Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EST Tue Dec 29 2020
Valid 12Z Sat Jan 02 2021 - 12Z Wed Jan 06 2021
...Deep late-week western Bering system with strong
winds/significant waves to weaken gradually as it tracks near the
Aleutians...
...Overview and Guidance/Uncertainty Assessment...
Today's guidance agrees well for the large scale pattern
evolution. Expect elongated trough/closed low energy over the
northern mainland to retrograde and possibly merge with Bering Sea
energy to yield a mean low/trough with a center near or a little
west of the Bering Strait by next Tue-Wed. Another feature
farther north in the Arctic could join the circulation but that is
still up for debate. The northern fringe of progressive North
Pacific flow should provide an eastward nudge for the initially
very deep low over the southern Bering Sea and most likely to
track near the Aleutians. Pacific flow will also feed into an
area of general upper troughing over/near the Gulf of Alaska with
at least a couple surface lows gravitating toward that area.
Operational guidance was similar enough in concept to allow for a
12Z model compromise to represent significant features for about
the first half of the period, with some modest manual enhancement
to offset the blend. Typical increase of uncertainty for some
details favored partial inclusion of the 12Z GEFS/00Z ECMWF means
for the latter half of the period.
Recent deeper trend in the ensemble means for the initial Bering
low is increasing confidence for depth closer to the operational
guidance late in the short range (upper 920s to lower 930s mb as
of 12Z Fri) as well as the start of the extended forecast (upper
940s mb at 12Z Sat). The most notable trend over the past day
with this feature is toward greater eastward progression, though
with some lingering east-west spread as the 12Z GFS/UKMET become
slowest and CMC fastest. An intermediate solution appears
reasonable at this time.
Over the northeastern Pacific/Gulf, a leading system that
represents a triple point wave breaking off from the Bering
system's front late in the short range should reach the Gulf
during the weekend. Then a second system should arrive during the
first half of next week. With this latter feature the operational
runs (with typical spread) generally take a slightly wider arc
than the means but ultimately end up in a similar place.
Confidence is much lower for a possible third wave, with the CMC
the only solution bringing it far enough north to have meaningful
effects.
As is usually the case for such an evolution, it will take
additional time to resolve the details of upper energy
retrograding across the northern mainland and possibly merging
with Bering flow. Model consensus has trended deeper with this
upper feature compared to 24 hours ago. Meanwhile the 12Z
ECMWF/ECMWF mean are consistent with their 00Z runs for a separate
Arctic feature dropping into the west side of the overall
circulation while other guidance is slower/less consistent.
...Weather/Hazards Highlights...
Strong winds and significant waves associated with the very deep
Bering system late this week into the early weekend will gradually
trend lower as the system most likely tracks eastward across the
southern Bering/Aleutians. Please consult the Ocean Prediction
Center for more information on wind/wave details late this week.
The low and surrounding flow will likely maintain periods of
precipitation across the Aleutians for a decent portion of the
period. Meanwhile at least a couple systems expected to track
into/near the Gulf will tend to focus rain/snow over the Panhandle
and southern coast this weekend and next week. Some locally
heavier amounts are possible depending on the exact strength and
track of each system.
Expect greater coverage of below normal temperatures (especially
for highs) over the northern/western three-fourths of the mainland
through the weekend and early next week as upper level energy
retrogrades across the area. Readings may moderate a little
toward midweek. Near to above normal temperatures (perhaps
locally well above normal for lows) are likely to prevail over the
Panhandle and Southcentral.
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,
Fri-Sat, Jan 1-Jan 2.
- High winds across portions of the Aleutians, Fri, Jan 1.
- High significant wave heights for coastal portions of the
Aleutians, Fri, Jan 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