Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
554 PM EST Sun Nov 17 2019
Valid 12Z Thu Nov 21 2019 - 12Z Mon Nov 25 2019
Models and ensemble means continue to show very good agreement on
fairly deep mean troughing aloft becoming aligned near or just
west of the western coast of the mainland for most of the period,
slowly weakening by early next week. This kind of pattern will
promote a stormy period featuring heavy precipitation along with
high winds/waves for some. In particular, a multi-day heavy
precipitation event appears likely over favored terrain along the
southern coast and the Panhandle.
The forecast period begins with lead shortwave energy crossing the
Panhandle on Thursday while a rather deep cyclone (near 960mb)
tracks across the Aleutians and into the eastern Bering Sea,
eventually settling across the southwest mainland by the start of
next weekend. Expect periods of high winds/waves for the Aleutians
and western mainland coast. Less predictable pieces of energy
should round the base of the mean upper trough into the Gulf of
Alaska providing continued support for heavy precipitation along
the southern coast and Panhandle through at least next weekend.
Models show remarkable agreement on this system and so a majority
blend of the latest 12z models (GFS/ECMWF/CMC) was used which
maintains good WPC continuity.
The biggest forecast differences arise not until the end of the
period as another system lifts north and east towards the Gulf of
Alaska. Models continue to struggle both with timing and intensity
of this with the 12z GFS the fastest with this next wave, while
the ECMWF is notably slower. The 12z CMC is also slower and far
deeper so the CMC was taken out of the blend after day 6. The
ensemble means would favor something slower and more in line with
the ECMWF. Behind this, another cyclone drops down towards/across
the far western Aleutians by day 8, and again, the deterministic
models show significant differences with timing/progression of
this. For both systems discussed, a blend of the agreeable
ensemble means (12z GEFS and 00z ECENS) was used for days 7-8. The
ECMWF also made up a small portion of the blend for days 7-8 to
add a bit of definition to the usual washed out means given that
it was the deterministic model that most closely resembled the
mean pattern.
Santorelli
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