Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
323 PM EDT Fri Mar 22 2019
Valid 12Z Tue Mar 26 2019 - 12Z Sat Mar 30 2019
The most common theme in today's guidance is for a strong upper
ridge over mainland Alaska/Gulf of Alaska and Yukon to become more
suppressed toward days 7-8 Fri-Sat while another ridge builds into
the western mainland and over/north of Siberia. There is also
decent consensus toward a deep trough settling over the far
western Pacific with an embedded upper low near Kamchatka. The
most prominent question marks involve how quickly the initial
ridge aloft drops southward, and related very uncertain details of
flow ahead of the developing western Pacific trough and continuing
around the initial ridge. A deep storm reaching the far
northwestern Bering Sea at the start of Tue will push a dying
front into the western mainland early in the period with mostly
light accompanying precipitation (versus some heavier activity
just before 12Z Tue). Beyond that feature one or more shortwaves
aloft may produce light/scattered precipitation but with low
confidence in specifics.
The Tue-Wed portion of the forecast reflects an operational model
blend of the 00Z GFS/ECMWF/UKMET. Ensemble means have been
trending stronger with the upper ridge over the past 1-2 days,
adding support for this model cluster versus the weaker CMC or
06-12Z GFS runs. The ECMWF mean has been leading the GEFS mean
for the stronger trend. Although the ridge has trended stronger
there is somewhat of a southward trend for the upper high center.
Confidence is quite low in details of shortwave energy across the
Aleutians/Bering Sea and around the initial ridge. By Thu the GFS
runs are on the faster side of the spread with the GEFS means
considerably toning down the shortwave which GFS runs bring into
the Bering at that time. Meanwhile by Fri-Sat the 00Z ECMWF
becomes rather amplified with its shortwave over the mainland and
Yukon.
By late week/early weekend the ensemble means show better than
average agreement in depicting the western Pacific trough anchored
by an upper low near Kamchatka and the ridge building over and
northwest from the western mainland. Like earlier in the period,
the ECMWF mean is stronger with the ridge than recent GEFS means
with a gradually stronger trend over the past couple days. The
00Z CMC mean also forecasts a strong ridge Fri-Sat. Thus
preferences lean more to the ECMWF mean late in the period with
some NAEFS also included to account for the CMC mean.
Above considerations Thu-Sat lead to the early period model blend
transitioning toward increasing ensemble mean weight, reaching 100
percent by day 8 Sat.
Rausch
Bullet points from the WPC Days 3-7 Hazards chart can be found at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
- High winds across portions of the Aleutians, Mon, Mar 25.
- Much above normal temperatures across portions of mainland
Alaska, Tue-Wed, Mar 26-Mar 27.
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