Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Mon May 03 2021
Valid 12Z Fri May 07 2021 - 12Z Tue May 11 2021
...Overview and Guidance Evaluation...
Most guidance continues to show a persistent deep-layer mean low
over the Aleutians/southern Bering Sea during the period with
variations among southerly mean flow and ridging aloft over the
mainland. Shortwaves carried along in the flow to the south and
east of the mean low should interact with the feature to produce a
series of wavy frontal systems that would produce the best
precipitation focus from the Aleutians through the Alaska
Peninsula and possibly to some degree eastward to the Panhandle.
Although the consensus mean evolution is fairly agreeable,
individual solutions suggest numerous uncertainties with the
forecast that would affect sensible weather--most notably over the
northeastern Pacific/Gulf of Alaska as well as for individual
system details/interaction from the western-central Pacific into
the Aleutians.
Over the northeastern Pacific/Gulf region, the 12Z GFS and prior
recent runs already start the forecast early Friday differing from
other guidance (including the GEFS mean), depicting an upper low
south of the Panhandle in contrast to a more pronounced split of
energy to the north and southeast. Then this feature interacts
with energy arriving from the west to produce a wave of low
pressure as the upper low is reinforced. The ensemble means and
ECMWF/CMC maintain a more east-west shortwave and thus show a mere
front, anchored by a wave whose track near the Alaska Peninsula
has varied somewhat over recent days. The UKMET is the one other
model that provides the potential for some of the upstream
shortwave energy to break off to the east during the weekend but
starts the period looking like the non-GFS majority. Then by the
weekend/early next week there is considerable spread for another
shortwave and possible surface reflection (which could include a
captured mid-latitude wave/front), with surface low tracks ranging
between the southeastern Bering Sea and northeastern Pacific.
Recent ECMWF runs have consistently been on the west side while
latest GFS runs and the 12Z CMC show a Northeast Pacific wave.
Meanwhile model/ensemble variation continues to keep confidence
fairly low for specifics of how waves emerging from the western
Pacific may evolve and interact with the mean low over the
Aleutians/southern Bering Sea. There is still a general hint of
one such case early in the period. After Saturday the specifics
become increasingly uncertain. Latest GFS runs are on the
southern/deep side with the initial upper system over the
Aleutians and thus force Pacific waviness to take a much wider arc
than other runs. On the other extreme, the CMC and faster CMC
mean take a more direct route toward the Aleutians--replacing or
merging with the initial low. Latest ECMWF means suggest a
merging/reinforcement of Aleutians low pressure to a greater
degree than the operational runs around next Monday. Then toward
the end of the period the past couple ECMWF runs bring yet another
system into the Aleutians while remaining models/means suggest
what feature exists should still be well westward.
Accounting for the majority cluster and continuity for meaningful
features, the 12Z ECMWF/CMC/UKMET provided the foundation of the
forecast early in the period. The blend emphasized the past two
ECMWF runs/12Z CMC (with slight introduction of ensemble means
while decreasing UKMET input) by early day 6 Sunday. Then
decreasing confidence in CMC and eventually ECMWF details with
time, along with preference for a better defined Aleutians/Bering
low in the ECens/CMCens relative to the GEFS mean (regardless of
how they get there), led to trending the forecast exclusively to
the means and much more ECens/CMCens weight than GEFS by day 8
Tuesday.
...Sensible Weather...
Based on the most likely scenario over the course of the period,
expect the highest precipitation totals to extend from along the
Alaska Peninsula to the Kenai Peninsula. The best-organized
events would be with two separate wavy frontal systems, the second
possibly bringing somewhat higher amounts than the first. Some of
the moisture with each system may extend farther eastward into the
southeastern coast and Panhandle. However note that confidence is
below average with a minority portion of the guidance envelope
showing a sufficiently different evolution to keep areas from the
eastern Alaska Peninsula eastward considerably drier. A band of
enhanced winds may also accompany these fronts. One or more
shortwaves lifting northward over the mainland may produce
episodes of precipitation with the best signal currently indicated
over parts of the eastern mainland. The Aleutians will likely see
unsettled conditions with periods of rain and variable winds
according to uncertain system details.
The Panhandle and portions of the southern mainland, along with
parts of the north-central mainland, should see below normal highs
through the period. Expect above normal highs to prevail
elsewhere. Morning lows will likely be above normal over much of
the state with just scattered pockets of modestly negative
anomalies being the only exception. In addition the increasing
cloud cover suggested by the pattern that becomes established may
lead to a warmer trend for lows with time at some locations.
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