Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
544 PM EDT Sat Jul 17 2021
Valid 12Z Wed Jul 21 2021 - 12Z Sun Jul 25 2021
...Overview...
No change in thinking here. It is still the case that there is
strong guidance signal indicating that 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 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...
Same as yesterday. The WPC Alaskan medium range product suite was
primarily derived from a blend of well clustered guidance of the
12 UTC GFS/ECMWF and GEFS/ECMWF ensemble means along with the 19
UTC National Blend of Models days 4-8. Predictability seems to
stay above normal through the period, albeit with a gradual
increase in forecast spread with time as addressed by minority
weighted inclusion of the ensemble means. WPC continuity is well
maintained, in part again due to subsequent manual adjustments to
this WPC blend to ensure offshore low depth consistent with
favorable upper support and lowered uncertainty in this pattern.
...Weather/Threats Highlights...
It remains the case that precipitation will be focused, especially
into midweek, 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. While the details of
embedded systems in this flow is not well established in guidance
that offers less than stellar run to run continuity upstream, this
emerging pattern will offer unsettled maritime conditions over the
Gulf and periods of locally enhanced rainfall into southern and
southeastern coastal areas mid-later next week.
Schichtel
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