Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
318 PM EDT Sat Sep 07 2019
Valid 12Z Wed Sep 11 2019 - 12Z Sun Sep 15 2019
A good majority of model and ensemble guidance agrees fairly well
that deep Bering Sea low pressure and associated upper low/trough
should gradually weaken by next Sat-Sun as the system reaches the
mainland and vicinity. The initial surface low reflects the
extratropical evolution of Typhoon Lingling. This system should
produce a broad area of fairly strong winds while leading
flow/moisture will likely focus significant precipitation across
the extreme southern mainland and into the Panhandle.
The most prominent detail question early in the period involves
Typhoon Faxai (currently to the southeast of
Lingling)--specifically if/when/how it interacts with the initial
Bering low/trailing front. Thus far recent and latest operational
model runs have varied with no clear high-probability scenario yet
apparent. An operational model blend including the 00-06Z GFS and
00Z ECMWF/UKMET provides the best starting point for the manual
forecast, indicating some potential for merging like the 00Z
ECMWF/UKMET but a bit less aggressively. The main theme is to
depict a deeper Bering system than shown in the ensemble means.
Note that the 00Z CMC begins to stray north of consensus with the
Bering system and in addition pulls the Wed-Thu system over the
northeastern Pacific well southward of consensus, so that model
could not be used.
By the latter half of the period upstream shortwave energy may
produce a surface wave from the North Pacific into the Gulf of
Alaska but again clustering and continuity among the operational
runs have not been that great and ensemble members are diverse
enough for a North Pacific wave not to show up in the means.
Meanwhile solutions gradually diverge for the exact position of
the Bering low that reaches the western coast of the mainland.
Trending the forecast increasingly toward the agreeable ensemble
means with time provides the best starting point, with day 7 Sat
still containing enough operational guidance to provide a hint of
waviness tracking to the south of the eastern Aleutians/Alaska
Peninsula. Low pressure over the Gulf of Alaska by day 8 Sun
would have support from some combination of North Pacific
shortwave energy and/or energy approaching from the eastern
Bering. The overall day 8 pattern has been consistent over
multiple GEFS/ECMWF mean runs so confidence in having a Gulf low
by day 8 is higher than the specifics of how it reaches that
position.
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
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