Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
700 PM EDT Sun Aug 30 2020
Valid 12Z Thu Sep 03 2020 - 12Z Mon Sep 07 2020
...Overview and Guidance/Predictability Assessment...
Guidance continues to evolve on some of the details but still
agrees on a pronounced amplification of a Bering Sea into central
Pacific mean trough and Northeast Pacific ridge, leading to a
surge of moisture reaching portions of the state by next weekend.
Two tropical systems forecast to move northward into Asia--Typhoon
Maysak and a trailing system that may develop--should
promote/maintain the strongly amplified pattern.
Over the past 12-24 hours the models and means have come into
better focus regarding the strong digging of Bering Sea energy and
flow to its west into the Pacific late this week, leading to a
stronger Bering Sea/Aleutians surface reflection as of early day 5
Fri. The amplifying energy should reach its southernmost point on
Sat, with associated vigorous low pressure tracking from the North
Pacific into the Bering Sea around Sun-Mon. There is still a
decent signal for a leading wave/warm front which will accompany
an initial push of moisture around Sat, but without much progress
compared to yesterday on resolving the wave's track which could
range between Bristol Bay and the Gulf of Alaska.
Today's forecast started with a 12Z operational model blend for
the first half of the period, followed by a quick transition to
half models (12Z GFS/ECMWF) and half means (12Z NAEFS/00Z ECens).
12Z GFS/GEFS runs represent a significant improvement over some
earlier weak runs for the central Pacific/Bering Sea evolution.
Still the GEFS mean is on the slow side of the guidance for the
primary system during the latter half of the period, supporting
use of the NAEFS compromise between the GEFS mean and fast CMC
mean instead. Late in the period questions arise over the extent
to which Maysak's energy rounds the far western Pacific upper
ridge. The ECMWF has backed off from its 00Z run somewhat but it
along with multiple ECMWF mean runs have consistently showed
associated low pressure reaching south of the Aleutians late in
the period, somewhat faster than most other guidance. This
feature is just outside of the forecast domain until late day 8
Mon but worth monitoring for possible downstream influence. At
the start of the forecast on Thu there is a slightly slower trend
for the modest low pressure/frontal system over/near the Gulf
along with the wave that should track south of the mainland.
...Weather Highlights/Threats...
Most of the state will see a brief drier trend from west to east
Thu-Fri with the weakening/departure of the low pressure and
frontal system initially from the southern coast of the mainland
into the Gulf of Alaska and Northeast Pacific. Then the
amplifying upper pattern will lead to a rapid increase of moisture
from the south-southwest during the weekend and likely continuing
to push farther north into the state early next week. A warm
front and possible anchoring wave reaching near the Alaska
Peninsula/Gulf on Sat-Sun will herald the leading surge of
moisture while flow ahead of the stronger system forecast to track
from the North Pacific into the Bering Sea Sun-Mon may produce a
more pronounced increase of moisture that would eventually extend
northward. Better-defined operational guidance suggests that
precipitable water values could reach 3-5 standard deviations
above normal for at least a brief time near the southern coast
with highly anomalous (but gradually decreasing) values reaching
northward into the mainland by early next week. Currently expect
the highest precipitation totals to be focused near or just east
of the Kenai Peninsula but with significant totals also possible
elsewhere between the Alaska Peninsula and the Panhandle. The
northward push of moisture early next week should also favor some
areas of focused activity over parts of the mainland.
A majority of the state will see below normal highs late this week
through the weekend, with the southern half of the mainland and
Panhandle seeing the greatest negative anomalies. Flow ahead of
the late-period system should lead to a warming trend by the start
of next week, with a mix of above/below normal highs depending on
location/elevation. For low temperatures expect modestly below
normal readings to expand across the southern half of the mainland
late this week into the weekend while northern areas remain above
normal. Then expect a trend to mostly above normal lows early
next week.
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