Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EST Thu Mar 07 2019
Valid 12Z Mon Mar 11 2019 - 12Z Fri Mar 15 2019
Today's guidance continues to show general agreement and decent
continuity for the overall pattern evolution, though with some
adjustments and spread for embedded details. Consensus keeps a
mean trough axis aloft along or near the western coast of the
mainland while Aleutians/North Pacific shortwave energy likely
closes off a system well south of Kodiak Island by midweek. As
the system develops and lifts northward expect downstream upper
ridging to build over western Canada into the mainland. The
pattern should open up/flatten late in the week. This storm
should bring the most pronounced focus for precipitation and
strong winds within an overall pattern that will favor highest
precip accumulations along the southern coast and Panhandle.
Elsewhere expect the southern periphery of Arctic troughing to
brush the extreme northern mainland early in the week before
retreating, while North Pacific flow should remain progressive
behind the aforementioned leading shortwave.
For the full five days of the forecast the 00Z ECMWF/06Z FV3 GFS
and 06Z GEFS/00Z ECMWF means provided the best overall clustering
for significant features and thus provided the primary basis for
the forecast. Days 4-5 Mon-Tue used an operational model blend
that included the 00Z CMC along with the 00Z ECMWF/06Z FV3 GFS.
Then the blend gradually increased ensemble mean input to 30
percent on day 6 Wed and 50 percent for days 7-8 Thu-Fri.
Operational GFS runs were also reasonable early in the period.
There are persistent question marks with the specifics of initial
Aleutians/North Pacific energy that ultimately serves to develop a
potentially strong storm that tracks northward near Kodiak Island
and likely continues northward into the mainland as it rapidly
weakens. Leading Aleutians surface wave should reach near the
southwestern Alaska Peninsula by early Tue but with rapidly
expanding spread for its track thereafter. Then by Wed-Thu the
past day of ECMWF/ECMWF mean runs have shown a slower/westward
trend for strong low pressure expected to track toward Kodiak
Island--toward some GFS/GEFS runs over the past 1-2 days. The
full ensemble spread has displayed a noticeable reduction of
members on the eastern side of the envelope over the past day.
These trends have brought the ensemble means closer together. The
00Z ECMWF and 06Z FV3 GFS are most similar to the means and thus
provide the best enhancement to the ensemble mean forecast. This
cluster falls between the faster/northwest 00Z GFS and
slower/southeast 06Z GFS. The 12Z GFS has come in with a track a
little east of the majority cluster but with timing more similar
to consensus than the prior runs.
There is still considerable spread for the details of one or more
waves/fronts across the Aleutians/Pacific. On the positive side
there is still a general theme in the models/means for a leading
wave tracking well south of the Aleutians while a trailing
low/front may reach closer to the region.
Guidance has diverged a bit for the Arctic trough aloft that
should extend down to the extrene northern mainland early in the
week before retreating as a ridge builds over western Canada into
the mainland. GFS runs have been on the fast/shallow side of the
guidance spread with the trough while the GEFS mean at least
hedges a bit slower than the operational runs. The preferred
blend tilts the forecast 2/3 toward the slower scenario. This
feature will likely push a front into the central mainland, with
the front lingering for a while before lifting northward late in
the week.
Rausch
Bullet points from the WPC Days 3-7 Hazards chart:
https://www.wpc.ncep.noaa.gov/threats/threats.php
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Mon, Mar 10-Mar 11.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sun, Mar 10.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Wed-Thu, Mar 13-Mar 14.
- High winds across portions of mainland Alaska and the Aleutians,
Sun-Mon, Mar 10-Mar 11.
- High significant wave heights for coastal portions of the Alaska
Panhandle and mainland Alaska, Thu, Mar 14.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can 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