Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
651 PM EDT Sat Aug 21 2021
Valid 12Z Wed Aug 25 2021 - 12Z Sun Aug 29 2021
...An Active Weather Regime for Alaska..
...Guidance/Predictability Evaluation...
The latest 12 UTC GFS/ECMWF/Canadian solutions seem reasonably
well clustered days 3-7 across much of Alaska and adjoining
waters, albeit with the ECMWF/Canadian more amplified with time
into the Interior within an energetic and unsettled Arctic stream.
Underneath in an also active mid-upper latitude stream from the
Bering Sea and northern Pacific into the Gulf of Alaska, a blend
of the more amplified 12 UTC GFS/ECMWF and now Canadian seems to
better fit upstream flow ridge amplitude than the somewhat more
progressive solutions from the 12 UTC UKMET and GEFS ensemble
mean. The 12 UTC ECMWF ensemble mean has also now trended toward
the suggested WPC guidance blend. Accordingly and considering
overall forecast spread, the WPC Alaskan medium range product
suite was primarily derived from a blend of the
GFS/ECMWF/Canadian, but also smaller inputs of the GEFS and now
especially the 12 UTC ECMWF ensemble mean to address lingering
uncertainties days 7/8. This forecast plan maintains good WPC
product continuity.
...Pattern Overview and Weather/Threats Highlights...
An amplified flow pattern next week should maintain wet and
unsettled conditions across much of Alaska.
Southerly fetch ahead of a series of energetic systems set to work
across the Gulf of Alaska next week will direct significant
moisture into southern Alaska fueling moderate to heavy rainfall
from parts of the southern Coast to the Panhandle. Higher
elevations will also see measurable snowfall. A lead system over
the northern and eastern Gulf midweek will give way to a more
complex system working from the Aleutians midweek that is expected
to solidify across the northern Gulf into later next week/weekend.
Meanwhile, potent shortwaves interacting with a persistent Arctic
frontal boundary will allow organized precipitation periodically
across northern and Interior Alaska. In particular, an organized
low pressure system reaching northwest Alaska Thursday will bring
increased chances for enhanced rainfall inland into later next
week. The pattern supports below normal maximum temperatures for
much of Alaska.
Schichtel
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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