Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
700 PM EDT Sat Apr 11 2020
Valid 12Z Wed Apr 15 2020 - 12Z Sun Apr 19 2020
...Overview and Guidance Evaluation...
Most models and ensembles still show a mean pattern aloft
featuring a ridge over the Northeast Pacific and extending into
mainland Alaska and western Canada, along with troughing farther
west. However today there is more spread and variability in the
guidance for some of the details. One prominent uncertainty
involves the specifics of flow aloft across Siberia and the Arctic
into the mainland, with resulting influence on wave/frontal
details over the northern-central mainland. The other uncertainty
regards flow upstream from the system forecast to reach the
Aleutians during the latter half of the week. Differences with
this flow ultimately lead to various ideas for low pressure
evolution/track across the North Pacific/Aleutians/Bering Sea by
next weekend. Even with the lack of confidence for specifics,
guidance still shows that it will be a fairly wet period for a
large portion of the state.
The manual forecast started with a 12Z operational model blend on
day 4 Wed. The rest of the period phased out/switched/added
various operational solutions as needed while steadily increasing
12Z GEFS/00Z ECMWF ensemble mean input with time. Total ensemble
input reached 70 percent by day 8 Sun.
The first required adjustment was to trade the 12Z GFS with the
06Z run from day 5 Thu onward. The 12Z version quickly developed
deeper upper dynamics over Siberia compared to other guidance and
then had additional digging flow from the north--leading to an
unsupported surface low reaching northwest of the mainland by 12Z
Fri and then northeastward progression of Aleutians/Bering low
pressure to an extent not seen in other guidance. The 06Z GFS run
compared better to other models/means in this regard. Meanwhile
over the northern half of the mainland the UKMET became more
suppressed than most other guidance at the surface and aloft by
late in the week, which went against ensemble mean trends from the
past couple days toward stronger upper ridging.
As for the Pacific/Aleutians/Bering stream, there is decent
clustering among the models/means into Fri for the system tracking
into the Aleutians during the latter half of the week. The 12Z
CMC was the one exception as it became deeper than other guidance
with dynamics reaching the western Bering. This led to its
Aleutians/Bering system being pulled farther northwest than the
majority cluster by late Thu-Fri. At the same time to the east
there is a relative consensus toward weakening shortwave energy
that supports a modest wave/frontal system near the Alaska
Peninsula/Gulf Wed-Thu. From about late Fri onward individual
solutions increasingly diverge for what happens to the upper
energy associated with the Aleutians system (most likely filtering
into/through the mean ridge in some fashion) and the character of
upstream flow. There is a general signal that a system emerging
over the western Pacific mid-late week will come into the picture,
with uncertainty over what interaction could take place among this
system and the leading one, or perhaps even with separate dynamics
that reach the western half of the Bering. The blend/ensemble
mean approach used by day 8 Sun led to some degree of interaction
and low pressure near the Aleutians at that time. The new 12Z
ECMWF mean shows a trend toward latest ECMWF/GFS runs that
maintain more separation and a low track to the south of the
Aleutians. Among the various possibilities the 12Z CMC seems to
be among the lower-confidence ones as its overall pattern becomes
out of phase with most other solutions by next Sun.
...Weather/Threats Highlights...
Even with the uncertainty in various details, there is decent
confidence that the pattern will lead to a fairly wet period for
most of the state including the Aleutians. For the five-day
period as a whole the best signal for highest totals is along the
southern coast of the mainland, centered near the Kenai Peninsula.
Somewhat less extreme relative maxima will be possible over other
areas across the southern and western mainland. A modest
wave/frontal system near the Alaska Peninsula/Gulf will provide
one focus for precip early in the period while flow ahead of the
leading Aleutians system may bring a period of focused activity
into the Peninsula/western mainland by late week/weekend.
The majority of the state will see above normal temperatures with
min temps tending to be more above normal than daytime highs. The
greatest positive anomalies should be across northern areas. The
Panhandle and localized pockets of the southern mainland may see
below normal highs on one or more days. There is some uncertainty
over precise temperatures over the northern/central mainland by
Fri-Sun as guidance spread develops for flow aloft and resulting
position of a wavy front that should be over the extreme northern
part of the mainland at least through Thu. This front may push
southward to some degree by the weekend.
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