Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
722 PM EDT Mon Aug 09 2021
Valid 12Z Fri Aug 13 2021 - 12Z Tue Aug 17 2021
...Heavy rain for parts of southeastern Alaska later this week...
...Overview...
Upper ridging near 155E and 140W will favor troughing in between,
roughly from the Arctic through the Bering Sea via several
systems. This will keep an active pattern over the high latitudes
into next week with both the northern and southern stream
affecting the 49th state. Lead system in the Gulf Friday will
bring a significant heavy precipitation threat to southeastern
Alaska out of the short range (Thu) where several inches of rain
may occur in favored areas. Indeed some skillful guidance shows
over 10 inches for the event maximum around and inland from
Yakutat.
...Guidance/Predictability Evaluation...
The ensembles remained in good agreement overall with some slight
timing/amplitude differences by next week over the Bering as a
couple more systems enter the picture. Overall, a blended solution
was preferred. This included the 12Z ECMWF which paired best with
the ensembles, followed by the 12Z GFS which was a bit removed
from, but still close enough to, the ensembles. The Canadian and
UKMET offered a bit of detail for the Friday forecast with the
Gulf system but otherwise drifted apart from the consensus. Later
this weekend into next week, systems south of the Aleutians,
containing the remnant moisture of Tropical Storms Lupit and
Mirinae, will tend to lift northeastward toward the
central/eastern Aleutians and AKPen next week, bringing in some
rainfall to the region. Timing differed in the guidance but was
within the expected spread for this time range.
...Weather/Threats Highlights...
Atmospheric river event for the end of the week could result in
hydrologic concerns for some areas especially near Yakutat despite
being used to heavy rain events. Ensemble Situational Awareness
Table from the GEFS shows "MAX" values over southeastern areas
signaling an unusually wet forecast relative to climatology. For
reference, Yakutat has had about 40 3-day 10" precipitation events
(some overlapping) in its 124-year station record (about 1 every
three to four years), usually in the cold season. By the weekend,
the rainfall will subside as it pushes southeastward.
To the north, deep upper low in the Arctic will slowly drag a cold
front out of the area, allowing another northern stream system out
of Russia to push eastward. This will bring in an expanding area
of precipitation to the northwestern corner of the state across
the Brooks Range next week.
Temperatures will generally be below normal across most of the
state, focused on northern areas. Some slow moderation is possible
next week but the continued troughing will likely result in only
small changes to the pattern through next Tuesday.
Fracasso
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
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Thu-Fri, Aug 12-Aug 13.
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