Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 PM EDT Mon Oct 07 2019
Valid 12Z Fri Oct 11 2019 - 12Z Tue Oct 15 2019
The latest model guidance generally agrees on a progressive flow
in the polar westerlies, with a series of systems moving across
the Aleutians, through the Gulf of Alaska and into the panhandle
region. To the north across much of interior Alaska, upper
ridging will predominate and result in mostly dry weather during
this medium range period. There are two main forecast challenges
today. The first is the timing and strength of the systems moving
through the polar stream, and the second is with the extratropical
transition of super typhoon Hagibis and its potential impacts on
the state later in the period.
Right at the beginning of our period, the models are struggling a
bit with the amplifying trough over the Gulf of Alaska on
Thursday-Friday. There has been a fair amount of run-to-run
inconsistency in exactly how this feature evolves, but there is
good agreement on its impact. Strong southerly flow ahead of the
developing surface low and cold front will draw deep moisture
northward and bring heavy precipitation (on the order of 1-2
inches into south-central Alaska and the panhandle late Thursday
through Friday. The main difficulty is with the timing of the
QPF, but right now am favoring the 12Z GFS, which is a compromise
between the latest ECMWF and Canadian.
Upstream, another closed low will push across the Bering Sea
Friday-Saturday and then through the northern part of the Aleutian
chain later Saturday into early Sunday. Models all are in pretty
good agreement with this feature until it gets in the Gulf of
Alaska later Sunday, and examining the latest guidance, it appears
that the differences with this system are at least partly tied to
the handling of Hagibis. All of the models show a very robust
surface system as the storm makes the extratropical transition,
with the Canadian and GFS showing a sub 950mb low tracking through
the Bering Sea. The Canadian is considerably faster and farthest
west with the system, and amplifies a short wave ridge of the
Aleutians and Gulf of Alaska early next week. This has the effect
of pushing the downstream upper low well southward into the
eastern Pacific. The GFS and ECMWF track the system farther
south, and allow the initial closed low over the Gulf of Alaska to
combine with another rapidly moving short wave that is tracking
ahead of Hagibis. Considering the uncertainty with the evolution
of Hagibis, the forecast confidence is relatively low for Days 6-8
over the Aleutians, south central and southeast Alaska is low. We
generally took a blend of the latest GFS, the 00Z ECMWF ensemble
mean, and the 06Z GEFS mean for Days 6-7, and mostly a 50/50 split
between the two ensemble means by Day 8. What does seem likely is
that a very strong low will spin up as Hagibis transitions and
could have sigificant impacts with respect to winds and waves over
the Bering Sea late this weekend through early next week.
Klein
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 rain is expected across portions of the Alaska Panhandle and
mainland Alaska, Thu-Fri, Oct 10-Oct 11.
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