Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Fri Oct 18 2019
Valid 12Z Tue Oct 22 2019 - 12Z Sat Oct 26 2019
The dominant focus of the forecast will be on two potentially
significant storm systems within moderately progressive and
amplified North Pacific flow, including some interaction with
northern stream/Bering Sea flow. Most guidance shows the first
system likely tracking somewhere between Bristol Bay and the Gulf
of Alaska around Wed-Thu followed by another system tracking near
the Aleutians Thu-Sat. The first system should bring a period of
locally heavy rainfall from the extreme southern mainland into the
Panhandle (possibly as far west as parts of the Alaska Peninsula)
along with strong winds/high waves. The late week Aleutians
system could produce areas of significant precipitation/wind/waves
as well.
The forecast of the leading system remains more uncertain than
desired. Individual models and ensemble members are having
difficulty in resolving exactly how or to what degree an emerging
western Pacific shortwave/surface low will interact with northern
stream/Bering Sea energy. On one end are the latest GFS runs and
some earlier CMC runs that are strongest and farther westward
toward Bristol Bay or at least the Alaska Peninsula. Other
guidance, including the 00-12Z ECMWF/CMC/UKMET, is farther east to
varying degrees. GEFS members generally lean toward the western
operational run's track while 00Z ECMWF ensemble members are so
diverse (including a few that keep the Pacific wave so suppressed
that it does not even reach the Gulf) that the ECMWF mean has the
weakest low pressure among all models/means. For the purposes of
a single deterministic forecast the preference is to depict a
track between the two ensemble means (discounting the
fast/westward 00Z CMC mean representative of some early CMC runs)
with surface low depth reaching down to around 980mb--a middle
ground between the very deep GFS and the low probability
weak/suppressed scenario. This provides decent track-timing
continuity during the first half of the system's evolution. Then
the latest cluster of guidance with the upstream Aleutians system
would support greater downstream ridging aloft which would lead to
quicker weakening of the Alaska Peninsula/Gulf system.
As for the Aleutians system, GFS/ECMWF/CMC model runs and the
ECMWF mean have been showing a well-defined feature for the past
couple days or so, with typical spread for the details given the
distant time frame. Latest GEFS/CMC means are starting to hint at
a system but in much weaker form at the surface and aloft than any
of the aforementioned cluster. Thus the preference is to use a
blend of the operational runs and ECMWF mean to represent this
system, yielding a deeper low than yesterday's forecast but with
fairly similar track and timing.
At higher latitudes there is general agreement with a weakness
aloft (containing one or more embedded upper lows) along/north of
the northern coast of the mainland. Some troughing may curl
around into the northwestern corner of the mainland with time. A
model/mean blend resolves the medium to smaller scale detail
differences that have low predictability at extended time frames.
Based on forecast preferences, today's blend started with the
06-12Z GFS and 00Z ECMWF/CMC through early Thu and then gradually
increased 00Z ECMWF mean weight to yield an even split between the
models and ECMWF mean by the end of day 8 Sat.
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, Wed-Fri, Oct 23-Oct 25.
- High winds across portions of mainland Alaska and the Aleutians,
Wed-Thu, Oct 23-Oct 24.
- High winds across portions of the Aleutians, Fri, Oct 25.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Wed-Fri, Oct 23-Oct 25.
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