Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
631 PM EDT Wed May 20 2020
Valid 12Z Sun May 24 2020 - 12Z Thu May 28 2020
...Overview and Weather Hazards/Threats...
The large scale flow pattern across Alaska is expected to remain
relatively unchanged through the extended forecast period. An
upper-level ridge is forecast to persist from the Arctic Ocean
across eastern Alaska and into northwest Canada, expanding through
time, resulting in above average temperatures (high temp anomalies
of +5 to +15 deg F) across much of the Interior. Farther south, a
broad region of cyclonic flow will remain in place across the
North Pacific from near/south of the Aleutians east to the Gulf of
Alaska, with mid/upper-level shortwave energy amplifying, and
associated surface low pressure systems deepening, as they pass
through this broad region of cyclonic flow. One such low pressure
system is forecast to slowly weaken across the eastern Aleutians
and southeastern Bering Sea Sun, with an elongated dissipating
occlusion extending east into the Gulf of Alaska. Widespread and
potentially heavy precipitation associated with this system is
forecast across portions of southern mainland Alaska through the
weekend. Precipitation should linger into Monday while gradually
diminishing in coverage as the front dissipates, however
persistent low-level onshore flow even after the front dissipates
will keep precipitation in the picture, especially for mountain
ranges in southern mainland Alaska where upslope flow will
persist. This pattern of general onshore flow should persist
through Tue, before another shortwave and low pressure system
approach, forecast to deepen as it passes well south of the
Aleutians and enters the Gulf of Alaska Wed-Thu, which will spread
precipitation and some gusty winds across the eastern Aleutians,
the Alaska Peninsula, and portions of southern mainland Alaska by
that time.
...Guidance/Predictability Assessment...
Overall predictability during the extended forecast period across
Alaska was above average, at least due in part to a relatively
stable and somewhat blocky large scale flow regime. The 12Z
ECMWF/GFS showed relatively similar solutions through most of the
forecast period, with some slight differences as to the
orientation/intensity of the upper-level ridge axis. Farther
south, the GFS and ECMWF were similar in the timing of the low
pressure systems, both the initial one Sun and the next one
approaching on Wed-Thu. Ensemble spread was below average, and
clustering of ensemble members surrounding the midweek low
pressure system was quite good considering the time range.
Confidence is increasing that a relatively deep low pressure
system may enter the Gulf of Alaska by the middle of next week,
although there remains some variability on the specific intensity
and timing/track. Given these considerations a blend of the 12Z
ECMWF/GFS was used as a forecast starting point during days 4-5
(Sun-Mon). After that, a gradual trend toward heavier ensemble
mean weighting was shown during days 6-8 (Tue-Thu), with majority
ensemble means on days 7-8.
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 southern Alaska, Sat-Sun,
May 23-May 24.
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