Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Sat May 01 2021
Valid 12Z Wed May 05 2021 - 12Z Sun May 09 2021
...Overview and Guidance Evaluation...
Today's forecast provides fairly good continuity for the general
pattern evolution along with some refinements that reflect recent
trends and/or improved guidance clustering for details. Consensus
continues to show a mean low aloft settling over the Aleutians and
southern Bering Sea after midweek. A leading Aleutians system
will establish this feature and then upstream energy (with
associated low pressure) should merge into/reinforce the Bering
Sea low. As some models were hinting before, latest runs are
coming together in suggesting that a wave along the leading
system's front will break off and track just south of the Alaska
Peninsula and Kodiak Island late in the week, followed by rapid
weakening as dynamics aloft likewise weaken over the northeastern
Pacific/Gulf. Meanwhile recent model/ensemble trends for a
stronger ridge aloft just west/northwest of the mainland mid-late
week lead to a better defined weakness between that ridge and a
Canadian ridge that extends into the eastern mainland. This
weakness should give way later in the period as upstream flow
encourages southerly flow or ridging. Over the northeastern
Pacific mid-late week, some detail uncertainties persist but the
model/mean majority still keeps the main surface system well south
near 50N latitude.
The starting blend incorporated the 12Z operational models from
early day 4 Wednesday into early day 6 Friday. After the end of
the UKMET's run, the remaining models maintained good enough
agreement and ensemble support to keep 80 percent operational
model weight in the blend up through early day 7 Saturday. Then
they diverged sufficiently to support increasing total ensemble
weight (12Z GEFS/00Z ECens) to 50 percent by day 8 Sunday.
Latest model runs are trending closer in showing how the initial
parent low over the far western Aleutians/Bering Sea as of early
Wednesday should give way to wave development near the central
Aleutians by early Thursday. Then the aforementioned frontal wave
should track near Kodiak Island by late in the week followed by
rapid weakening. There is still the typical ambiguity and thus
low confidence over specifics of how the trailing system may
interact with/merge into the Bering Sea mean low. Over the
northeastern Pacific, the inverted surface trough provided by the
composite blend over recent days provides a placeholder along
which an embedded wave could exist (even if not the main low that
is expected farther south). Expect very slowly weakening high
pressure to prevail over the Arctic with a general axis of lower
pressures over the central/south-central mainland.
...Sensible Weather...
The best-defined precipitation focus should be from the Aleutians
into parts of the Alaska Peninsula and Kodiak Island mid-late week
with an Aleutians system whose leading front should develop a wave
that ultimately tracks just south of Kodiak Island. Expect some
moderate enhancement of winds and precipitation for a time, but
increasing confidence in the evolution of the frontal wave to the
east is reducing the potential for southerly flow that would have
led to higher rainfall totals over the Alaska Peninsula. Some of
this moisture may continue into the southern coast and Panhandle
by next weekend. Expect a broad area of light precipitation
behind this system into the weekend. Meanwhile upper level energy
and an inverted surface trough could bring some light
precipitation into the Panhandle and vicinity after midweek but
with low confidence for coverage. The weakness aloft over the
mainland mid-late week may promote episodes of scattered
precipitation. Northwestern parts of the mainland should remain
fairly dry though. The southwestern mainland will see periods of
precipitation from late week into the weekend, first with moisture
ahead of the initial Aleutians system and then with persistence of
the Bering Sea upper low.
Temperature anomalies across the state should remain consistent
for most of the period. Central latitudes and possibly areas
along the northern coast should see above normal highs while the
southern mainland and Panhandle will tend to be below normal. A
majority of the state will see above normal lows, with some
pockets of below normal min temperatures between the Panhandle and
the Kenai Peninsula as well as near Kotzebue Sound and possibly a
small part of the northeastern mainland.
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