Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Tue Jun 04 2019
Valid 12Z Sat Jun 08 2019 - 12Z Wed Jun 12 2019
The large scale pattern across Alaska and the North Pacific is
expected to remain relatively stable through the extended forecast
period. Ensembles show consensus that positive height anomalies
should persist across the Bering Sea through the middle of next
week. Upper ridging is also expected to become reestablished
across western Canada, with an axis of positive height anomalies
extending north/west across mainland Alaska to the Bering upper
high. This setup will keep the predominant storm track/strongest
upper jet suppressed to the south of Alaska and perhaps just south
of the Aleutians. Models showed significant detail differences
despite overall good large scale agreement. Shortwave energy
traversing the strong North Pacific upper jet was one point of
contention. The ECMWF was not favored in this area as it seemed to
over amplify upper shortwave energy passing south of the Aleutians
Sat-Sun, in between the two larger scale troughs (one across
eastern Russia and another approaching the Gulf of Alaska). The
GFS kept flatter flow or even slightly anticyclonic flow in this
area, which made more sense conceptually. The solution across this
area has downstream implications on the timing of a low pressure
system expected to move into the Gulf of Alaska early next week
(the ECMWF was much faster with this feature due to its somewhat
less blocky flow at 500 hPa).
Farther north, the ECMWF was generally weaker than consensus with
the upper ridge axis across interior Alaska, which would allow
more arctic shortwave energy to dig south into the interior than
the GFS (and CMC) which keep a stronger ridge in place with less
potential for energy from higher latitudes to move south. There
was some consensus evident by the end of the period that an upper
low should potentially approach the western Aleutians, with the
potential for an associated relatively deep surface low. Despite
broad consensus on this feature, detail differences were quite
large on the timing, track, and intensity of such a system, and
thus a consensus/ensemble approach was preferred. Given these
considerations, the GFS was favored among the deterministic
guidance, and was used heavily during days 4-5 (Sat-Sun), with a
shift to majority GEFS/NAEFS ensemble mean weighting for days 6-8.
Ryan
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
- No significant hazards are expected over Alaska during this
forecast period.
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