Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
727 PM EDT Wed Mar 25 2020
Valid 12Z Sun Mar 29 2020 - 12Z Thu Apr 02 2020
...Below to much below normal temperatures to continue over parts
of Alaska into next week...
...Overview...
Guidance continues to show significant spread and variability for
various aspects of the forecast. The most prominent discrepancies
involve the evolution of the upper ridge to the west of the
mainland and an upper low to the south of the Aleutians at the
start of the period early day 4 Sun. Handling of the upper
ridge/surrounding flow, the North Pacific upper low, and fairly
low-predictability progressive upstream flow will all play a role
in the specifics of individual surface lows within a favored area
of mean low pressure over the Gulf of Alaska/Northeast Pacific.
...Guidance Evaluation/Preferences...
For the upper ridge initially west of the mainland, over the past
day the GFS has made a pronounced trend toward recent ECMWF/ECMWF
mean runs that have been closing off a high by Sun with this high
continuing across the Arctic with time. GEFS mean runs have been
closing off a high as well but with a path somewhat farther
south/east. Alternative ideas persist, such as the
open/progressive ridge in the 12Z CMC/CMC mean and a closed high
which then opens up in the 12Z UKMET. Continuity and the current
array of guidance would favor the GFS/ECMWF cluster for the upper
ridge.
Meanwhile even among that cluster (including the past two ECMWF
runs) there are significant differences for shortwave/upper low
energy that travels to the south of the upper high and upstream
energy that flows into the northeastern Pacific from the North
Pacific/Bering Sea. Both streams may have a significant influence
on Gulf of Alaska/northeastern Pacific surface low evolution. At
this time prefer a blended approach to provide a conservative
depiction of the energy/possible upper low that could travel over
the southern half of the mainland, as well as the additional
energy from farther west.
As for the North Pacific upper low the short range portion of the
12Z GFS splits the feature in a way not shown by other guidance,
leading to the remaining upper low hanging back much farther west.
The 06Z GFS run compared better in principle. Still the GEFS/CMC
ensembles are quite slow and the CMC/UKMET ultimately drop the
upper low far enough south for it to be a non-factor in the Alaska
forecast.
By Tue-Thu there is still a weak signal for a possible wave
crossing the Bering Sea while latest trends from some of the
operational runs place more energy on a system tracking south of
the Aleutians and Alaska Peninsula. Ensemble means have been less
defined and generally keep lowest pressures over the Bering. The
12Z ECMWF mean is starting to hint at a trend toward the
operational scenario. Interestingly some of the best agreement
among the means during the entire period is for low pressure to
consolidate to the south of the Alaska Peninsula by day 8 Thu.
ECMWF runs and the 06Z GFS compare well to the means with this
aspect of the forecast.
The various considerations noted above led to a starting mass
field blend that included the 06Z GFS and 12Z/00Z ECMWF runs along
with the 00Z ECMWF mean/12Z GEFS mean. Operational model weight
was 70 percent early-mid period and 50 percent by days 7-8 Wed-Thu
while ECMWF mean input was somewhat higher than that of the GEFS
mean.
...Weather Highlights/Threats...
Below to much below normal temperatures should persist into next
week over a considerable portion of the state, with greatest
negative anomalies likely to be over interior and eastern areas.
Exact anomalies and any potential moderation (likely delayed
somewhat) will depend on very uncertain specifics of flow aloft.
Locations over and near the North Slope region may see some above
normal readings after the early part of next week. The most
coherent signal for highest five-day precipitation totals is
confined to the southern Panhandle, in association with multiple
surface lows expected over the northeastern Pacific/Gulf of Alaska
during the period. Some of this moisture may extend back to the
southern coast of the mainland but with low confidence for timing
or amounts. The Aleutians should see one or more episodes of
precipitation while activity associated with the system currently
forecast to be south of the Peninsula next Thu will be very
sensitive to exact storm track.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Sat, Mar 28 and Mon-Tue, Mar 30-Mar 31.
- High winds across portions of the Alaska Panhandle, mainland
Alaska, and the Aleutians, Sat-Mon, Mar 28-Mar 30.
- Much below normal temperatures across portions of mainland
Alaska, Sat-Tue, Mar 28-Mar 31.
- Much below normal temperatures across portions of mainland
Alaska, Sat, Mar 28.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Sat, Mar 28.
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