Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Tue Aug 13 2019
Valid 12Z Sat Aug 17 2019 - 12Z Wed Aug 21 2019
With the details of southeast progression of a compact upper
low/trough expected to be over eastern Siberia on Wed-Thu
seemingly resolved, the extended forecast now looks somewhat less
contentious. There are still the typical low-predictability
embedded shortwave issues and by later in the period question
marks over the relative strength/position of the larger scale
features.
Model/ensemble consensus shows a strong eastern Bering upper high
as of early Sat being pinched/pushed northwestward, between
northwestern North America/northeastern Pacific troughing
(anchored by the upper low that has been meandering over the
western Canadian Archipelago) and a northwestern Pacific upper
low. The Pacific low should reach the Aleutians around day 8 next
Wed. The downstream ridge may lead to rising heights over the
mainland/Gulf of Alaska by the end of the period. Teleconnections
relative to the upper low's negative height anomaly center support
the ensemble means and some operational runs that bring some
ridging into the mainland/Gulf by next Wed.
During the first half of the period a mostly operational model
blend places more weight on the 00Z ECMWF relative to the 00Z/12Z
GFS or 00Z CMC as recent ensemble mean trends favor a stronger
Bering ridge aloft. The blend phases out the CMC fairly quickly
due to questionably far northward extent of the North Pacific low.
At the same time ensemble means provide the greatest support for
the 00Z ECMWF/GFS/CMC forecast of the northwest Canada upper low
versus the farther eastward 06-12Z GFS runs. There is still a
fair degree of uncertainty over path of this low given model
variability over the past few days. The blend provides fairly
good continuity for surface low pressure tracking near/southeast
of the Panhandle. Expect a brief period of brisk winds behind this
system. The 12Z GFS downplays a low-confidence wave that prior
GFS runs had south of the Aleutians around Sun, meriting modest
inclusion of that run in the overall forecast.
Later in the period a model/ensemble mean blend provides a
reasonable starting point as guidance continues to cluster
decently. Operational input helps to enhance the means somewhat
for the North Pacific/Aleutians system (which should be
accompanied by enhanced winds and areas of rainfall) while the
means dampen operational model differences farther northeast. The
12Z GFS may become too fast with the Aleutians system after Tue.
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
- Heavy precipitation across portions of mainland Alaska, Fri, Aug
16.
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