Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
500 PM EDT Sun May 24 2020
Valid 12Z Thu May 28 2020 - 12Z Mon Jun 01 2020
...Guidance/Predictability Assessment...
Alaskan medium range forecast predictability remains above average
due to a relatively stable and blocky larger scale flow regime,
but there has been more uncertainty with offshore lows. Run to run
continuity has improved over the last two days, but forecast
spread still increases steadily after Thursday with small-mid
scale weather features embedded within the larger scale flow.
Accordingly, the WPC Alaskan medium range product suite was
primarily derived from a composite blend of the 12 UTC GFS/ECMWF
for improved detail along with the GEFS/ECMWF ensemble means and
the 19 UTC National Blend of Models for better consistency.
Increased blend weightings toward the ensemble means Friday onward
amid growing uncertainty, but applied manual adjustments to ensure
offshore low strengths consistent with upper support. This acts to
maintain decent WPC continuity.
...Overview and Weather Hazards/Threats...
An amplified upper-level ridge is forecast to persist from the
Arctic Ocean across eastern Alaska and into northwest Canada this
week. This will result in above average temperatures with
anomalies upwards of 15 deg F over northern Alaska. This will
enhance snow/ice melt, so please refer to your local office for
any seasonal river ice flow issues. This ridge may tend to spread
increasingly over a warm Interior later period.
A system well south of the state over the eastern Pacific
mid-latitudes should organize early week. This system has been
difficult to track in guidance. However, models and ensembles now
agree to lift a deepened low northward into the eastern Gulf of
Alaska by Thursday. Maritime wind/rain impacts may also affect
Southeast Alaska. The system should slowly weaken and meander over
the northern Gulf of Alaska Friday into next weekend, blocked
underneath the upper ridge. Guidance has trended slightly more
offshore and holds onto an organized circulation more than shown
yesterday.
Meanwhile, a surface system will continue to weaken and lift
northward on the western periphery of the upper ridge from the
northern Bering Sea to the Arctic Ocean Thursday into Friday. This
system offers modest lingering precipitation. Another low in the
wake of this system into Thursday-Friday is expected to hold/spin
over the southwest Bering Sea where it should dissipate.
There is also still a strong guidance signal in support of
subsequent development of a deep Pacific low that is slated to
slowly track along with wrapping winds/rains near the Aleutians
into next weekend/early next week that would also present an
organized maritime hazard.
Schichtel
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