Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
751 PM EDT Sun May 02 2021
Valid 12Z Thu May 06 2021 - 12Z Mon May 10 2021
...Overview and Guidance Evaluation...
Aside from the occasional stray model run, the majority of
guidance is consistent with the large scale pattern forecast. An
upper low reaching the Aleutians/southern Bering Sea after midweek
will likely persist into next Sunday-Monday though it could weaken
somewhat by that time. Downstream over the mainland expect mean
flow aloft to become more southerly with the possibility of
ridging to develop at times. This flow may carry along a couple
shortwaves into the mainland from over/south of the Gulf. Between
Thursday and Saturday expect a front (with one or more embedded
waves possible) anchored by initial Aleutians low pressure to
progress eastward toward the northeastern Pacific. Upstream
energy may support one or more additional systems that interact
with or merge into the mean low over the Bering Sea/Aleutians.
Meanwhile high pressure over the Arctic will drift a bit northward
with time and an axis of general surface troughing/low pressure
will likely prevail over parts of the southern/central mainland
after Thursday.
The early portion of today's forecast blend incorporated the 12Z
ECMWF, 06Z GFS, and 12Z CMC/UKMET in order of most to least
weight. Typical divergence of details later in the period led to
increasing ensemble input (more 00Z ECMWF mean versus 12Z GEFS due
to the former being deeper with southern Bering low pressure) to
40-55 percent by days 7-8 Sunday-Monday while remaining
operational runs included the 12Z/old 00Z ECMWF along with the 06Z
GFS.
The 12Z GFS departed from consensus in multiple ways: straying
weaker/northeast with the upper ridge initially just northwest of
the mainland, becoming slow/southward with shortwave energy likely
reaching the Alaska Peninsula and Kodiak island by around early
Saturday (with corresponding issues at the surface), and wobbling
south of other guidance for the Aleutians upper low during the
latter half of the period. Some 06Z GFS details over the
northeastern Pacific were not ideal early and late but the weight
in the blend was low enough to downplay those. Meanwhile the past
couple CMC runs have been most aggressive in bringing a strong
western Pacific system toward the Aleutians by late in the period,
so the forecast excluded that model after Saturday. Even among
other guidance there is considerable spread and run-to-run
variability for the details of a wave that could emerge from the
western Pacific. 06Z GFS/12Z ECMWF components of the blend
provided a suppressed wave that serves as a starting point while
awaiting better agreement.
Farther east, details continue to differ for specifics of upper
low/shortwave energy initially south of the mainland and expected
to lift northward while latest models appear to have diverged
somewhat for details of frontal waviness that has been forecast to
reach near the Alaska Peninsula/Kodiak Island by around
Friday-Saturday. The model blend accounts for these uncertainties.
...Sensible Weather...
The best focus for meaningful precipitation during the period
should extend from the Alaska Peninsula to the Panhandle.
Moisture along a possibly wavy front will likely progress eastward
across at least the Peninsula late this week and should extend
farther eastward to some degree. With lower confidence, another
area of moisture may reach the Alaska Peninsula and nearby areas
during the weekend and early next week. Through the period there
is still a fair degree of uncertainty over exact totals.
Shortwaves lifting northward may produce episodes of precipitation
from the southern coast and Panhandle into portions of the
mainland.
Expect the Panhandle and portions of the southern mainland, as
well as the north-central mainland, to see below normal highs for
most of the period. Above normal highs should prevail between
these two areas. Most of the state is likely to see above average
readings for morning lows with mostly just scattered pockets of
modestly negative anomalies. In addition lows may trend a bit
warmer with time given increasing cloudiness promoted by the large
scale pattern that becomes established.
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