Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Fri Jul 06 2018

Valid 12Z Tue Jul 10 2018 - 12Z Sat Jul 14 2018

Relatively progressive flow is expected across Alaska during the
medium range forecast period, with a couple low pressure systems
of significance expected to affect the state. At the start of the
forecast period on Tue, models/ensembles show good agreement on a
relatively deep and largely barotropic low along the western
Alaska/Bering coast, which should begin to weaken by Wed-Thu as
the energy moves east across the northern Gulf. Model agreement
early on in the medium range was sufficient to justify use a of a
multi-model, majority deterministic blend including the
ECMWF/GFS/CMC for days 4-5. From day 6 onward, spread begins to
quickly increase in the guidance with respect to additional energy
approaching the western Aleutians in the form of potentially a
closed upper low, and interactions that occur between this feature
and additional shortwave energy of Arctic origin. Models have
shown a fairly large degree of run-to-run variability, but in
general, preferred a slower arrival of the upper low and evolution
as it interacts with the Arctic flow. This excluded the 00Z ECMWF
which was the quickest of all deterministic solutions with this
process (the 12Z ECMWF has gone back toward a slower solution).
The 00Z/06Z GFS were preferred over their faster 12Z counterpart.
Thus, the forecast for days 6-8 was trended quickly to heavier
ensemble mean (00Z ECENS/06Z GEFS) weighting, with a minority
component of the 00Z CMC/06Z GFS continued through the period.

In terms of sensible weather impacts, this pattern will result in
a period of generally below average temperatures along with
frequent precipitation for much of the state. Height falls
resulting from colder air aloft will support areas of snow,
especially for portions of the North Slope. Rain could be heavy at
times across southern mainland as well as Southeast Alaska as
frontal systems pass overhead.

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
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html