Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EST Wed Dec 20 2023
Valid 12Z Sun Dec 24 2023 - 12Z Thu Dec 28 2023
...A weekend surface low will spread heavy precipitation to
Southcentral and Southeast Alaska along with a maritime wind
threat, with another round of heavy precipitation early next week
for Southeast...
...Overview...
A mean upper trough/low will settle across the junction of the
eastern Bering Sea/southwestern Mainland/Bristol Bay centered
around Nunivak Island this weekend into much of next week.
Downstream ridging over western Canada may extend into the
mainland through the weekend before shifting away. This evolution
should produce a strong surface low pressure system tracking just
south of the Alaska Peninsula into Kodiak Island this weekend,
leading to areas of strong winds across the North Pacific
especially Saturday, along with a threat of heavy precipitation
along the southern coast of the mainland into Southeast Alaska. As
another surface low tracks through the northeast Pacific and
pivots, rounds of precipitation should persist across these areas
into next week.
...Guidance Evaluation/Preferences...
Model guidance shows reasonably good agreement on the large scale
with the pattern described above, with typical differences in the
individual details. The first model to diverge from consensus was
the UKMET, which showed a surface low near the Panhandle Sunday
secondary to the one near Kodiak Island that models generally
agree upon, and this split of the low seemed suspicious. Then
there is some spread regarding another shortwave coming eastward
through the Pacific along the southern side of the upper trough
and then pivoting north a bit, which supports the next surface low
similarly tracking east and then north through the Pacific. Models
have converged better (but not perfectly) compared to a day ago
with the placement of the surface low through Monday, other than
once again the UKMET that tracks much farther east. By late Monday
into Tuesday another surface low over the Northeast Pacific may
complicate matters. Aloft, the 12Z GFS was a bit displaced east
with the upper low centroid compared to consensus and the ensemble
means Monday-Tuesday. While deterministic models vary considerably
with these mentioned and and other shortwaves, their overall
pattern with the upper low seems reasonably locked in. The GEFS
and EC ensemble means also show good agreement, though the 00Z CMC
mean seemed off with some Pacific low features by
Wednesday-Thursday.
A model blend favoring the 06Z/12Z GFS, 12Z ECMWF, and 12Z CMC
with a bit of GEFS and EC ensemble means worked for the early part
of the forecast period, with this blend serving to average out
some individual model differences. Increased the proportion of the
ensemble means in the blend with increasing lead time to around
half by days 7-8.
...Weather/Hazards Highlights...
After spreading areas of brisk to strong winds across the North
Pacific on Saturday, a surface low is likely to be weakening
slightly by Sunday and more so by Monday near the Kenai Peninsula.
This storm track will direct a weak atmospheric river into the
region and focus heavy precipitation along the southern coast from
the Kenai Peninsula eastward across the southeast Mainland into
the Panhandle particularly on Saturday to early Sunday. For
Southcentral Alaska, forecast questions remain regarding how far
inland the precipitation type would be something other than snow.
In Southeast Alaska areas of snow and rain are likely, but this is
more typical there. Later Sunday into Monday there looks to be a
brief break at least in the heaviest precipitation intensity, but
renewed heavy precipitation concerns are likely later Monday
(Christmas Day) into early Tuesday for Southeast as another
surface low will track through the northeast Pacific and direct
another round of moisture there. After that it generally looks
like an unsettled pattern for southern parts of the state, but
amounts are expected to be much lower. Elsewhere, the Aleutians
and vicinity will see unsettled conditions for most of the period
with frequent light precipitation, with some possible snow farther
north into the southwestern Mainland especially this weekend.
Lighter snow may be possible for western and then northern parts
of the Mainland, with some continued uncertainty over whether much
of the moisture reaching the south will reach farther northward.
Another area that winds could be enhanced is across the Bering
Strait, with some northerlies on the backside of the stacked
Mainland low.
The mean trough across the western part of the state will promote
below normal temperatures for the Aleutians, Alaska Peninsula, and
western Mainland this weekend and continuing to cool into the
first half of next week. Conversely, mean southwesterly flow and
upper ridging initially poking into the East should lead to above
normal temperatures on average for the eastern two-thirds of the
state. Anomalies for lows should generally be greater relative to
those for highs. However, some areas of near normal to lightly
below normal highs may be possible for parts of the central
Interior, particularly closer to midweek.
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