Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Sun Oct 14 2018

Valid 12Z Thu Oct 18 2018 - 12Z Mon Oct 22 2018

...Multi-day heavy precipitation possible across
southern/southeastern Alaska next week...


The large scale flow pattern across Alaska and the north Pacific
will remain relatively stable and unchanged during the extended
period with persistent/blocking upper ridging across western North
America and a mean trough centered across the north Pacific just
south of the Alaska Peninsula. A number of shortwaves/low pressure
systems will rotate through this mean trough, with a nearly
constant plume of Pacific moisture directed into southern and
southeastern Alaska. While models show broad agreement, with the
stable pattern lending to above average forecast confidence at the
large scale - it is a different story entirely when it comes to
the specific details of individual systems. One significant
surface low appears likely to approach the Alaska Peninsula or
southern mainland Alaska Thu-Fri, with another (perhaps stronger)
one by early next week. Some degree of clustering was evident in
ensembles with respect to the first system, and model consensus
was sufficient to begin the forecast on day 4 (Thu) based on a
slight majority of deterministic guidance (GFS/ECMWF), along with
the ECENS/GEFS ensemble means. From day 5, however, a gradual
trend toward heavier weighting of ensemble guidance was warranted
given the increasing spread. For the second significant system
early next week, models agree of the existence and general timing
of the system, but differ on the track, timing, and intensity.
Some ensemble members are quite intense, as indicated by ensemble
mean SLP values in the 980-984 hPa range, with ensemble spread
values indicated a number of deeper members.

The stable upper-level flow will result in a sustained period of
southerly mid-level flow into Alaska from the subtropics, with
each low pressure system enhancing precipitation potential. Models
show general agreement on a multi-day heavy precipitation event
for portions of southern and southeastern Alaska with several
inches (liquid equivalent) of precipitation looking likely,
especially at windward locations near the coast. The flow regime
will prevent any arctic air from making its way into Alaska, thus
expect temperatures to be near or slightly above climatological
norms through the extended period.


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