Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
626 PM EDT Tue Jul 21 2020
Valid 12Z Sat Jul 25 2020 - 12Z Wed Jul 29 2020
...Heavy rain possible for Southeastern Alaska Sunday...
...Overview and Guidance Evaluation...
Breakdown/pattern shift across the western/central Pacific has
left the guidance a bit lost in how things evolve this weekend
into next week. With the weakening/shift westward of the upper
high over northeastern Russia, lower heights to its east (near the
North Slope into Beaufort Sea) will move westward and/or
northwestward, potentially in two pieces. To the south, a system
out of the central Pacific may move east-northeastward and slip
into the southwest side of an upper low over the Gulf, or fall
just short and meander to the north. The combination of two or
more scenarios in the northern/mid-latitudes has made for some
disparate solutions in the recent guidance.
In light of the uncertainty, favored the 12Z GFS/ECMWF to start
with better agreement near the Aleutians on a slower system that
may at least hold back its sfc low if not frontal structure. Kept
the front progressive enough to move toward the Gulf before
dissipating by Tue. This would then not favor the 12Z ECMWF that
builds in ridging across much of the interior next week though
keeping some percentage in the blend tempered the GFS (which kept
the stronger trough moving eastward). Trended toward an ensemble
mean blend with some lowering GFS/ECMWF weight to balance the
larger discrepancy to the north. There, The ensembles showed
strong agreement on lowering heights into NW Alaska but the
deterministic models split the upper low into two pieces and move
the northern component to the northwest. Such a divide between the
deterministic models and ensemble means at only a day 5 lead time
is pretty rare but not unexpected for a rather strong pattern.
...Weather Highlights/Threats...
The best potential for highest five-day precipitation totals
should be over the southeastern mainland and Panhandle. This
should come with the reinforcing system over the Gulf of Sunday
(leading wave on Saturday) though it will remain progressive into
Canada late Sun. Lighter showers will follow in its wake as the
old parent low dissipates in the Gulf. Temperatures will generally
be below normal over much of the state owing to the upper
trough/low. By the end of the period, temperatures should moderate
somewhat but the pattern remains quite uncertain.
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
Hazards:
- Heavy rain across much of the Alaska Panhandle into southern
Alaska, Sun-Mon, Jul 26-Jul 27.
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