Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
652 PM EDT Mon May 17 2021
Valid 12Z Fri May 21 2021 - 12Z Tue May 25 2021
...Overview...
Messy multi-stream flow off Asia in the short term will transition
toward a bit more definition in the medium range as a Gulf and
then Bering system traverse the region. With upper ridging
forecast to build across the Sakhalin Oblast on Sunday, downstream
troughing will dig through the Bering and ridging over the
Interior. This favors a warm-up across the Tanana Valley with
rainfall focused over southern/coastal areas of Southcentral this
weekend and the Aleutians to southwestern Alaska by next week.
...Guidance/Predictability Assessment...
The ensemble systems (GEFS/Canadian/ECMWF) showed good agreement
through the period even as the deterministic models continued to
stray from their consensus and relatively good continuity. The 12Z
GFS was well-removed from the consensus even before the forecast
period (Thu) over the Bering as it took a defined system
southeastward toward Bristol Bay -- much stronger and farther
south than the other guidance or previous runs. Followed the
forecast cluster of the ECMWF/UKMET/Canadian to start before the
Canadian took the southern stream Pacific system northward to the
Bering rather than allowing it to meander south of 50N. The 12Z
ECMWF offered a most reasonable depiction of the systems
progressions until next Sun-Mon where it developed a low in the
southern stream toward the Aleutians. Given its low
predictability, opted to rely solely on the ensemble means (12Z
GEFS mean was best defined) and a simple occluded system in the
Bering early next week until the upstream pattern comes into
better focus.
...Weather/Threats Highlights...
Weak troughing over the mainland will promote scattered showers
(snow over higher elevations) across the Interior/North Slope for
the first part of the forecast, with isolated convection as
temperatures aloft remain cool (850T near 0C). A defined system
just south of the AKPen on Friday will bring widespread light to
modest rain for the eastern Aleutians into southwestern Alaska as
the occlusion skirts eastward. Enough moisture could get drawn
northward toward Kodiak and the southern/coastal area of
Southcentral to bring a couple inches of rain to those areas with
southern exposure. Lighter rain is expected for the Panhandle as
the system progresses steadily eastward late Saturday. Next system
in the Bering could bring more light rain to the Aleutians early
next week and perhaps into the western mainland thereafter.
Temperatures will be within several degrees of normal over much of
the state, warming into the weekend over the Interior to the 60s
and 70s (10-20 deg above normal), but near to below normal along
coastal areas of the Gulf where clouds/rain are more likely.
Fracasso
Hazards:
No significant hazards are expected over Alaska during this
forecast period.
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