Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
339 PM EDT Tue May 14 2019
Valid 12Z Sat May 18 2019 - 12Z Wed May 22 2019
Today's ensemble means agree well for large scale flow evolution.
They show initial Aleutians through northeastern Pacific energy
(with one or more embedded lows) trying to consolidate near the
eastern Aleutians/southwestern Alaska Peninsula. Western Pacific
mid-latitude trough energy should gradually progress into the
central Pacific with some interaction possible. Farther
east/northeast, the means expect upper ridging over Canada to
build northwestward, eventually covering the northeastern mainland
and possibly connecting with an upper high that reaches the Arctic
in the short range time frame.
Not surprisingly the spread among individual model runs and
ensemble members would suggest much lower confidence in specific
details. For the upper ridge, operational GFS runs continue to be
on the stronger side of the spectrum. The 06Z FV3 GFS offers a
compromise between the GFS and weaker means/ECMWF. Across the
North Pacific and Bering, ensemble members lose any semblance of
clustering after day 5 Sun over the former and already after early
day 4 Sat over the latter. There is a general hint among the most
compatible solutions that northeastern Pacific low pressure in
some form may try to track back to the west, providing a period of
enhanced--though not exceptional--low level easterly
flow/precipitation to the southwestern coast/Alaska Peninsula.
Meanwhile another uncertainty is the extent to which
western-central Pacific troughing/associated surface reflection
might interact with the mean upper low near the eastern Aleutians.
The ECMWF/CMC ensemble means provide enough support for at least
a conservative depiction of phasing in the manual forecast.
Sporadic operational runs (e.g., 12Z/13 ECMWF and 00Z/14 GFS) have
displayed much stronger low pressure but there is not yet enough
agreement/continuity to depict in a single deterministic forecast.
Energy that may drop southward over the Bering provides another
wild card in the forecast.
The manual forecast started with a model/ensemble mean blend
consisting of the 06Z GFS/FV3 GFS and 00Z ECMWF along with the 06Z
GEFS/00Z ECMWF means (plus some 00Z NAEFS late day 6 Mon onward
due to having better low pressure definition vs the GEFS). The
blend leaned 70 percent toward the models early in the period and
then transitioned to 70 percent means by day 8 Wed, with minor
manual adjustments as needed.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
No significant hazards are expected over Alaska during this
forecast period.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html