Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EST Sun Dec 13 2020
Valid 12Z Thu Dec 17 2020 - 12Z Mon Dec 21 2020
...Guidance and Uncertainty Assessment...
From late week into the weekend the guidance continues to reflect
the combined uncertainties associated with shortwaves carried
along in progressive North Pacific flow and features embedded
within a weakness aloft between the Bering Sea and northern
Canada. The axis of this weakness may drift northward a little
with time as the strong upper ridge/high over the Arctic gradually
retreats. A large scale storm system forecast to track into the
Aleutians by next Sun-Mon may also help to raise heights to its
east--as well as hopefully provide some improvement in
predictability.
Today's models continue to signal fairly vigorous storm
development during the latter half of the week from near the
Alaska Peninsula into the Gulf of Alaska but with some adjustments
on the specifics. The 12Z GFS/ECMWF/UKMET, with support from the
12Z GEFS/CMC means, have all adjusted toward keeping the parent
low to the south of the Peninsula at the start of the period early
Thu. Thus the days 4-5 Thu-Fri forecast used majority weight of
the 12Z GFS/ECMWF with modest UKMET input (due to the latter
having less agreeable upstream features). The 12Z CMC system
along the Peninsula appears questionably slow given how close it
is to faster Pacific mean flow.
Even though the CMC was not preferred early-mid in the period, it
along with the 12Z ECMWF and 12Z GEFS/00Z ECMWF means provided the
best clustering for low pressure expected to linger over the Gulf
of Alaska through the weekend into Mon. A mid-latitude Pacific
wave could reach into the northeastern Pacific and/or merge into
the lingering Gulf low by Sat (currently expected to be weaker
than the 12Z GFS), followed by gradual weakening of Gulf low
pressure. The preferred cluster also worked well for the system
expected to track into the Aleutians late in the period.
Operational model runs have been suggesting potential for more
than one surface low within the general area of low pressure but
recent GFS runs have tended to be more fragmented than the other
models. As anticipated the ensemble means are more consolidated
and merit half weight in the blend given typical detail
uncertainty that far out in time and very good agreement in the
large scale evolution. Consensus has trended somewhat faster
compared to 24 hours ago. Based on above comparisons day 6 Sat
starts transitioning away from GFS input and adding some of the
means on the way to days 7-8 Sun-Mon reflecting a nearly even
proportion of the ECMWF/CMC and GEFS/ECMWF means.
...Weather/Hazards Highlights...
Trends for late this week show the best potential for low pressure
to track from just south of the Alaska Peninsula into the Gulf of
Alaska. This path may brush the Peninsula with moderate
precipitation (amounts very sensitive to exact track) and then
likely initiate a period of enhanced precipitation along the
southern coast and Panhandle. Precip should continue through the
weekend as Gulf low pressure lingers, whether from the arrival of
another system and/or upstream energy aloft helping to maintain
low pressure in place. Expect max amounts to decrease somewhat
compared to late this week though. The leading system could
produce some brisk/strong winds but most likely to the south of
the low track. Much of the mainland will likely see dry
conditions or light/scattered precipitation. The Aleutians should
see an increase in organized precipitation and some areas of
strengthening winds with the approach/arrival of the storm system
next weekend/early next week.
Below normal high temperatures will prevail over most of the
northern two-thirds to three-fourths of the mainland, while some
above normal readings are possible over the far south. Eastern
parts of the Interior may see the coldest anomalies. For low
temperatures the south should see warmer anomalies than for highs
while the rest of the mainland may see some pockets of above
normal readings within the broader cold pattern. The Panhandle
should see a mix of above/below normal temperatures depending on
location.
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