Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
639 PM EDT Mon May 04 2020
Valid 12Z Fri May 08 2020 - 12Z Tue May 12 2020
...Guidance/Predictability Assessment...
Models/ensembles show relatively good consensus at the large
scales through the extended forecast period, with a persistent
trough/upper-level low south of Alaska across the North Pacific,
and an upper-level ridge axis building from western Canada into
mainland Alaska next week. This setup will result in a Rex Block
pattern, that will persist through the end of the extended
forecast period. Even at smaller scales, deterministic model
consensus is above average during days 4-5 (Fri-Sat) as one low
pressure system slowly weakens south of the Gulf of Alaska and
another stronger system crosses the western Aleutians and moves
into the western Bering Sea. The above average consensus during
this time frame lent to the use of a 12Z ECMWF/GFS blend during
days 4-5. By days 6-8 (Sun-Tue), the preferred storm track does
not change substantially, but guidance spread gradually increases
through time. Spread is highest days 7-8 with respect to another
surface low forecast to move into the western Bering Sea. Also by
that time, the GFS was a bit quicker than other model/ensemble
guidance to weaken the western side of the upper ridge, allowing
more shortwave energy to affect western portions of mainland
Alaska. This solution does not have overwhelming support among
ensemble guidance at this time, and thus emphasis on the GFS was
reduced substantially through time. Given the decent large scale
agreement among the ensembles, opted to gradually increase
weighting of ECENS/GEFS ensemble means through time, with majority
weight on the means by days 7-8.
...Weather/Hazard Highlights...
Persistent low pressure south of the Gulf of Alaska will keep
onshore flow and precipitation, along with gusty winds, in the
picture for the Alaska Peninsula and portions of southern mainland
Alaska through the forecast period, especially Fri as a weakening
frontal boundary approaches. The western/central Aleutians will be
subject to rain and gusty winds Fri-Sun in association with a low
pressure system. Rising heights across mainland Alaska as the
upper ridge builds will promote warming temperatures. High
temperatures are forecast to be 5 to 15 deg F (even 20 deg in
some cases) above average across much of mainland Alaska through
much of the forecast period, with temperatures forecast to rise
into the 60s (possibly even surpassing 70) for portions of the
Interior.
Ryan
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
Hazards:
- Heavy precipitation across portions of mainland Alaska, Thu-Fri,
May 7-May 8.
- Heavy precipitation across portions of mainland Alaska and the
Aleutians, Fri-Sat, May 8-May 9.
- Much above normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Fri-Mon, May 8-May 11.
- Much above normal temperatures across portions of mainland
Alaska, Thu-Mon, May 7-May 11.
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