Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Sat Jul 10 2021
Valid 12Z Wed Jul 14 2021 - 12Z Sun Jul 18 2021
...Overview...
The large scale pattern should gradually become a little less
blocky with time as an upper high initially south of the Bering
Strait drifts southward and likely opens up by next weekend to
yield a ridge axis that reaches the eastern Aleutians/far southern
mainland. It remains to be seen what proportion of Bering Sea
upper low/shortwave energy to the southwest of the initial ridge
will travel to the south of the ridge and into the North Pacific
versus around the western side of the ridge and eventually into
the mainland. The persistent upper low over the Northeast Pacific
should meander toward a position slightly offshore the southern
Panhandle/Haida Gwaii. Expect unsettled weather at times over the
northern mainland as a mean frontal boundary oscillates from day
to day in response to shortwaves embedded within progressive
Arctic flow. Meanwhile there is still a decent signal for low
pressure to track over Kamchatka early in the period but the
ultimate eastward extent of this system remains uncertain. Flow
ahead of the system should at least affect parts of the Aleutians.
...Guidance/Predictability Evaluation...
Consensus position of the front over the northern/eastern mainland
as of Wednesday, in response to an upper trough heading into the
Canadian Archipelago, has held up fairly well over recent days.
Then over the past 24 hours or so the operational models have been
improving their agreement for the details of Arctic flow aloft and
associated frontal evolution later in the week. The majority
scenario now has a wave and a trailing frontal system helping to
lift the initial mainland front northward as a warm front,
followed by the trailing front settling back over the northern
mainland. This improves confidence in the forecast through about
Friday but solutions become more diverse next weekend. Of
particular note the 12Z GFS appeared too deep with its upper
trough reaching into the northwestern mainland next weekend, with
the 18Z run trending less extreme and faster. The 12Z CMC strayed
faster than consensus late in the week.
Elsewhere, an intermediate solution provides reasonable continuity
for the Northeast Pacific upper low. One question mark is how
much northern stream energy could reach far enough southward to
interact with/displace the initial low. In general the ensemble
means have favored a middle ground between western/eastern
solutions for upper low position over the past couple
days--favoring a blend/compromise approach. Guidance trends have
become more mixed for low pressure emerging from Kamchatka. Prior
adjustments had been away from the fast side of the envelope but
the 12Z CMC leads other guidance already by Friday and then the
12Z ECMWF catches up to it during the weekend as it tracks into
the Bering Sea. On the other hand the 00Z ECMWF and recent GFS
runs have kept the low to the south/southwest of the western
Aleutians through the weekend. Current preference goes to a
slightly enhanced version of the ensemble means that are between
these two extremes and more stable from run to run.
Today's guidance comparisons led to a blend consisting of 12Z
operational guidance early in the period followed by removal of an
already minimal CMC weight. Later in the period the forecast
trended more to the means, reaching 70 percent total 12Z GEFS/00Z
ECens by day 8 Sunday. At that time the remaining weight went to
the last two ECMWF runs whose differences over some areas averaged
out closer to the means. The questionable 12Z GFS forecast over
the Arctic led to its removal late in the period.
...Weather/Threats Highlights...
The southeastern quarter of the mainland should see organized
precipitation on Wednesday followed by a drier trend, while the
Panhandle may see periods of light to locally moderate
precipitation one or more days depending on the exact path of the
Northeast Pacific upper low. Expect the northern mainland to see
one or more periods of precipitation in association with the mean
frontal boundary likely to meander over or just north of the
region from midweek into the weekend. The gradient ahead of low
pressure emerging from Kamchatka should bring an increase of
precipitation and winds into at least parts of the Aleutians. The
eastward extent of moisture by next weekend becomes very
uncertain, with possibilities ranging from having it stay over the
western Aleutians or reaching as far east as the western mainland
(also aided by any lingering Bering Sea energy picked up by flow
aloft).
The Panhandle and southern coast will tend to see below normal
temperatures for most of the period. The North Slope should see
below normal readings around midweek followed by some variability
in temperatures as the mean front over the region oscillates.
In-between expect upper ridging to support above normal
temperatures though the gradual weakening/opening of the upper
ridge should lead to a cooling trend over western portions of the
central-southern mainland.
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