Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Tue Apr 13 2021
Valid 12Z Sat Apr 17 2021 - 12Z Wed Apr 21 2021
...Overview...
Latest guidance continues to show a broad area of mean ridging
aloft from the Bering Sea across Mainland Alaska and western
Canada. This ridge will support high pressure for most of the
period over the Bering, central-northern parts of the mainland,
and northwestern Canada. To the north of the mean ridge expect
progressive flow to carry along a series of fronts that should at
least brush northern areas, with the possibility that one or more
of these fronts could extend farther south. Meanwhile an inverted
upper trough will rotate across the southeastern Bering early in
the weekend and then a closed low (with corresponding low pressure
at the surface) should lift into a position south of the Alaska
Peninsula/eastern Aleutians by next Monday-Wednesday.
...Guidance/Uncertainty Assessment...
The progressive nature of northern stream flow lends itself to low
predictability for resolving specifics of individual shortwaves
and thus associated waves/frontal systems. By day 6 Monday the
12Z UKMET becomes the strongest solution with its shortwave
reaching the mainland so preference is to phase out its
contribution to the forecast at that time. After Monday ECMWF/CMC
runs (CMC deeper and later than the ECMWF) have been suggesting
that some shortwave energy will drop into a weakness in the mean
ridge, over the eastern Bering and/or the mainland. On the other
hand the GFS maintains an east-west ridge. GEFS/ECMWF/CMC
ensemble means all depict a modest weakness in some fashion, a
reasonable intermediate solution given the full spread. Best
consensus indicates that the front reaching the mainland by
midweek may reach farther southward than its predecessors.
Continuity/trends in the ensemble means over the past couple days
have tended to favor the stronger side of the spread for the
overall area of upper ridging. This is particularly the case for
the western Bering Sea, where the GEFS mean has been notably
lagging most other guidance in ridge strength.
Farther south and from the start of the period early Saturday,
there is a strong majority cluster showing that a system near the
western Aleutians should drop southward more quickly than in the
GFS/GEFS mean. This precludes use of the GFS during the weekend.
The model is more acceptable as part of a blend/compromise for
North Pacific low pressure by Monday-Wednesday though.
The above considerations led to starting the blend with the 12Z
ECMWF/UKMET/CMC during the weekend. Then the forecast phased out
the UKMET while bringing in the GFS by early day 6 Monday. The
rest of the way, ensemble mean input (more 00Z ECMWF mean than 12Z
GEFS mean) increased to yield a nearly even blend of means and
models (last two ECMWF runs plus 12Z GFS) by midweek. ECMWF/GFS
runs essentially canceled out over the mainland to reflect the
means while providing some detail elsewhere. Depth of the CMC
trough over the mainland favored removing its influence by day 8.
...Weather/Hazards Highlights...
The relative dominance of high pressure will keep many areas dry
during the period. The primary focus for some precipitation is
most likely to be over the Alaska Peninsula and eastern Aleutians
early-mid week in the brisk flow around North Pacific low
pressure. Lower confidence specifics of flow aloft will determine
if any moisture extends farther northward-northeastward and/or
becomes locally heavier, or alternatively is suppressed farther
south. Scattered light precipitation may accompany northern
stream fronts that reach the mainland.
Expect most of the state to see above normal temperatures into
Monday with anomalies for morning lows tending to be greater than
for daytime highs. Also the North Slope should see higher
anomalies than other areas. On the other hand a few pockets of
slightly below normal highs may exist over the southeastern half
of the mainland and into the Panhandle. The potential break in
mean ridging aloft may promote a cooling trend by next
Tuesday-Wednesday.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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