Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
549 PM EST Thu Nov 26 2020
Valid 12Z Mon Nov 30 2020 - 12Z Fri Dec 04 2020
...Deep North Pacific storm to bring a broad area of strong winds
along with heavy precipitation focused over the southern
coast/Panhandle early-mid week...
...Overview and Guidance/Predictability Assessment...
Today's guidance continues to show a transition from an amplified
western mainland trough/western Canada ridge configuration with
high predictability (aside from minor details, forecast with
remarkable agreement and continuity since eight days out) toward
gradual relaxation of the pattern and decreasing predictability.
The decline in predictability extends across the Pacific mid-late
week as well.
For the deep storm initially south of the Aleutians early Mon,
multi-day trends have favored a slower and farther south track
which ultimately leads to a delayed arrival into the southern
coast and today's forecast blend reflects this. The southern
trend in the GFS over the past 2-3 days is particularly noticeable
while the ECMWF has averaged farther south but temporarily
reversed the trend in the 12Z/25 run. By day 6 Wed the 12Z
GFS/GEFS mean/CMC stray to the western side of the full guidance
spread, which recently has not held up well in successive cycles.
Preferred forecast follows closer to the past two ECMWF runs for
track/timing while while leaning closer to the deeper GFS/UKMET
for depth. GFS input transitions to the more compatible 06Z run
by this time while ECMWF input starts to include a little 00Z
ECMWF as well, heading into the less confident late week forecast.
There are multiple late week uncertainties. One involves exactly
how the upper trough over/near the far western mainland weakens,
whether by ejection of initial energy or filling in place. There
is also a signal from sporadic operational runs for a possible
frontal wave reaching near the Panhandle (strongest in the 12Z
ECMWF). Upstream there is a general area of shortwave energy that
reaches east-central parts of the Pacific late in the week, with
decent agreement that surface development should occur in some
fashion but clustering and run-to-run consistency have been quite
poor thus far. CMC/CMC mean runs have tended to be on the fast
extreme with low pressure generated by the leading edge of this
energy. Therefore the forecast quickly increases ensemble mean
weight, reaching 70 percent total by day 8 Fri. At that time the
most common theme is for low pressure to consolidate close to
Kodiak Island and greater consistency of the ECMWF mean in recent
runs has merited a little more weight versus the GEFS mean whose
12Z run is closer to the ECMWF mean than in some prior runs.
...Weather/Threats Highlights...
The deep storm to the south of the Aleutians at the start of the
week will bring a broad area of strong winds from the southern
Bering Sea/Aleutians/North Pacific into the Gulf of Alaska and
southern coast/Panhandle. Accompanying precipitation will extend
from the Aleutians eastward, with the greatest moisture feed
likely to focus over the Panhandle and to some degree the southern
coast around Tue. The flow of moisture may become somewhat less
concentrated mid-late week but the general pattern still favors
the potential for meaningful to significant precipitation over
some locations along the Panhandle/southern coast. Confidence is
low regarding one or more potential waves that could end up
providing renewed focus of heavy precipitation for at least a
brief period of time. Lesser totals will reach farther north into
the mainland though with some sensitivity to exact surface low
track.
Much of the North Slope as well as the eastern two-thirds of the
remainder of the mainland should see above to well above normal
temperatures through the period with highest anomalies likely to
be for lows (20F or more above normal in some cases). The
Panhandle will tend to be above normal as well except for some
below normal areas that are most likely early in the week. Expect
the best potential for below normal temperatures over the
west/southwest part of the mainland and through the
Peninsula/Aleutians.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Wed, Nov 29-Dec 2.
- High winds across portions of mainland Alaska and the Aleutians,
Sun-Mon, Nov 29-Nov 30.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Mon-Tue, Nov 30-Dec 1.
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