Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 PM EDT Sun Jun 24 2018

Valid 12Z Thu Jun 28 2018 - 12Z Mon Jul 02 2018

Models and ensembles showed good consensus at the start of the
forecast period (Thu) across Alaska, with an upper low drifting
eastward near the Alaska Peninsula, and an upper ridge axis in
place across the interior. A relatively active upper-level jet
across the North Pacific will initially be suppressed well south
of Alaska, while Arctic flow from Russia/northeast Asia will also
be kept largely at bay due to the mainland Alaska ridging.

Two significant areas of model discrepancy were noted during the
medium range. The first being how quickly the upper low initially
near the Alaska Peninsula moves eastward across the Gulf. The
ECMWF continues to lead the pack, being the fastest solution to do
so, while the GFS was on the slow side, with the CMC representing
somewhat of a compromise. At this time, was inclined to support a
compromise solution relatively close to the 00Z CMC. The second
area of uncertainty involves an upper low initially in place over
the Arctic Ocean north of Siberia. The ECMWF continues to suggest
the potential for the feature to move southward into the Bering
Sea, bringing much lower heights and the potential for more
interaction between the Arctic and polar streams. The GFS and CMC
solutions continue to keep the feature confined to the Arctic,
with the strong support of ensemble means (including the 00Z
ECENS). Thus, the ECMWF solution was not preferred beyond day 5
(Fri). Given these considerations, the WPC forecast was initially
based on a heavily deterministic blend including the 12Z GFS/00Z
CMC/00Z ECMWF (with a bit more weight toward the GFS/CMC). From
day 6 onward, ensemble weighting (00Z ECENS/06Z GEFS) was
gradually increased, and comprised a majority of the forecast,
with the ECMWF excluded entirely given its southward excursion of
the Arctic low into the Bering Sea.

In terms of sensible weather, this will be a relatively wet
pattern for much of southern mainland Alaska as well as Southeast
Alaska, given persistent onshore flow ahead of the low pressure
system which eventually moves into the Gulf. Temperatures should
remain close to seasonal norms through much of the forecast
period, with cooler temperatures in areas receiving more precip
and warmer temps in relatively dry areas.

Ryan


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