Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
628 PM EDT Thu Jul 15 2021
Valid 12Z Mon Jul 19 2021 - 12Z Fri Jul 23 2021
...Overview...
The current blocky pattern will start to break down and move east
next week, but with continued/renewed troughing into an unsettled
Bering Sea/Strait then western/northwestern Alaska. Meanwhile,
ample undercutting storm energies will increasingly work through
the Aleutians/Southwest/AKpen to the Gulf of Alaska. A lead upper
low initially south of Haida Gwaii Monday will very slowly move
into British Columbia as weakening ridging over the
Interior/Southcentral moves into the Yukon as the pattern relaxes.
...Guidance/Predictability Evaluation...
The WPC Alaskan medium range product suite was primarily derived
from a composite blend of now well clustered guidance from the 12
UTC GFS/ECMWF/UKMET/Canadian along with the 19 UTC National Blend
of Models and WPC continuity days 4-6 (Monday-Wednesday).
Predictability seems to stay above normal into days 7/8, albeit
with a gradual increase in forecast spread addressed by adding to
the composite some 12 UTC GEFS/NAEFS/ECMWF ensemble mean data.
...Weather/Threats Highlights...
Precipitation will be focused, especially early-mid next week,
over western/northwestern Alaskan areas during the period under
southwest flow with Bering lows approach/height falls, as well as
across the Aleutians along the tail-end of the frontal boundaries.
Widespread tenth to a quarter inch with focused half to one inch
rainfall (locally more) is possible over these areas over a 1-2
day period with each system. Some precipitation will also
overspread the North Slope/Brooks Range with inland system
translation and weakening.
Meanwhile, ample undercutting energy flow through the
Southwest/AKpen and into the Gulf of Alaska will lead to deepening
northern Gulf low and frontal system genesis. This emerging
pattern will offer unsettled maritime conditions over the Gulf and
some potential for modest rainfall into southern and southeastern
coastal areas mid-later next week.
Schichtel
Hazards:
Heavy rain across portions of northwestern Alaska, Tue-Wed, Jul
20-Jul 21.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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