Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Tue May 04 2021
Valid 12Z Sat May 08 2021 - 12Z Wed May 12 2021
...Overview and Guidance Evaluation...
Most guidance continues to show a deep-layer mean low over the
Aleutians/southern Bering Sea that persists through the weekend
before weakening early next week. Shortwaves ejecting from the
mean low should produce a series of wavy frontal systems that
cross the Aleutians through the Alaska Peninsula and Gulf of
Alaska, producing precipitation across these areas, and eastward
along the southern coast/Panhandle. Although the consensus
clusters a little better than average, individual solutions
suggest numerous uncertainties with the forecast that would affect
sensible weather over the coastal areas, and islands.
The first wave ejecting from the main low drifts north across the
Gulf of AK Day 4 in its weakening phase, decaying on Saturday.
On Sunday, the next of the series of wave moves north across the
Gulf of AK, with the 12z GFS and ECMWF trending a few hours faster
than their prior runs.
The next cyclone develops Sunday south of the main Aleutians low.
On Monday to Tue the low moves northeast,gradually merging with
the old parent low. The 12z GFS becomes an outlier in moving the
low further north/faster,so more weighting was given to the 00-12z
ECMWF/Canadian/12z GEFS Mean cyclone position.
By mid week, the next 500 mb low moves from south of Kamchatka
east near 50N, spawning cyclogenesis, with the low approaching the
lower Aleutians and possible arriving by the middle of next week.
There is better than typical agreement the low will develop, with
typical timing/location differences. The agreement lead to a
blend of the last 2 ECMWF and GFS runs, along with the ECMWF
Ensemble Mean to mitigate the differences among solutions.
...Sensible Weather...
Expect the highest probability of precipitation and moderate
totals to extend from along the Alaska Peninsula to the Kenai
Peninsula, with another max in southeast Alaska/panhandle in
tandem with each wave of low pressure/associated fronts. Scattered
diurnal convection is possible inland as a quasi-stationary front
lingers over interior AK. The Aleutians will likely see unsettled
conditions with periods of rain with each low later this week and
again in the middle of next week.
With above normal heights across interior and northern AK,
temperatures are favored to be above normal in most areas, with
near normal in far northern AK. The Panhandle and coastal areas
west to the Aleutians may see temperatures near normal in periods
of clouds and showers (but below normal highs on cloudier days).
Petersen
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