Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
542 PM EST Sat Dec 05 2020
Valid 12Z Wed Dec 09 2020 - 12Z Sun Dec 13 2020
...Guidance and Uncertainty Assessment...
Models and ensembles are suggesting the large scale pattern aloft
during Wed-Sun will be fairly complex and blocky. The best
consensus of guidance shows closed lows for at least a portion of
the period over the North Pacific and northern/western Alaska
mainland while ridging evolves, with one or more possible embedded
closed highs, over the Bering Sea/Siberia as well as over the
Arctic. Uncertainties with each individual feature and their
possible interactions already lower confidence in some forecast
specifics, while shortwave/interaction issues in the Northeast
Pacific also complicate the forecast.
The first significant discrepancy of note arises by day 5 Thu and
involves the surface pattern over the Gulf of Alaska/northeastern
Pacific. The full spectrum of guidance ranges from maintaining
emphasis on southern coast/northern Gulf low pressure from Wed, to
weakening this feature in deference to a stronger system that
develops to the southeast (tracking anywhere from offshore the
Panhandle to Vancouver Island)--depending on low-predictability
specifics of leading shortwave energy. Preferred operational
model blend early in the period (12Z GFS/UKMET/CMC and 12Z/00Z
split of ECMWF input) provides an intermediate solution to enhance
flexibility pending future guidance changes.
Model runs diverge by late week on potential evolution of the
early-period upper low over the mainland, and then next weekend
for specifics of Bering/Siberia upper ridging and how/whether an
initially deep North Pacific upper low/storm system may weaken and
ultimately become part of the upper trough surrounding the
mainland upper low. Especially by next weekend the GEFS/ECMWF/CMC
ensemble means provide the most agreeable and consistent evolution
of the upper low center curling into the western mainland while
the broader circulation incorporates the weakening North Pacific
feature. Thus the forecast trends more to the means in principle,
with a transition of operational model components toward the 00Z
ECMWF and 06Z GFS that compare best to the means. The 12Z ECMWF
ultimately brings the upper low farther south than most solutions
while the 12Z GFS is weaker/northward with initial mainland upper
low energy as it builds more ridging in from the south. Meanwhile
the past couple CMC runs have wanted to pull the upper low into
the Gulf. The preferred evolution should lead to surface low
pressure evolving near/just southeast of Kodiak Island and the
Kenai Peninsula by next weekend, but again with modest confidence
given model spread and pattern complexity. The one aspect of the
forecast having better agreement/confidence is the high pressure
expected over the Arctic through the period.
...Weather/Hazards Highlights...
Surface details over the Gulf/northeastern Pacific become more
uncertain already by Thu, but the general pattern favoring one or
more surface lows over this region should promote some periods of
focused precipitation over the southern coast and Panhandle
between midweek and next weekend. Precip should be somewhat
lighter and more scattered farther north across the mainland.
Meanwhile during mid-late week a strong North Pacific storm will
bring precipitation to the Aleutians and vicinity. The gradient
between this storm and strengthening high pressure over higher
latitudes will also support a period of strong winds over the
Aleutians and Bering Sea.
The most likely pattern evolution during the period should promote
below normal temperatures over southwestern/central portions of
the mainland, while above normal readings prevail over the North
Slope and southeastern mainland. The Panhandle may see a mix of
above/below normal anomalies depending on location with min temps
more likely to be above normal.
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