Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Wed Sep 05 2018

Valid 12Z Sun Sep 09 2018 - 12Z Thu Sep 13 2018

...Guidance/Uncertainty Assessment...

An impressive mid/upper-level ridge sets up over the Bering Sea
partially in response to the extratropical remains of Jebi. This
cut-off upper high will remain a fixture in the forecast for much
of the period with 12Z GEFS height anomalies around 3 to 4 sigma
relative to early September climatology. Bounding this region to
the south will be a pair of cut-off lows, one south of the western
Aleutians with the other off the western Canadian coast. This
configuration should remain in place through Day 5/Monday before
upstream height falls from the eastern Siberian Sea approach the
mean ridge axis. Ultimately this will force the synoptic-scale
pattern eastward allowing for the ridge to set up over mainland
Alaska by the middle of next week. The overall guidance has a
reasonable handle on the evolution of the forecast through early
next week. The process of pushing the ridge downstream remains
somewhat in question as a few operational models attempt to squash
the northern extent of the ridge. Given its stout nature,
preferred keeping intact throughout the period. This afforded a
multi-model consensus of the 12Z GFS and 00Z CMC/ECMWF early on
before moving toward a more ensemble mean influenced forecast for
Days 6-8, September 11-13.


...Weather Highlights/Hazards...

Early precipitation threats should focus across the Alaska
panhandle in response to the upper low just offshore. Eventually
the focus will shift across northern Alaska as a cold front sweeps
through the state. Otherwise, expect a rather dry period given the
dominance of the upper ridge. The windiest conditions can be
anticipated over the Aleutians given closer proximity to the
strong cyclone centered to the south.


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