Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
650 PM EDT Fri Aug 21 2020
Valid 12Z Tue Aug 25 2020 - 12Z Sat Aug 29 2020
...Overview and Guidance Evaluation/Preferences...
The primary focus of the forecast continues to be on two dominant
features. The first is an upper low/associated trough tracking
from the Bering Sea through the Gulf of Alaska/Northeast Pacific,
currently expected to cross the Panhandle around late Thu-early
Fri. A surface low of moderate strength and trailing front will
accompany the upper dynamics. Farther west there is still a
decent signal for low pressure development between the North
Pacific and Bering Sea in association with consolidating energy
that originates from eastern Asia. However models/ensemble
members continue to show a lot of spread for exactly how the
system will evolve. Also of note, leading energy aloft will
support an early-period wave over the Gulf of Alaska and a system
behind the North Pacific/Bering feature may begin to affect the
western Aleutians by day 8 Sat.
The 12Z ECMWF/GFS/UKMET represent the majority cluster for initial
Gulf low pressure and the system that tracks east-southeast from
the Bering Sea, with reasonable support from the ensemble means.
The CMC strays to the slow/deep side of the spread, as has been
its tendency over the past day or so. The non-CMC consensus has
trended a bit slower over the past 12-24 hours though.
Guidance spread has not really improved for North Pacific into
Bering Sea evolution. ECMWF/ECMWF mean runs have been consistent
in showing fairly early and strong consolidation of upper
dynamics, leading to a deep Aleutians/Bering Sea system. The 00Z
ECMWF may have been too deep with some 500mb height anomalies
reaching at least four standard deviations below normal. The new
12Z run is less extreme. 06Z/12Z GFS runs take longer to develop
and ultimately bring a more suppressed North Pacific wave into the
far eastern Bering as it deepens while the GEFS mean is slower and
quite weak. The 12Z CMC tracks southeast of the GFS after Fri.
For another alternative, the CMC mean holds low pressure back over
the western Aleutians. With low confidence today's preference is
to emphasize a solution closest to the ECMWF/ECMWF mean--providing
better continuity than other options. The main adjustment is a
moderate southward trend around day 6 Thu.
ECMWF/CMC runs, some GFS runs, and the 12Z CMC mean all indicate a
late-period system that may reach the far northwestern Pacific and
possibly affect the western Aleutians by day 8 Sat. The 06Z GFS
was much more comparable to the majority cluster's western Pacific
pattern than the 12Z run late in the period.
Combining preferences for features of interest, the starting blend
used the 12Z ECMWF/GFS/UKMET/CMC (ordered from most to least
weight) early in the period and then transitioned to mostly 12Z
ECMWF/00Z ECMWF mean emphasis, while minority GFS input switched
from the 12Z run to 06Z run around mid-period.
...Weather Highlights/Threats...
The combination of a leading Gulf of Alaska wave and trailing
system tracking east-southeast from the Bering Sea will favor
highest precipitation totals along the southern coast and
Panhandle, while other portions of the mainland also see episodes
of precip with varying intensity. Expect a drier trend from west
to east late next week. From Thu into next weekend an area of
enhanced precipitation and stronger winds should spread across
parts of the Aleutians/Bering Sea and eventually into the western
mainland with developing North Pacific-Bering Sea low pressure.
Continued significant spread for exact system evolution keeps
confidence fairly low for determining the coverage, timing, and
intensity of sensible weather effects.
Below normal high temperatures will be most likely over the
southern mainland and Panhandle for most of the period. Other
areas should vary between above or below normal highs depending on
location/elevation. Meanwhile a large percentage of the state
should see above normal lows. Expect the warmest anomalies to be
over the northern mainland. There may be a few pockets of
slightly below normal min temps over central/southern areas.
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
No significant hazards are expected over Alaska during this
forecast period.
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