Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Sun Nov 25 2018

Valid 12Z Thu Nov 29 2018 - 12Z Mon Dec 03 2018

...Large-Scale Overview...

The flow pattern remains an active one across the North Pacific
with numerous systems and interactions/phasing between multiple
streams, which introduces increasing chaos into the models from
day 6 (Sat) onward. D+8 mean 500 hPa anomalies from the ECENS and
GEFS show a mean Rex Block extending from near the North Pole into
Siberia, with mean troughing across the northwest Pacific. The
result will be a persistent source of shortwave energy exiting
east Asia and moving near the Aleutians. Potential phasing of
polar/arctic energy in this region will give at least the
potential for some systems to undergo rapid/significant
intensification, as some of the guidance indicate toward the end
of the extended period. By next weekend, models/ensemble suggest
an upper-level ridge building north across the eastern Gulf and
into southern mainland and Southeast Alaska. This evolution is
supported by hemispheric teleconnections and should result in a
gradual westward shift of the predominant storm track toward the
Aleutians and Bering Sea.


...Guidance/Predictability Assessment...

Model solutions continue to show moderate consensus during the
early part of the extended period (Thu-Fri). A low pressure system
should quickly move across the southern Gulf and onshore in
British Columbia during that time period. Another significant
shortwave/low pressure system should enter the Bering Sea Wed
night/Thu with a front sweeping east across the Aleutians. Given
relatively good consensus observed in the guidance, a blend of the
ECMWF/GFS along with the ECENS/GEFS means served as a good
starting point for the forecast during days 4-5.

Predictability begins to degrade by Sat as another shortwave
crosses the North Pacific, with models/ensembles backing off a bit
from the idea of a rapidly deepening cyclone approaching the
Aleutians on Sat. Ensembles continue to show rather large spread
focused around the means, however, indicating that a number of
members do continue to show a much deeper system, even if the
probability of that scenario seems to have decreased a bit.
Another point of contention was introduced by the 12Z GFS and CMC,
which show a period of westerly/less amplified flow from the
Bering Sea into Alaska Sat-Sun, as opposed to more amplified flow
shown by the ECMWF. This would result in much weaker warm
advection and colder temperatures for southern mainland Alaska if
this solution were to materialize. Teleconnections associated with
blocking 500 hPa flow extending from Siberia to the North Pole
support more amplified flow across the North Pacific.
Additionally, the extratropical transition of now Typhoon Man-yi
across the northwest Pacific by the middle of next week should
support some degree of flow amplification downstream closer to
Alaska. By next Sun/Mon yet another highly amplified trough/upper
low appears likely to enter the Bering Sea or North Pacific in the
vicinity of the Aleutians. Models/ensemble show somewhat better
consensus surrounding this system, at least at the large scale,
but do show expected timing/intensity differences with the surface
low. Given the increased uncertainty, the ECENS/GEFS means were
most heavily weighted in the forecast for days 6-8 (Sat-Mon).


...Sensible Weather Highlights/Hazards...

Precip should become relatively widespread across the Aleutians
during the extended period as a couple low pressure systems affect
the island chain. Southerly/onshore flow will also increase precip
chances across southern mainland Alaska by Fri night/Sat ahead of
the incoming frontal system. Conditions will be relatively dry in
the interior, the result of the persistent southerly flow and
moisture struggling to make it past the coastal mountain ranges,
as well as building upper ridging by next weekend. Temperatures
will be near to slightly above average for most of the state, as
the active North Pacific flow regime will inhibit any significant
southward incursions of arctic air across Alaska. Gradually
warming temperatures are possible across southern mainland Alaska
by next weekend as somewhat warmer Pacific air advects into the
region and the mean upper trough axis gradually shifts farther
west (although this would not be the case if the more zonal
GFS/CMC solutions were to verify).


Ryan


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html