Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
644 PM EDT Sat May 16 2020
Valid 12Z Wed May 20 2020 - 12Z Sun May 24 2020
...Guidance/Predictability Assessment...
Latest models and ensemble means display better than average
agreement and continuity with most aspects of the overall pattern
from midweek through next weekend. The dominant focus will be on
a tightly wound and deep storm system that is forecast to track
into the Aleutians/southern Bering Sea Wed-Thu and slowly weaken
as it drifts gradually eastward toward the southwestern Alaska
Peninsula. The flow around this system will produce areas of
strong winds. In addition the slow system motion and leading
front(s) with possible embedded waves could support a period of
strong precipitation enhancement most likely to focus along the
Kenai Peninsula/eastern Alaska Peninsula and possibly parts of the
southwest corner of the mainland. Between this system and a
quickly departing Gulf of Alaska upper low as of early Wed, an
upper ridge will likely build into the Gulf and the mainland by
Thu. This ridge should push northeast with time to yield an axis
from western Canada to the northeast corner of the mainland and
into the Arctic next weekend.
Guidance preferences sided with a blend of the 12Z GFS/ECMWF with
modest input from the 12Z CMC from day 4 Wed into early day 6 Fri.
After early Fri the blend phased out the CMC as it strayed
farther northwest than most solutions for the southern Bering
system. Meanwhile detail uncertainties favored a gradually
increasing contribution from the 12Z GEFS/00Z ECMWF means,
reaching 30-40 percent total by days 7-8 Sat-Sun. Late in the
period the 00Z/12Z ECMWF runs became mixed over which run compared
better to the model/mean consensus depending on location, so the
blend included both runs next weekend. Maintaining a fair degree
of operational GFS/ECMWF input helped to strengthen the downstream
ridge aloft versus the means, reflecting recent stronger trends in
most guidance. The primary adjustment from continuity for the
Aleutians/Bering Sea storm was a slight northward trend. The GFS
was in the southern part of the spread (close to continuity) while
the UKMET tracked north of the ECMWF (which along with its mean
started the northward trend overnight). Within the fairly
high-confidence large scale pattern, confidence decreases later in
the period for one or more possible frontal waves reaching the
Gulf. Such details may take a number of days to resolve
satisfactorily.
...Weather/Hazard Highlights...
The initially deep storm system reaching the Aleutians/Bering Sea
will first bring precipitation and areas of strong winds across
the Aleutians. Then for a period of time after Wed heavier
activity should focus more narrowly over southeastward-facing
terrain along the Kenai Peninsula/Alaska Peninsula, where models
and ensembles show the most common signal for highest five-day
totals. Kodiak Island and parts of the southwestern mainland
could see some enhancement as well. By next weekend the
location/duration of any lingering moderate-heavy
precip--including the degree of eastward extent--will depend upon
lower confidence small scale wave details over the Gulf of Alaska.
While the general surface pattern over the mainland looks stable
through the period, a stationary front draped across the central
or south-central mainland through midweek should quickly dissipate
as upper ridging builds overhead and to the east, leaving a mere
trough that remains through the weekend. Highs will tend to be
above normal over the central mainland and somewhat cooler than
normal over the North Slope region and southern
mainland/Panhandle. Expect greater coverage of above normal
anomalies for morning lows.
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