Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Thu Nov 07 2019
Valid 12Z Mon Nov 11 2019 - 12Z Fri Nov 15 2019
The best clustering of guidance shows strong height falls
progressing across the North Pacific and Aleutians early-mid week,
supporting a potentially strong storm for which the full range of
guidance has been very diverse for precise strength and track.
This energy/storm system will help to push leading northeast
Pacific/Gulf of Alaska shortwave energy into the long-term mean
ridge aligned over the British Columbia coast and Alaska
Panhandle. Meanwhile expect an upper high to close off northwest
of the mainland while cyclonic flow develops to farther east and
including the northern mainland. Later in the period details
become murky within the general col region depicted over parts of
the mainland/eastern Bering Sea. Finally, a progressive system
should track into the western-central Pacific over the course of
the week.
Over the past day guidance has been trying to converge toward a
common solution for northeastern Pacific/Gulf of Alaska low
pressure during the first half of the week. In particular GFS
runs have trended eastward while the ECMWF has adjusted farther
north. The best clustering of ensemble members would favor a
track closest to the 12Z GFS/00Z ECMWF. However latest CMC/UKMET
runs and some GEFS/ECMWF ensemble members still offer potential
for a more southern track, associated with stronger energy aloft.
Prefer the GFS/ECMWF cluster given that stronger/southern
solutions may not be ideal for a feature heading into a mean
ridge. The 12Z ECMWF has come in similar to its previous run.
Arrival of the new 12Z guidance seems to be adding some clarity to
the deep storm system that may develop over the North Pacific and
possibly reach the eastern Aleutians and/or Alaska Peninsula.
Some GFS runs had stayed from consensus in tracking the storm well
northwest into/through the Bering Sea but have been consistent
with depicting a very strong system. Other models have varied
more in strength but at least would keep the storm farther
southeast. The new 12Z GFS fell in line quite well with the
ensemble means (which have adjusted southeastward some from
yesterday), favoring a 12Z GFS/00Z ECMWF and 06Z GEFS/00Z ECMWF
mean compromise as a reasonable starting point. The 12Z UKMET/CMC
have come in with a very deep system along a similar path, so now
there is somewhat more confidence in going with a deeper system.
The new ECMWF is also quite deep though a bit east of the majority
into Wed.
The general signal for the upstream western-central Pacific system
has good continuity. However over the past day most models and
ensemble means have made a pronounced southward shift in dynamic
focus/low pressure, with less influence on the western Aleutians
than forecast by the 12Z GFS.
With time, confidence decreases in specifics of flow aloft over
the mainland as a weakness develops southeast of the closed upper
high north of eastern Siberia. The most common idea shared by
latest model/ensemble guidance is for some energy within the
cyclonic flow over the northeastern mainland to begin stretching
southwestward late in the week. Fortunately the surface pattern
continues to exhibit strong clustering regardless of specifics
aloft, with a front dropping south over the mainland while the
surface gradient strengthens between high pressure over the Arctic
and low pressure near the Alaska Peninsula.
Based on the above preferences through arrival of the 12Z GFS, the
forecast blend started with the 12Z GFS/00Z ECMWF and 06Z GEFS/00Z
ECMWF means through the period. 70 percent weight went to the
operational runs early-mid week followed by a trend to 50/50
model/mean weight. The northeastern Pacific/Gulf system will
bring the best potential for enhanced precipitation to parts of
the Panhandle. The deep storm tracking near the Peninsula should
be accompanied by areas of strong winds over/near the North
Pacific and support a period of significant precipitation from the
eastern Aleutians and/or Peninsula into the southern coast and
Panhandle. The latter areas should see highest totals with best
flow of moisture ahead of the storm.
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
- Much above normal temperatures across portions of mainland
Alaska, Sun-Wed, Nov 10-Nov 13.
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