Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Fri Oct 11 2019
Valid 12Z Tue Oct 15 2019 - 12Z Sat Oct 19 2019
The majority of guidance shows similar ideas for the large scale
flow evolution over the course of the Tue-Sat period but with a
number of meaningful question marks for embedded features.
Consensus shows an axis of low heights (containing one or more
closed lows) from the Bering Sea to the northeast Pacific/Gulf of
Alaska into late week while an upper ridge initially near the
northern coast of the mainland lifts into the Arctic. Flow around
the southern side of the ridge should bring one or more bundles of
energy into/across the mainland. There is reasonable agreement on
amplification of western-central Pacific troughing aloft Thu-Sat
with the combination of the trough and persistent Bering upper low
energy providing support for low pressure development
Right from the start models/ensembles have been diverse and
inconsistent for the path of extratropical Hagibis. At the time
of forecast preparation (arrival of the 12Z GFS) there was
pronounced spread in forecasts valid early Tue, highlighted by 00Z
ECMWF ensemble members ranging from the northern Bering Sea to the
North Pacific. Ensemble means had adjusted somewhat toward the
slow Bering Sea scenario but the 00Z ECMWF/CMC and to some extent
00Z GFS represented the faster southern cluster that looked more
probable before guidance became more mixed over the past couple
cycles. Thus the initial forecast was based on the 00Z
ECMWF/CMC/GFS early in the period. However the new 12Z ECMWF
added to the northern Bering cluster, leaving the CMC as the only
southern solution. So now there is increased confidence in the
12Z GFS/ECMWF/UKMET and ensemble means.
After Tue gradually increasing weight of the 06Z GEFS/00Z ECMWF
means looked reasonable to balance the agreeable aspects of the
large scale pattern with less confident details. Some combination
of extratropical Hagibis and fast-moving upstream energy should
reflect as low pressure over the Bering on day 5 Wed with energy
feeding into low pressure that lingers over or near the Gulf of
Alaska. Then by Thu-Sat individual models and ensemble members
differ considerably for low pressure evolution in association with
the amplifying Pacific upper trough. The means do show better
than average clustering for the surface low, reaching around
Bristol Bay next Sat--which favors tilting the forecast more
toward the means at that time.
The upper ridge forecast to be near the northern coast of the
mainland as of early Tue has trended a little slower on its way
into the Arctic. Guidance seems to diverge for the ridge's path
more than was the case 24 hours ago. To the south of the ridge
there is also considerable uncertainty for the details of
retrograding shortwave energy across the mainland. These issues
also recommend an initial model blend that transitions more toward
the ensemble means with time.
Extratropical Hagibis and the generally active nature of North
Pacific flow should produce one or more episodes of strong
winds/high waves, while precipitation of varying intensity will
likely extend across the Aleutians/Alaska Peninsula and southern
coast/Panhandle.
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
Alaska Hazards...
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Oct 15-Oct 16.
- High winds across portions of the Aleutians, Mon-Thu, Oct 14-Oct
17.
- High significant wave heights for coastal portions of the Alaska
Panhandle, Wed-Thu, Oct 16-Oct 17.
- High significant wave heights for coastal portions of the
Aleutians, Mon-Thu, Oct 14-Oct 17.
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