Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Sat Oct 19 2019
Valid 12Z Wed Oct 23 2019 - 12Z Sun Oct 27 2019
The extended forecast for Alaska continues to highlight two strong
storm systems, the first one most likely near the Alaska Peninsula
at the start of the period early day 4 Wed and the second tracking
into the Aleutians during the latter half of the week. Both
systems will bring the potential for areas of heavy precipitation
with locations from the Alaska Peninsula to the Panhandle seeing
highest 5-day totals. Expect strong winds and high waves to
accompany these systems as well.
For the leading system near the Alaska Peninsula to start the
period, guidance continues to show meaningful track and timing
differences but with a slow tendency toward convergence. One
favorable trend is that today more ECMWF ensemble members are
showing a strong surface low near the Peninsula than 24 hours ago,
helping its ensemble mean to be somewhat deeper--but likely still
excessively weak. Fewer ECMWF members have a suppressed wave that
would miss the Peninsula/Gulf of Alaska. Consensus of latest
operational guidance is providing increased confidence that the
central pressure should reach at least into the 970's mb while 12Z
runs in particular are adding to potential for the upper half of
the 960's--still not quite as deep as GFS runs. The GFS through
the 12Z cycle is still a bit on the western side of the spread
(albeit supported by GEFS means thus far), while the 12Z
UKMET/ECMWF are positioned between the GFS and farther east CMC.
The new ECMWF represents a fractional westward shift from its 00Z
run. Based on guidance behavior thus far and data available
through arrival of the 12Z GFS, an intermediate solution among the
06-12Z GFS and 00Z ECMWF/UKMET seemed to be the best way to guide
the forecast. The system should rapidly weaken over the following
couple of days.
The front trailing from this storm will extend into the
northeastern Pacific and possibly contain a wave that could have
some influence on the Panhandle during the latter half of the
week. Guidance shows a wide spread for this wave. Among 12Z
solutions the UKMET is deepest/farthest northwest while the GFS is
next closest (was the farthest northwest before the UKMET's
arrival). Other solutions are weaker and/or southeastward, while
ensemble spread is sufficient for the means to be fairly
ill-defined. Predictability is low due to the feature's small
scale and sensitivity to finer details of shortwave impulse(s)
aloft.
Over the past day guidance clustering has dramatically improved
for the depiction of deep low pressure reaching the Aleutians by
Thu-Fri and then weakening gradually as it tracks into the eastern
Aleutians/Bering Sea next weekend. A significant part of this
improved agreement relates to the resolution of how recurving
Typhoon Neoguri interacts with mid-latitude dynamics. Over recent
days the ECMWF mean, and with varied details operational runs,
have provided the best signal for a significant storm while it
took until the past 12 hours or so for the GEFS/CMC means to come
around to this idea.
Guidance generally agrees upon a weakness aloft near/north of the
northern coast of the mainland mid-late week. This weakness
should be pushed northward next weekend as the deep Aleutians
storm helps to build a downstream ridge aloft into the mainland.
A blend among the 06-12Z GFS/00Z ECMWF (plus the 00Z UKMET through
day 5 Thu) provided the best operational template for today's
forecast. A small amount of the 00Z ECMWF mean (deeper than the
GEFS for extratropical Neoguri) was incorporated by Fri.
Increasing spread of operational runs led to increasing the
ensemble weight to 40-50 percent on day 8 Sun.
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