Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Mon Aug 16 2021
Valid 12Z Fri Aug 20 2021 - 12Z Tue Aug 24 2021
...Overview...
Continue to expect an axis of multi-stream mean troughing aloft to
extend from the Arctic through the Bering Sea/western Mainland
Alaska into the northern Pacific, leading to a cool and unsettled
period over a majority of the state. Recent guidance had been
quite varied among each other and in consecutive runs for
important embedded details but improved clustering in the latest
cycle provides some hope for improved stability going forward.
Once an upper trough and surface system drop southeastward after
early Friday, the main features of interest will be a North
Pacific system expected to track into the southeastern Bering and
then the mainland along with an Arctic system that will bring a
trailing front into the mainland during the weekend with portions
of the front continuing southward thereafter.
...Guidance/Predictability Evaluation...
The multiple streams in play and uncertainties over their
interaction still temper predictability somewhat. However
confidence has improved compared to previous days as latest models
and means have strongly gravitated to a more common solution.
With the slower CMC from recent days appearing to be the guru
model at the moment, consensus now shows a fairly slow-moving
closed upper low and associated surface system tracking from the
North Pacific into the southeastern Bering Sea Friday-Sunday.
This supports enough of a downstream shortwave ridge to allow the
feature initially over the mainland on Friday to drop more
southeastward as it tracks into western Canada. It still remains
to be seen how upstream Siberia/Bering energy and then perhaps
trailing flow may interact with the Pacific/Bering system as it
most likely continues into the mainland by early next week.
Meanwhile trends are to the fast side of the prior spread for the
compact upper low/surface system tracking across the Arctic and
pushing a front southeastward into the mainland.
The improved agreement among the operational models allowed for a
12Z model blend to represent the latest consensus from Friday into
early Sunday. Details still go astray to some degree by the
latter half of the period as Arctic and eastern Asia/Bering energy
feed into the overall mean trough aloft, which favored a
transition toward a mix of models (12Z GFS/CMC and 12Z-00Z ECMWF
runs) and means (12Z GEFS/00Z ECens). Total ensemble mean weight
reached 50-60 percent by day 8 Tuesday.
...Weather/Threats Highlights...
A broad area of precipitation across the southern three-fourths of
the mainland should taper off from west to east Friday-Friday
night. Pockets of enhanced totals are possible over some
southern/eastern locations. The system now expected to track from
the North Pacific into the southern-southeastern Bering Sea late
this week into the weekend should bring organized rainfall into
the Aleutians and the Alaska Peninsula. Some of this moisture
will likely extend into the mainland from late weekend into next
week as low pressure continues along. There are still significant
detail differences that affect the coverage and intensity of the
precipitation at that time. Some areas of moderate to heavy
totals are possible with the best signal currently along/near the
southern coast into the Panhandle. Meanwhile precipitation should
accompany the cold front dropping into the northern mainland but
with lighter totals. By the first part of next week there will be
a question of how much moisture with the system to the south may
interact with the front.
The forecast continues to show below normal high temperatures over
much of the state through the period but with some daily
variability with system progression. Anomalies are likely to
trend colder over northern areas during the latter half of the
period behind the Arctic cold front. Lows should be more moderate
with various areas of above/below normal readings. Best potential
for above normal lows will be over some southern areas early and
late in the period and over the North Slope on Saturday ahead of
the Arctic front.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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