Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
629 PM EST Sat Nov 16 2019
Valid 12Z Wed Nov 20 2019 - 12Z Sun Nov 24 2019
Models and ensemble means agree on fairly deep mean troughing
aloft aligned near or just west of the western coast of the
mainland for most of the period. This pattern will promote a
stormy period with high winds/waves along with heavy
precipitation. In particular a multi-day heavy precipitation
event is likely over favored terrain along the southern coast and
Panhandle.
A leading system should reach the mainland by the start of the
period early Wed with a front trailing into the northeastern
Pacific. Guidance shows a fair degree of scatter for surface low
track relative to the time frame, favoring a compromise approach.
There is some suggestion of a frontal wave reaching the Gulf
around Wed-Wed night but the 12Z GFS is a notable extreme with a
deep/western wave due to dynamics aloft hanging back farther west
than consensus. The 18Z GFS appears to correct this issue.
Guidance expects the next storm to become quite deep as it tracks
from the near the Aleutians to the southwestern mainland during
the latter half of the week. An intermediate solution again looks
most reasonable given existing spread for surface low track and
timing. The 12Z GFS has one of the most southern tracks and is in
the southern part of the full ensemble envelope (18Z GFS adjusted
a little farther north) while the 12Z ECMWF is slower than the
past couple runs and most other models/means. Based on data
available through the 12Z cycle, the 06Z GFS run compared best to
consensus for GFS input.
Aside from aforementioned detail issues, guidance exhibits good
enough consensus in principle to allow the initial blend to
include exclusively operational models--12Z/00Z ECMWF, 06Z GFS,
and 00Z CMC in order of higher to lower weight--from day 4 Wed
into day 6 Fri. Overall agreement was still rather good into day
7 Sat leading to only modest 12Z GEFS/00Z ECMWF mean inclusion at
that point. However by day 8 Sun the steady weakening of the mean
trough aloft and rapidly increasing detail spread across the North
Pacific favored rapid increase of ensemble guidance to 50-60
percent. In the days 7-8 period there is a signal for Pacific
energy to produce an area of low pressure somewhere between the
Alaska peninsula (12Z ECMWF) and Gulf of Alaska (12Z CMC), but
with very poor clustering/continuity among the full range of
guidance. Thus the manual forecast was on the
conservative/progressive side in depicting this feature.
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