Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
643 PM EST Sat Feb 29 2020
Valid 12Z Wed Mar 04 2020 - 12Z Sun Mar 08 2020
...Overview and Guidance/Predictability Assessment...
Latest models and ensembles still display better than average
agreement and continuity for significant features during the days
4-8 Wed-Sun period. There have been some differences/trends with
various specifics, typical for forecasts several days out in time.
The unusually good consensus in principle allowed for an
operational model blend (12Z GFS/ECMWF/UKMET/CMC) to start the
forecast days 4-5 Wed-Thu and maintaining 70 percent total
operational GFS/ECMWF weight relative to the 12Z GEFS/00Z ECMWF
means even into day 8 Sun.
An amplifying upper trough, anchored by a closed low likely to be
tracking near the Kenai Peninsula very early Wed, should progress
southeastward with time and encourage a similar path for one or
more surface lows over the Gulf of Alaska/Northeast Pacific.
Exact details for low pressure are still murky though. Consensus
has held steady for the initial low off the Kenai Peninsula early
Wed but most models aside from the 12Z UKMET have backed off
somewhat from the development of separate low pressure to the
south and reaching a position south of the Panhandle. The
aforementioned model blend provided a reasonable account for the
ongoing uncertainty. By early day 5 Thu this yielded a surface
low a bit west of the ensemble means--which admittedly have been
more stable than individual model runs over the past day.
Meanwhile the ensemble means are still in the process of catching
up to the operational runs in representing the definition of the
system forecast to affect the Aleutians/Bering Sea and parts of
the Alaska Peninsula Thu-Fri. The latest model average has
trended somewhat stronger from yesterday. 12Z ECMWF trend to
divert a greater proportion of upper dynamics and surface low
pressure to the southeast after early Fri aligns well with most
other latest guidance. The 12Z CMC strays from the majority
cluster after early Fri with minimal other support for its idea of
low pressure tracking into the western mainland. Thus the CMC was
dropped from the forecast blend after 12Z Fri.
Given the days 7-8 time frame, the GFS/ECMWF runs and
GEFS/ECMWF/CMC ensemble means are remarkably similar and
consistent for the storm forecast to emerge over the western
Pacific around midweek and track into the western/northern Bering
Sea next weekend. Consecutive cycles have varied a bit for
track/timing but an overall average has provided good continuity.
The unusually good guidance clustering (not counting the
slower/weaker CMC) allowed for keeping greater than usual
operational model weight through the end of the period. Expect
the upper ridge ahead of this system to progress from the Bering
Sea through the mainland late week through the weekend.
At higher latitudes expect northwesterly flow aloft early in the
period followed by more westerly progressive flow on average with
increasingly uncertain shortwave details over the Arctic and into
the mainland.
...Weather Highlights/Threats...
Corresponding to the progress of low pressure over the
Gulf/northeastern Pacific, precipitation over the southeastern
corner of the mainland and Panhandle should gradually decrease in
coverage and intensity from northwest to southeast during the
latter half of the week. An area of precipitation and brisk winds
will accompany the system expected to affect the Aleutians/Bering
Sea and parts of the Alaska Peninsula around Thu-Fri. The system
likely to track into the western/northern Bering Sea next weekend
should bring a broad shield of moisture and large area of brisk to
strong winds across the Aleutians/Bering Sea and eventually into
the mainland. Some locations in the southwestern half of the
mainland will see the best potential for focused precipitation by
the latter half of the weekend. The change in pattern over the
course of the period will lead to a pronounced warming trend from
initially well below normal temperatures. The North Slope region
should be first to see readings rebound to the above normal side
of the spectrum, followed by warmer air spreading from west to
east over central/southern areas. Some locations over the extreme
east/Panhandle could still be below normal into next Sun though.
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