Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EDT Sat May 15 2021
Valid 12Z Wed May 19 2021 - 12Z Sun May 23 2021
...Overview...
Troughing is generally favored along 50N from the Aleutians to the
Gulf, with weak ridging building into interior Alaska later in the
week. This will maintain a fairly quiet pattern over much of the
state with showers and some convection over the Alaska Range to
the Brooks Range with daytime heating (50s and 60s to near 70 in
the valleys) and still cool air aloft to start. More organized
rainfall will be along parts of the Aleutians to Kodiak, coastal
Southcentral, and eventually to the Panhandle late next week.
...Guidance/Predictability Assessment...
Multiple streams in the upstream jet have complicated the pattern
for several days running, with the latest 12Z guidance no
exception. Best course of action has been to remain in the middle
of the spread to minimize changes in the following forecast,
absent any clear trends. Aside from the UKMET, the 12Z
GFS/Canadian/ECMWF were clustered fairly well with the Pacific
system south of the Aleutians Wed that moves eastward through the
rest of the week. Weaker systems upstream have less
predictability. By next Fri-Sun, GEFS mean and ECMWF ensemble mean
diverge in timing, but the GFS/Canadian/ECMWF mostly stayed in
between the two ensemble camps (somewhat like the 12Z Canadian
ensembles, but with much more definition). Despite the
uncertainty, used a majority deterministic blend to get the
preferred intermediate-timing solution until next Sun when those
models diverged too much. This at least maintained a reasonable
system progression.
...Weather/Threats Highlights...
Weak troughing over the mainland will promote scattered showers
(snow over higher elevations) across the Interior/North Slope
through much of the week, with isolated convection as temperatures
aloft remain cool (850T near 0C to start). A more defined system
around the Aleutians on Wednesday will bring generally light to
perhaps modest rain for the island chain 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. Temperatures will be within several
degrees of normal over much of the state, warming into next
weekend over the Interior to near 70, but near to below normal
along coastal areas of the Gulf where clouds/rain are more likely.
Fracasso
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