Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Mon May 02 2022
Valid 12Z Fri May 06 2022 - 12Z Tue May 10 2022
...Overview...
Expect somewhat ill-defined flow aloft over the mainland late this
week into the weekend with a tendency toward weak ridging over the
Interior. From late weekend into early next week a mean trough
may develop near the west coast of the mainland and perhaps drift
a little inland. Surface high pressure should track from the
Arctic into the Canadian Archipelago while a wavy front will
likely meander over the central mainland. Farther south, forecast
details remain very uncertain as guidance shows a wide array of
possibilities for how northern stream flow aloft could interact
with an initial mid-latitude Pacific system. Greater interaction
would lead to low pressure and associated moisture having more of
an effect on some areas from the eastern Aleutians/Alaska
Peninsula through the southern coast and Panhandle, while the
opposite side of the guidance envelope with minimal interaction
could lead to fairly dry conditions. Currently the most probable
outcome is for precipitation to reach some of these locations with
pockets of at least moderate totals.
...Guidance/Predictability Assessment...
The 12Z GFS appeared to provide a resolution to lingering question
marks with the upper low near the eastern Aleutians as of early
Friday, trending toward the greater progression which the
ECMWF/UKMET/CMC have been advertising over the past couple days.
The 12Z GEFS mean still held it back some like prior GFS runs, and
now the 18Z GFS has reverted to having it reach no farther east
than the Alaska Peninsula. Preference at the time of forecast
preparation was a compromise timing among 12Z models, with the
ECMWF input split between the last two runs since the 12Z run was
fastest.
The combination of issues with this upper low and subsequent
differences in how northern stream flow may interact with
mid-latitude Pacific low pressure continues to result in widely
divergent solutions for what ultimately happens to the system and
what the effects between the eastern Aleutians and Panhandle would
be. The 12Z ECMWF jumped to a more separated scenario that keeps
the Pacific system and its moisture well southwest, in a manner
similar to some prior CMC runs. This contrasts with the prior 00Z
ECMWF that brought the system into optimal position to direct a
pronounced flow of focused moisture into the southern coast.
Regardless of details of the leading upper low, recent GFS runs
and GEFS means plus ECMWF means through the 00Z cycle have
generally favored bringing the system along a path south of the
Alaska Peninsula and into the Gulf of Alaska, spreading moderate
to locally enhanced areas of precipitation into the southern coast
and Panhandle. The 12Z CMC/CMC mean and now the new 12Z ECMWF
mean offer an alternative idea, keeping the Pacific system off to
the southwest but still having enough dynamics aloft to develop a
weak leading wave that could track over or near the Gulf and
produce some precipitation. Based on guidance available through
arrival of the 12Z ECMWF, above average uncertainty favored
holding onto as much continuity as possible by way of trending the
initial operational model blend toward 70 percent total means (12Z
GEFS/00Z ECens) and the rest 12Z GFS by day 8 Tuesday.
Guidance has been inconsistent for specifics of flow farther north
over and near the mainland. The 12Z ECMWF strayed toward the
faster side of the spread for the mid-late period upper trough but
the new 12Z ECMWF mean trended somewhat east as well. The
forecast from the aforementioned blend provided a conservative
approach which at least reflects somewhat more definition than
yesterday.
...Weather and Hazard Highlights...
Mostly light precipitation from the eastern Aleutians across the
southern coast and Panhandle late this week should become more
scattered heading into the weekend. Significant uncertainty with
flow details over and near the North Pacific maintains low
confidence for the ultimate path of Pacific low pressure (and/or
possible leading wave) and thus for how much moisture may reach
areas from the eastern Aleutians through the Panhandle from later
in the weekend through early next week. Possibilities range from
fairly dry conditions to a period of significant moisture aimed at
the southern coast. The most probable outcome at this time is for
some precipitation to spread across areas from the Alaska
Peninsula through the southern coast and Panhandle, with some
locations reaching at least moderate intensity for a time.
Scattered precipitation will be possible over the rest of the
mainland during the period, tending to be more over the south late
week into the weekend but then possibly expanding as an upper
trough develops/approaches from the west.
Temperatures should be fairly stable through the period. Expect
below normal highs across a majority of the southern mainland and
Panhandle as well as the North Slope, with moderately above normal
highs over Interior areas. Morning lows will tend to have warmer
anomalies, leading to greater coverage of above normal readings.
The Panhandle, parts of the far eastern mainland, and areas near
the northern coast have the best potential for below normal lows.
Sufficiently warm temperatures combined with the remaining snow
pack could present flooding concerns across portions of the Yukon
River Basin in eastern Alaska.
Rausch
Hazards:
- Flooding possible across portions of eastern interior Alaska,
Thu-Mon, May 5-May 9.
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