Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Thu Sep 27 2018

Valid 12Z Mon Oct 01 2018 - 12Z Fri Oct 05 2018

...Wet Pattern for the upper Aleutians/southwest Alaska with
locally heavy rains on Sunday/Monday...

...Anomalous warmth in store for the region underneath the
powerful upper ridge...


...Pattern Overview and Guidance/Predictability Assessment...

Model guidance continues to depict a 500-mb height anomaly on the
order of over 4 sigma above climatology over the state that will
shift northward toward the Arctic Ocean through the extended
period. A broad area of lower heights will move from the Bering
Sea into the Aleutians region forming an omega block late in the
period. Before this occurs, a potent shortwave is primed to lift
up toward the Aleutians on Monday accompanied by a rather deep
area of low pressure. While this system shears into the downstream
ridge, other embedded shortwaves may aid in further cyclone
passages later next week. Significant spread remains with the
evolution of what is now Typhoon Trami, resulting in a lower
confidence forecast.

Its extratropical transition depicts a potent surface low with
pressures possibly below 950 mb, however due to the model spread,
continuity using an ensemble approach was maintained. Initially a
combination of the 12Z/06Z GFS and 00Z ECMWF with inclusion of
relevant ensemble means was chosen with increasing amounts of the
06Z GEFS and 00Z ECMWF/NAEFS ensemble means from Day 5 and beyond.
Given the heavy use of ensembles, the resultant extratropical
pressure of Trami is likely being highly underdone.


...Weather/Hazards Highlights...

Much of the state will have warmer temperatures this weekend under
the strong ridge; but as it retreats northward, the temperatures
across the southern portion of the state will cool 5 to 10 degrees
while the northern locations will remain above seasonal averages.
The Aleutian chain and a vast portion of southern Alaska will be
in a prolonged cloudy and wet pattern as a robust low pressure
system slowly tracks east. This will become reinforced with what
is now Typhoon Trami.
A steady period of moderate to locally heavy rainfall into the
upper Aleutians.  Gradually the focus for precipitation will
migrate eastward into the early/middle part of next week as the
robust cyclone emerges out of the north-central Pacific. Given
reasonably strong pressure gradients with each low, expect
multiple periods of gusty winds over the Aleutians into
southwestern/western Alaska.


Campbell/Rubin-Oster


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