Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
723 PM EDT Sun Apr 18 2021
Valid 12Z Thu Apr 22 2021 - 12Z Mon Apr 26 2021
...Overview and Guidance/Uncertainty Assessment...
Latest guidance continues to depict a fairly blocky regime during
much of the extended range period, with upper ridging/high
pressure dominating much of mainland Alaska. To begin the period
on Thursday, the guidance agrees an upper low/surface low should
settle into a position just south of the eastern
Aleutians/Peninsula region, but weaken as the main low shifts
north/west into the Bering and energy rounding the base of the
trough drives the cold front through the north Pacific into the
weekend. By Sunday, another upper low should approach the
Aleutians and move eastward with time, with the associated
organized surface cyclone drifting near or just south of the
Aleutians.
Generally speaking, the models show fairly good agreement on the
overall large scale pattern as depicted above, but diverge greatly
on the details as well as timing of various systems, especially
past about day 5. The GFS continues to be the model most
consistent in holding onto the blocky pattern across the region,
while the ECMWF has shown more run to run variability on how
quickly to break down the ridge. The ensemble means also support
the GFS in holding onto the mainland ridge, so a preference
towards stronger ridging is preferred at this time. The WPC
forecast for days 4-5 uses a mostly deterministic model blend
(between the 12z GFS/ECMWF/CMC), increasing contributions from the
ensemble means thereafter to account for the increased uncertainty
with timing and details of the deterministics. Did continue to
carry roughly a third of the GFS/ECMWF combined into day 8 though
just for some added definition to the otherwise fairly washed out
means. This approach fits well with yesterdays WPC continuity as
well as the preference downstream over the CONUS.
...Weather/Hazards Highlights...
Most of Alaska should remain warm and dry through the period, with
any precipitation confined to the Aleutians/Alaska Peninsula and
vicinity associated with the couple of surface cyclones, first on
Thursday, and then again next weekend. Mainly light to moderate
amounts are expected, though some locally higher totals could be
possible depending on exact strength and track of systems. Each
system may also bring a period of gusty winds to parts of the
region as well.
The vast majority of the state should be well above normal in
terms of temperatures through much of the period, with the warmest
anomalies over the North Slope region and western areas of the
Mainland (with even more anomalous warm morning lows). Lowering
heights across Alaska next weekend should help to moderate
temperatures a bit, especially for eastern areas. The exception to
all this warmth will be in the southeast quadrant and much of the
Panhandle where temperatures should be near or below normal the
entire medium range period.
Santorelli
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