Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
624 PM EST Thu Dec 19 2024
Valid 12Z Mon Dec 23 2024 - 12Z Fri Dec 27 2024
...Stormy pattern into next week with multiple episodes of
southern coast into Panhandle precipitation and hazardous maritime
conditions over the North Pacific/Gulf of Alaska...
...Overview...
Latest models and ensembles continue the recent theme of mean
ridging aloft, perhaps flattening to southerly flow at times, over
western Canada into the mainland during next week while an upper
trough axis persists over or near the western coast of the
mainland. This trough may deepen some during the latter half of
next week. The last in a series of north/northwestward tracking
systems should reach the southern coast next Monday. Then a large
and strong upper/surface low should track east across the North
Pacific/Aleutians Monday-Wednesday, with this system likely
settling over the Alaska Peninsula or western Gulf of Alaska late
next week. These systems will produce focused precipitation along
the southern coast while the North Pacific and then the Bering Sea
into Aleutians could see periods of strong winds. The North
Pacific storm's leading front could also produce a band of strong
winds near the Panhandle on Wednesday/Christmas Day. The forecast
pattern evolution will likely support above normal temperatures
over most areas aside from parts of the North Slope early in the
week, followed by increasing coverage of below normal temperatures
over the west as upper troughing deepens.
...Guidance Evaluation and Preferences...
For the system likely to reach just east of Kodiak Island by early
Monday, there is better than average agreement among most
dynamical and machine learning (ML) models for such a track. The
12Z GFS starts to stray farther west on Sunday and is notably west
of consensus by Monday. The 12Z GEFS mean is a compromise between
the GFS and other guidance. The 06Z GFS compares better to the
majority. Behind this system, the 12Z GFS shows another wave
reaching the Gulf of Alaska but other guidance does not show this
feature. Farther south, a separate developing system will likely
track near Haida Gwaii around Tuesday with fairly low influence on
the southern Panhandle.
The average of guidance, and the ensemble means in particular,
have displayed better than average continuity for the North
Pacific/Aleutians storm in terms of track and depth. The consensus
path is just a bit south of the Aleutians into midweek, reaching
the Alaska Peninsula or western Gulf of Alaska by late week. The
ensemble means and ML models show lowest central pressure reaching
the mid 950s to 960 mb range with operational models reaching a
couple mb on either side of 950 mb. There is a fair amount of
spread for precise details among the parent low and one or more
potential leading frontal waves, so there is a lurking potential
for meaningful forecast changes in specifics. There are also
differences in how much upper trough energy could drop south
through the Bering Sea (per recent ECMWF runs and a minority of ML
guidance), affecting surface low details to some degree. At least
the 12Z ECMWF trended weaker with its Bering Sea energy, allowing
for inclusion as part of a blend. Meanwhile the 12Z GFS is a
prominent misfit versus other guidance, holding back the primary
upper/surface low from Tuesday onward. Again the 06Z GFS compared
better to other guidance.
Recent guidance has shown considerable spread and variability for
the depth of upper troughing along and west of the western coast
of the mainland by the latter half of the week. GFS runs are
deepest with the upper low while the 12Z GFS and 12Z ECMWF are
most pronounced with height falls reaching into the western half
of the mainland by late next week. The 12Z CMC keeps the upper low
well westward. ML models are mixed but generally lean toward a
more moderate upper trough, while recent GEFS mean runs have been
trending somewhat deeper and the new 12Z ECMWF mean (available
after forecast preparation) is a little deeper after the past few
runs showed no trend at all. A conservative model/mean blend looks
like a reasonable starting point for this trough by late next week.
Guidance comparisons led to starting the first half of the
forecast with an even weight among the 06Z GFS and 12Z ECMWF,
UKMET, and CMC. Then the forecast increased ensemble mean weight
(divided among the 12Z GEFS/CMCens and 00Z ECens) to 40 percent by
day 7 Thursday and 60 percent on day 8 Friday, with lingering
minority contributions from the 06Z GFS and 12Z ECMWF/CMC.
...Weather/Hazards Highlights...
On Sunday, before the start of the extended forecast period, there
will be a strong gradient between low pressure near Bristol Bay
and high pressure over or north of Siberia. This gradient will
support strong winds across the northern Bering Sea and parts of
the northern mainland. Reflecting the best guidance probabilities
for hazardous winds, the Days 3-7 Hazards Outlook depicts high
wind areas for St Lawrence Island and some coastal areas of the
Seward Peninsula and just south of western Norton Sound. In
addition, flow ahead of a storm lifting through the northeastern
Pacific on Sunday will bring the threat of high winds especially
over the far southern Panhandle as highlighted in the Hazards
Outlook. This storm will likely reach into the Gulf of Alaska by
early Monday, bringing enhanced precipitation to Southcentral and
Southeast Alaska with a weak to moderate atmospheric river. Winds
to the east of this system will likely be strong over other parts
of Southeast Alaska beyond the southern Panhandle with potential
enhancement of barrier jets and gap winds over favored areas, even
if not reaching hazardous thresholds. Then focus will turn to the
potentially deep North Pacific/Aleutians storm that may reach near
the Alaska Peninsula/western Gulf of Alaska by mid/late week.
This system should produce moderate amounts of precipitation
across the Aleutians with somewhat greater enhancement eventually
possible into the southern coast. Precipitation totals should not
reach hazardous thresholds but multi-day totals may still be
significant. Expect a broad area of strong winds as well,
including on the north side of the system early in the week
followed by northerly flow in the storm's wake from the Bering Sea
into North Pacific. Also there may be a band of strong winds ahead
of the storm's leading front reaching into the northeastern
Pacific on Wednesday/Christmas Day. As was the case for Sunday,
the southern Panhandle has relatively higher potential for winds
to reach hazardous thresholds--and thus is included again in the
Hazards Outlook. Otherwise, there may be a northeast-southwest
axis of light snow farther north of the mainland while light
precipitation will likely linger over the Aleutians into mid-late
week.
Early in the week expect above normal temperatures over the
southern two-thirds of the state while parts of the North Slope
see below normal readings. In general anomalies for min
temperatures should be warmer than those for highs. By mid-late
week some North Slope locations will remain below normal and the
western mainland will likely trend below normal as upper troughing
deepens near or just west of the western coast, though with
uncertainty as to the magnitude of this trend. Southern and
eastern areas should remain above normal into late week.
Rausch
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at: The WPC Hazards Outline
depicts these threats.
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