Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Sat Sep 28 2019
Valid 12Z Wed Oct 02 2019 - 12Z Sun Oct 06 2019
Amplified yet progressive upper pattern will trend toward renewed
troughing in the Gulf of Alaska by next weekend. While the
longwave pattern remains generally predictable through about
Friday, the northern/southern stream interaction/orientation has
resulted in some mixed deterministic model signals in the interim.
By next weekend, ensembles trend out of phase out of Asia into the
Bering with the ECMWF ensembles flatter while the GEFS/Canadian
members show ridging.
North Central Pacific system Wednesday will lift toward the
eastern Aleutians/Kodiak around Thursday but likely turn eastward
into the southern Gulf. Ensemble spread remains both along and
across the preferred track (mostly near the 00Z ECMWF) but a
middle of the road approach seemed to fit the upper pattern. At
the end of the week, uncertainty increases west of the Dateline
with a northern stream system diving southeastward through
western/southwestern Alaska and into the Gulf. ECMWF ensembles
lift the main low northeastward but all ensembles ultimately
showed Gulf redevelopment (triple point development). Further
complicating the flow will be the likely extratropical transition
of current tropical storm Mitag. JTWC forecast takes this across
southern South Korea near the 00Z ECMWF track. Timing of and
interaction with a northern stream trough (or multiple troughs)
over northeastern Russia will ultimately determine the downstream
flow. Though the ECMWF ensembles indicated less interaction/more
separation and flatter flow across the Bering, opted to lean more
on the GEFS/Canadian ensembles that showed more interaction and
downstream ridge response into the Bering given the low
predictability area (Siberia) of northern stream flow. Confidence
is low in the western portion of the area next weekend.
Fracasso
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