Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
709 PM EDT Wed Oct 23 2024
Valid 12Z Sun Oct 27 2024 - 12Z Thu Oct 31 2024
...Rounds of active weather including heavy precipitation and high
winds concerns are likely across southern coastal Alaska next
week...
...Overview...
Upper and surface lows will be weakening as they track into the
Panhandle by Sunday, with some lingering precipitation and windy
conditions. Mean troughing aloft should prevail over most of the
mainland as the period begins, anchored by an Arctic upper low,
while a weak ridge is forecast to move across the Aleutians to
Bristol Bay and surrounding land areas. Into the workweek, the
ridge is forecast to move east across the Southern Coast, but
meanwhile energies over the Bering Sea will combine to create an
upper trough just upstream, which is forecast to deepen across the
western Mainland to Alaska Peninsula to Southcentral Tuesday and
move through the Gulf Wednesday. A surface low should accompany
this trough but with variations in the details. Another
upper/surface low that could be strong looks to move across the
Aleutians/southern Bering Sea by next Wednesday-Thursday.
...Guidance Evaluation/Preferences...
Model guidance is reasonably agreeable on this progressive pattern
over the state next week, with some typical variations in the
details. Models begin the period Sunday with indicating low
pressure entering near the southern Panhandle or Haida Gwaii
generally in the low 980s, so weakened from its earlier depth in
the northeast Pacific. There remains some scatter for
position--nothing out of the ordinary, but the exact positions may
matter for sensible weather impacts like winds and precipitation.
Meanwhile a weak ridge is forecast to be atop Bristol Bay to the
western Mainland, while a closed low spins north of the state.
After some spread during the last couple of days, models are more
agreeable now that this Arctic upper low should remain to the
north of Alaska with some consensus for it to meander a little
eastward at times.
Models show an upper low tracking across the Bering Sea early week
and moving atop the Mainland Tuesday-Wednesday. Vorticity should
spill south to create troughing extending south from this low
moving quickly across the Gulf Tuesday-Wednesday. While this
pattern is more agreeable today, the placement of an associated
surface low is more variable. GFS runs have been farther north
with the low position Monday-Tuesday with fairly good consensus
from the GEFS members, and the CMC has trended north (even jumping
farther north than the GFS). The ECMWF is the main holdout with a
southern position, with even most of the EC-based AI models
farther northeast. Thus favored a more northern position for the
low with less ECMWF that brings more precipitation from the Alaska
Peninsula to Southcentral than the previous forecast.
Toward the end of the medium range period, guidance is agreeable
in principle that an upper low atop a surface low may both be
relatively strong as they move near or over the Aleutians.
Deterministic models show the surface low in the 950s or even 940s
around midweek, with some timing and placement differences that
are certainly within the typical spread for Days 7-8.
The WPC forecast used a blend of the 12Z GFS/ECMWF/CMC for the
early part of the period. The UKMET was excluded as it was farther
west than consensus with the Bering Sea/western Mainland to
northern Pacific trough Monday-Tuesday. Lessened the proportion of
particularly the ECMWF by Monday and the GFS/CMC gradually later
in the period in favor of the GEFS and EC ensemble means, with the
means reaching just over half the blend by Day 8.
...Weather/Hazards Highlights...
The surface low moving from the southeastern Gulf toward the
Panhandle will lead to wind and wave threats offshore, but some
high winds will possibly come onshore for parts of Southeast
Saturday. Moisture inflow should also provide support for some
moderate precipitation over the weekend and lingering in southern
portions of the Panhandle Sunday. Light snow may wrap back into
southeastern parts of the Mainland as well. Southeast Alaska
should trend drier by Sunday or Monday as low pressure weakens and
moves eastward. Meanwhile some light snow may reach coastal parts
of the North Slope near the Arctic upper low.
Another round of organized precipitation is likely to track from
the Aleutians and entering the western Mainland Sunday, eastward
into the Alaska Peninsula and southwestern Mainland to
Southcentral Monday, and into Southeast Tuesday considering the
upper trough and surface low track. Forecast precipitation amounts
have come up somewhat from the Alaska Peninsula to Southcentral,
but the best consensus for heavy precipitation is across the
Panhandle. Gap winds look to be a concern behind the surface low
near Kodiak Island and the Kenai Peninsula, while strong southerly
winds east of the low could also be a concern. Then by midweek,
strong winds and heavy rain are possible over the Aleutians with
the next strong low.
The trough atop most of Alaska through the early part of next week
will promote generally below normal temperatures, with some
exceptions. The North Slope can expect temperatures that are
warmer than average. Warmer southerly inflow into southwestern
Alaska should lead to above average highs and lows there as well.
A warming trend is likely across the Mainland as the week
progresses with a mix of above and below normal temperatures.
Tate
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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