including these solutions along with the 00Z ECENS/6Z GEFS
means served as a good starting point for the WPC forecast through
Tuesday.

By the middle of next week, there is general consensus in the
guidance that a vigorous shortwave in the westerlies, and its
associated surface low should approach the Aleutians and the
southern Bering Sea.  Models show the typical timing and strength
differences with this system as would be expected for a forecast
at this time range.  Although the GFS has been a fast outlier with
this system for a few runs now, the latest 12Z run has trended a
little closer to the consensus, and the slower timing shown by the
ECMWF and ensemble means was preferred.  Given these
considerations, the WPC forecast was gradually trended toward
heavier ECENS/GEFS weighting for Wednesday and into Thursday.

In terms of sensible weather, rainfall prospects are highest for
southern mainland Alaska, the Aleutians, and the Panhandle region
for much of the forecast period owing to the proximity of the Gulf
of Alaska low and the southerly flow ahead of the next system by
mid week.  Mainly dry conditions are expected across the Interior,
with isolated showers possible.  Temperatures will be near or
slightly above average, especially for areas across the interior
that don't get any rainfall.  Not surprising for this time of
year, the coldest readings will be found north of the Brooks
Range, with high temperatures struggling to reach the freezing
mark for the Arctic Coast.

D. Hamrick


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