Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
708 PM EDT Mon Aug 31 2020
Valid 12Z Fri Sep 04 2020 - 12Z Tue Sep 08 2020
...Overview and Guidance/Predictability Assessment...
Latest models and means continue to show the large scale pattern
settling into a very amplified regime between eastern Asia and
western North America, aided in part by two tropical cyclones
(Typhoon Maysak and newly formed T.S. 11W behind Maysak) that will
track northward into Asia. The combination of mean troughing
aloft from the Bering Sea into the central Pacific along with a
strong upper ridge building over the Northeast Pacific and into
eastern Alaska-western Canada will lead to increasing
precipitation over portions of Alaska by the weekend and early
next week. Best potential for highest totals will be from near
the Kenai Peninsula to the Panhandle. There is good agreement and
consistency with the overall pattern but guidance continues to
have trouble resolving important embedded details that will affect
precipitation coverage/intensity/duration.
Consensus evolution still shows digging of initial western Bering
Sea/Aleutians energy and surface low pressure into the North
Pacific late this week into the weekend, followed by progression
into the Bering Sea and western coast of the mainland by days 6-8
Sun-Tue. In addition there should be a leading wave that helps to
push a warm front to its east into the Gulf. Over recent days
guidance has not made any real progress in resolving the
evolution/track of the leading wave that will influence the timing
and extent of the leading surge of moisture. Latest GFS/ECMWF
runs represent ongoing continuity with a track reaching at least
as far northeast as Kodiak Island. CMC/UKMET runs have generally
held the wave farther west. Ensemble means that have tended to
divert their wave (whether in explicit or implied form) westward
of the GFS/ECMWF are quicker to build the downstream upper
ridge--certainly a possibility--but at the same time lose the
finer shortwave details that operational models tend to resolve
better. The main difference for the North Pacific to Bering Sea
low is that the 12Z GFS has slowed down toward the GEFS mean which
has been on the slower side of the spread for at least the past
day. A compromise tilted a bit more to the faster ECMWF cluster
provides a good account for the full guidance spread and
continuity.
Over the past couple days the ECMWF-based guidance has trended
considerably slower and weaker with possible low pressure now
expected to come into the picture near/south of the western
Aleutians by day 8 Tue. Earlier runs had been fast to bring out
Maysak's energy. There is a gradually improving signal for energy
to start reloading the central Pacific mean trough late in the
period though with uncertainty over what proportion could be
initially associated with Maysak versus separate northern stream
flow.
Forecast considerations led to a starting blend that incorporated
mostly 12Z operational guidance into day 7 Mon (total 12Z GEFS/00Z
ECMWF mean weight reaching only 25 percent at that time). The
blend removed the UKMET after Fri due to its divergence from other
guidance for important features. Greater weight of the ECMWF/GFS
led to only a modest westward adjustment from yesterday for the
leading wave. Day 8 adjusted to half models/half means, with the
means quite agreeable for the surface/upper pattern at that time
and ECMWF/CMC runs more comparable than the GFS.
...Weather Highlights/Threats...
Expect scattered precipitation of mostly light to moderate
intensity on Fri. Then a warm front/anchoring wave likely to
reach into the Northeast Pacific/Gulf and trailing deeper low
tracking from the North Pacific into the Bering Sea and the
western coast of the mainland should bring a pronounced increase
of moisture into the state during the weekend and early next week.
There are still significant uncertainties over the details of the
leading frontal wave and precise details of flow ahead of the
stronger storm system--thus tempering confidence in the specifics
of the precipitation forecast. The most common signal is for
highest totals through the period to be between the Kenai
Peninsula and the Panhandle. Areas farther west as well as
eventually extending north through the mainland may see some
locally significant precipitation as well. Within the axis of
greatest deep moisture the ensemble means have generally been
showing precipitable water values up to 2-3 standard deviations
above normal with operational GFS/ECMWF runs tending to reach as
high as 3-5 standard deviations above normal.
Most of the state will see below normal high temperatures late
this week followed by a gradual trend toward a mix of moderately
above or below normal readings depending on location/elevation.
For low temperatures expect below normal readings over the
southwest quadrant of the mainland on Fri with these negative
anomalies moderating as they drift eastward into the weekend.
Northern areas will see above normal lows. By early next week the
flow ahead of the North Pacific into Bering storm will spread
increasingly warm min temperatures across most of the state.
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 rain across portions of the Alaska Panhandle and mainland
Alaska, Sat-Mon, Sep 5-Sep 7.
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