Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
648 PM EDT Sat Aug 29 2020
Valid 12Z Wed Sep 02 2020 - 12Z Sun Sep 06 2020
...Guidance/Predictability Assessment...
Model and ensemble guidance agrees upon maintaining a mean trough
aloft from west of the mainland into the central Pacific, with a
general amplifying trend from midweek into next weekend.
Incorporation of tropical cyclone Maysak into an upper trough over
Asia will aid the evolution toward greater amplitude. Northeast
Pacific upper ridging should also build by next weekend, becoming
anchored by a 594-600 dm high forecast to be south or southwest of
Haida Gwaii by day 8 next Sun.
There are two primary features of interest and both involve
details that are small enough in scale to yield low predictability
in resolving differences among the guidance. Early in the period
low pressure near the Kenai Peninsula may anchor a front that
attaches to a Northeast Pacific wave which is most likely to track
south of the Panhandle. The overall pattern at the surface and
aloft has fairly good continuity but there are still noticeable
differences with track/timing of the southern wave. Then most
guidance expects a wave (with leading warm front) to track
somewhere between Bristol Bay and the Gulf of Alaska next weekend.
The 12Z CMC along with the 12Z GEFS-CMC/00Z ECMWF means and some
earlier ECMWF/GFS runs favor the Bristol Bay route while latest
GFS runs and the 12Z ECMWF are farther east. Also of note,
consolidating North Pacific low pressure may reach far enough
north to have some effect over/east of the eastern Aleutians at
some point next Sun.
A 12Z operational blend represented the consensus/continuity
evolution early in the period. Trending to a nearly even blend of
models and means (including a little 00Z ECMWF in addition to the
12Z run for that model's contribution) by days 7-8 Sat-Sun
provided a good depiction of the agreeable amplified pattern while
staying conservative with finer details. The resulting
PMSL/thickness pattern next Sun provided more support for the
eastern cluster with the frontal wave at that time, though a
separate southeastern Bering wave could anchor a forming
stationary front that extends across the southern mainland.
...Weather Highlights/Threats...
Expect areas from the Kenai Peninsula to the Panhandle to see the
highest precipitation totals for the Wed-Sun period. Low pressure
near the Kenai Peninsula during mid-late week will provide the
first focus for activity. Then another surge of moisture will
likely accompany an approaching warm front and possible anchoring
wave next weekend. Other parts of the mainland will see some
episodes of precip with varying intensity though with lighter
total amounts. Most of the mainland should see below normal highs
through the period with the greatest negative anomalies over the
south and Panhandle. There may be very localized pockets of above
normal readings on one or more days over the central/northern
mainland or northern coast. Meanwhile moderately below normal low
temperatures pushing into the southwest quadrant of the mainland
and then across the central mainland will trim some of the
coverage of above normal lows with time.
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