Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Tue Sep 25 2018

Valid 12Z Sat Sep 29 2018 - 12Z Wed Oct 03 2018

...Wet Pattern for the upper Aleutians/Alaskan Peninsula/Southwest
Alaska with locally Heavy Rains this weekend...

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

The synoptic-scale pattern across the state will be dominated by a
powerhouse ridge with 500-mb height anomalies maximizing around 4
sigma above climatology. This should keep its stranglehold on the
region into early next week before gradually giving way to the
upstream approach of a persistent longwave trough. With a
semi-permanent axis of lower heights over the Bering Sea and
points westward, a series of embedded shortwaves are forecast to
lift through the Aleutians before shearing upon encounter with the
ridge. Models show rather marked lowering of pressures with each
cyclone passage with a focus over the upper Aleutians into the far
eastern Bering Sea. On the other side of the upper ridge, the
guidance continue to depict a series of waves dropping southward
from the Arctic down toward the Yukon Territory. This entire
pattern remains intact into early next week before operational
models show intermittent attempts at eroding this ridge. On the
other hand, ensemble means seem to maintain this ridge, albeit in
a flatter state, through the middle of next week. Of course the
extratropical transition of Super Typhoon Trami will also play a
role in the forecast ahead. Given the overall pattern assessment,
preferred to remove operational influences out of the picture
beyond Day 6/Monday and move toward a ensemble-based approach.
This featured a combination of the 06Z GEFS and 00Z ECMWF/NAEFS
ensemble means.


...Weather/Hazards Highlights...

Temperatures should gradually come down throughout the period
given the primary upper ridge lifts up toward extreme northern
Alaska. Most areas across the southern half of the state will
shift back down into the low to mid 50s into early next week while
near freezing air sits closer to the Arctic Ocean. The wettest
area during the period should reside across the upper Aleutians
given the slow eastward progression of the enhanced moisture
channel. Each accompanying wave will also lead to periods of gusty
winds given the tightening pressure gradient in the vicinity of
the Aleutians.


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