Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 PM EDT Sat Aug 17 2019
Valid 12Z Wed Aug 21 2019 - 12Z Sun Aug 25 2019
A blend of the 00Z EC ensemble mean and the 06Z GEFS mean,
together with a smaller portions of the deterministic 00z
ECMWF/00z Canadian/06z GFS were used to generate the 500 mb height
forecasts and sea level pressures/fronts and gridded probability
of precip and wind fields for days 4 to 8.
Following the Wed low moving onshore into southeastern Alaska
Wednesday, the next system upstream develops in the north Pacific
south of the Aleutians.
On Thu, the low drifts east with a possible triple point low
developing well south of Kodak Island. The 12z GFS there in an
outlier solution of a rapidly deepening low further east with its
low forecast at 00z Fri near 54n 138w.
I would prefer to wait for better continuity and model and model
agreement on this development, so the 12z GFS was not used in the
blend.
The ECMWF and 06z GEFS ensembles indicate the possibility of a few
waves of low pressure in the Gulf of AK to southeast AK next
weekend. The gfs holds the axis of low pressure int he north
Pacific further southwest than the other models.
With a general cluster of lows over north Gulf of AK and northern
Pacific, a general ensemble mean and model blend was sued to
mitigate differences.
Over the Arctic Ocean, persistent low heights and a few waves of
low pressure are forecast this period. Several models and
ensembles indicate a wave on the front developing on days 4-5 as
the system drifts east. The 12z GFS and 00z Canadian ensemble
mean even have another wave developing at the base of the trough
next weekend, so the area will be monitored for these
developments. The 00z ECMWF was given less weight as it moves the
upper trough east faster than most models/ensemble members. More
weighting was given to the 06z GEFS Mean and 00z ECMWF Ensemble
Mean as a result.
Petersen
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