Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
629 PM EST Sun Dec 29 2019
Valid 12Z Thu Jan 02 2020 - 12Z Mon Jan 06 2020
The best clustering of today's guidance continues to show a mean
pattern that features troughing over the mainland and/or Gulf of
Alaska depending on the day, a western Pacific trough anchored by
an upper low most likely to track west and then south of
Kamchatka, and varying degrees of mean ridging in-between. The
portion of the ridge over/near Siberia early in the period may
ultimately drift into the Arctic and end up to the northwest or
north of the mainland by the start of next week. Models/means
still show a progressive North Pacific/Aleutians stream extending
out from flow around the western Pacific trough. It is this flow
that has been producing some of the greatest model spread and
variability in recent days. The uncertainty in shape and position
of the Siberia/Arctic ridge aloft leads to further challenges in
determining specifics over and near the mainland.
At the start of the period early Thu there is decent agreement for
low pressure to be near the Kenai Peninsula/Kodiak Island, with
this low drifting off to the east/east-southeast while the
corresponding upper low makes modest progress as well. Recent GFS
runs had been eager to track the surface low farther
south/southeast than model/mean consensus but the 12Z run compares
better.
Issues with upstream flow across the North Pacific/Aleutians into
the southern Bering arise already by day 5 Fri and become
increasingly pronounced thereafter. Operational models have been
erratic with individual waves/frontal systems while ensemble
spread has been sufficiently broad that their means have been
fairly modest with the depth of any wave. At least from Fri
onward the 06Z/12Z GFS runs generally fit into the theme of the
GFS/ECMWF means, tracking low pressure over the Aleutians/far
southern Bering and then into the area of long-term mean low
pressure over/near the Gulf of Alaska. Differences between the
12Z ECMWF and its 00Z predecessor are particularly dramatic with
systems becoming 180 degrees out of phase by Sun-Mon. At that
time the 12Z ECMWF does not compare well to other guidance
including its mean so it had to be excluded from the forecast
blend after mid-period.
Confidence decreases in resolving the character of upper troughing
over the mainland by the latter half of the period due to combined
dependence on details of the low-predictability North
Pacific/Aleutians flow as well as the ridge over Siberia and the
Arctic. Currently the most likely scenario is for lingering
energy to yield an upper low over the mainland after the
early-period Gulf upper low weakens. Recent GFS/GEFS mean runs
have shown a gradual trend toward higher heights within the
overall trough, so the weaker ECMWF mean could merit some
consideration.
Late in the period models/means show enough broadening and/or
progression of the western Pacific trough aloft to push a strong
surface gradient into the Aleutians with a corresponding increase
of wind speeds.
Based on the above forecast considerations the initial blend
started with an average of the 12Z GFS/ECMWF early day 4 Thu.
Developing spread in the North Pacific stream required the
forecast to begin incorporating the 12Z GEFS/00Z ECMWF means after
early Thu, reaching nearly half total weight by early day 6 Sat.
Questionable attributes of the 12Z ECMWF led to its removal by
days 7-8 Sun-Mon. During those last two days the ensembles
reached 70 percent emphasis while the GFS component included both
06Z and 12Z runs to mitigate detail differences.
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
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