Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
648 PM EDT Tue May 19 2020
Valid 12Z Sat May 23 2020 - 12Z Wed May 27 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, 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 Sat-Sun, with an elongated 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 approaches the Gulf of Alaska Tue night/Wed, which
will start to spread precipitation and some gusty winds across the
eastern Aleutians and the Alaska Peninsula by that time.
...Guidance/Predictability Assessment...
Overall predictability during the extended forecast period across
Alaska was at least slightly 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 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 Sat-Sun and the next one
approaching on Wed. Ensemble spread was perhaps a bit below
average, but forecast confidence is not quite as high as the
GFS/ECMWF similar solutions would suggest, as both models have
shown some degree of run-to-run variability. Given these
considerations a blend of the 12Z ECMWF/GFS was used as a forecast
starting point during days 4-5 (Sat-Sun). After that, a gradual
trend toward heavier ensemble mean weighting was shown during days
6-8 (Mon-Wed), 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 mainland Alaska, Fri-Sun,
May 22-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