Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 PM EDT Sun Sep 09 2018

Valid 12Z Thu Sep 13 2018 - 12Z Mon Sep 17 2018

...Guidance/Uncertainty Assessment...

A 3 to 4 sigma above climatology mid-level ridge will be in the
process of advancing eastward toward mainland Alaska on Thursday.
This is in response to a significant lowering of heights over the
Bering Sea as advertised by all of the guidance. A compact
negatively-tilted shortwave should initially cross the Aleutians
early Thursday while a more pronounced upper low tracks in from
west of the Bering Sea. Given the anomalous strength of the
persistent upper ridge, would prefer to keep it intact as much as
possible. The 00Z ECMWF depicts a strong upper low across the
Arctic which drops southward and ultimately affects the downstream
ridging pattern. Scenarios are quite variable when considering
operational models on a run-to-run basis as previous ECMWF cycles
did not show such an evolution. Will keep some operational
influences in the picture early on before models break down with
how to handle the cascade of shortwave energy. A combination of
the 12Z GFS along with the 06Z GEFS and 00Z ECMWF/NAEFS ensemble
means was utilized throughout much of the period with a small
amount of the 00Z ECMWF incorporated given did not want to
completely dismiss its scenario.


...Weather Highlights/Hazards...

A significant cooling trend is expected in the forecast given the
loss of the anomalous ridging aloft. Initial temperatures should
be in the mid/upper 60s across south-central Alaska before numbers
drop back into the 50s by the weekend and early the following
week. Regarding precipitation threats, a northwest-southeast
oriented convergence zone will set a focus for a mixture of rain
and snow across far northeastern Alaska. This should quickly wind
down as the boundary shifts toward the Yukon Territory. Eventually
a more expansive shield of precipitation should spread from west
to east across the Aleutians into mainland Alaska. This coincides
with the degradation of the mean ridge as strong forcing moves in
from the Bering Sea.


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