Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
704 PM EDT Thu May 21 2020
Valid 12Z Mon May 25 2020 - 12Z Fri May 29 2020
...Overview and Weather Hazards/Threats...
A blocky large scale flow pattern over Alaska is expected to
remain relatively unchanged next week. An amplified upper-level
ridge is forecast to persist from the Arctic Ocean across eastern
Alaska and into northwest Canada. This will result in above
average temperatures with anomalies upwards of +15 deg F across
the Interior into the Brooks Range. This will enhance snow/ice
melt, so please refer to your local office for all seasonal river
ice flow issues.
A closed upper low and deepened surface system will gradually
weaken while lifting northward on the western periphery of the
upper ridge from the Aleutians/southern Bering Sea Sunday to the
Arctic Ocean by next Thursday. A second closed upper low and
deepened surface system will meanwhile organize over the northern
Pacific early next week and is slated to slowly lift northward
over the western Gulf of Alaska mid-later next week into/around
the blocking upper ridge.
Each of these systems offers a lingering wind/rain threat and an
open water maritime hazard.
...Guidance/Predictability Assessment...
Overall larger scale predictability across Alaska was above
average due to a relatively stable and blocky flow regime.
However, forecast spread is still quite evident with ridge
evolution specifics and with significant developmental details of
especially the second main low/system. Ensembles and WPC
continuity lifted the low more rapidly over the west Gulf of
Alaska through next midweek compared to the deterministic higher
resolution 12 UTC model suite. The 12 UTC GFS seemed to best
cluster with the GEFS/ECMWF ensemble means into early-mid next
week before diverging more significantly. Accordingly, the WPC
Alaskan medium range product suite was primarily derived from a
composite blend of the 12 UTC GFS and GEFS/ECMWF ensemble means
along with the 19 UTC National Blend of Models for days 4-8. The
slower northward lift trend for the second main low in this
scenario seems reasonable considering the nature of the blocking
upper ridge.
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
Hazards:
- Heavy precipitation across portions of southern Alaska, Sun, 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