Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
348 PM EDT Thu Oct 10 2019
Valid 12Z Mon Oct 14 2019 - 12Z Fri Oct 18 2019
Most guidance shows a very strong northern Pacific stream to the
south of an axis of low heights aloft (containing multiple upper
lows) from the Bering Sea into northeastern Pacific/Gulf of
Alaska. Meanwhile an initial upper ridge over the mainland should
drift to the north or northwest of the mainland by midweek. The
overall pattern should favor highest five-day precipitation totals
from the eastern Alaska Peninsula and Kodiak Island to the
Panhandle with lower but still meaningful amounts across the
Aleutians.
The most prominent system of interest will be the extratropical
evolution of current Super Typhoon Hagibis, likely reaching the
western Aleutians by early day 4 Mon and tracking generally
eastward thereafter. There are still track and timing
uncertainties for this system which will likely produce an episode
of strong winds and high waves especially to the south of the low
track. Through the 00Z cycle recent trends/ECMWF continuity led
to a majority guidance cluster showing a track near the Aleutians
and then into the northeastern Pacific--versus a farther north
path into the Bering per some earlier ensemble means. However
06Z-12Z guidance is a little more mixed with the GFS and 12Z UKMET
having adjusted slower/northward. The 12Z GEFS mean still shows
faster timing than the 12Z GFS/UKMET after early Tue though, and
the new ECMWF/CMC maintain the faster/south scenario. Based on
data available through the arrival of the 12Z GFS, the initial
blend started with the 00Z GFS/ECMWF/UKMET that represented the
best cluster to provide reasonable detail. Even an intermediate
approach yielded a surface low deepening below 960mb near the
eastern Aleutians by early day 5 Tue.
Stemming in part from differences with extratropical Hagibis
earlier in the period, the operational models and ensemble means
differ for overall low pressure expected to settle over the
northeastern Pacific/Gulf of Alaska by mid-late week. Preferred
to trend the forecast toward a model/mean blend (00Z GFS/ECMWF and
06Z GEFS/00Z ECMWF mean) given decreasing confidence in any
particular solution for that time frame.
Farther upstream some guidance is signaling potential for a system
reaching the Bering by day 6 Wed. The ECMWF had been fairly
consistent for multiple cycles through the 00Z run while the less
consistent GFS agreed in its 00Z run (as did the CMC), offering
support for a fair amount of operational model input to enhance
the ill-defined means. The full array of 12Z guidance lowers
confidence in specifics as the ECMWF/UKMET are weaker and faster.
Then another area of general low pressure may develop over the
Bering/Pacific by day 8 Fri as an upper trough develops over the
western Pacific.
Upper flow over the mainland will tend to lack definition (with
considerable uncertainty in the fine-scale details) between the
ridge that settles into the Arctic and upper lows over the Bering
and northeastern Pacific. New 12Z guidance suggests the upper
ridge could be a little stronger than the 00-06Z model/model-mean
blend that served as the basis for today's forecast.
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
Alaska Hazards...
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Oct 15-Oct 16.
- High winds across portions of the Aleutians, Mon-Wed, Oct 14-Oct
16.
- High significant wave heights for coastal portions of the Alaska
Panhandle, Thu, Oct 17.
- High significant wave heights for coastal portions of the
Aleutians, Mon-Wed, Oct 14-Oct 16.
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