Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Thu Mar 11 2021
Valid 12Z Mon Mar 15 2021 - 12Z Fri Mar 19 2021
...Overview and Guidance/Uncertainty Assessment...
Today's models and ensembles continue to advertise an increasingly
blocky pattern from the Bering Sea through a large portion of the
eastern Pacific. However after about early Tuesday solutions
diverge for the shape and position of significant features, and
the differences appear to be at least as great if not more so than
over the past couple days. Early in the week there is decent
continuity with the axis of a sharpening and digging upper trough
reaching about 160W longitude by early Tuesday and possibly
including an embedded low not far from Bristol Bay. After that
time the forecast complexity increases with the inter-related
issues of how upper ridging that builds into the Bering Sea and
vicinity evolves, what proportion of Tuesday trough energy drops
into an upper low well south of the mainland, as well as the shape
and position of that upper low. Issues with the ridge also affect
the character of higher latitude flow and its influence on the
mainland.
Early in the period the 12Z GFS has trended favorably toward (but
not yet quite to) other models that have been more progressive
than the GFS with the short range wave that develops near the
Alaska Peninsula and then continues into the Gulf. Already by day
5 Tuesday some greater surface differences arise from
low-predictability shortwave details on the east side of the upper
trough, with the 12Z CMC more aggressive than other operational
runs with the idea of another wave tracking north into the Gulf.
The 12Z parallel GFS and 06Z operational GFS had variations of
such a wave though. Then the ECMWF/ECMWF mean/UKMET are quickest
to drop the greatest proportion of trough energy into the Pacific
upper low and to position this low farther south. The GFS/GEFS
mean and to varying degrees CMC/CMC mean have more energy
lingering near the southwestern mainland/Peninsula for a time,
leading into an upper low that is generally farther northward and
in some cases more east-west elongated. These differences
correspond to the ECMWF/ECMWF mean building the Bering ridge
partially into the mainland with a high center getting close to
the western coast of the mainland, in contrast to GFS/GEFS/CMC
mean runs that are quicker to close of the Bering ridge and have
the upper high drifting into or near Siberia. In terms of
sensible weather, which direction the pattern consolidates toward
(or between the two extremes) will determine if the southern coast
trends drier (ECMWF) or there is a continued potential for one or
more waves that will bring episodes of wind/precipitation to areas
over and near the Gulf of Alaska.
Given the uncertainty of forecast specifics and if anything more
divergence in solutions over the past day or so, the deterministic
forecast blend trended toward an even mix of the 12Z GFS/GEFS mean
and 12Z ECMWF/00Z ECMWF mean by the latter half of the period to
reflect an intermediate solution for the Bering Sea ridge/Pacific
upper low as well as flow to their north along with corresponding
surface features. Early in the week the forecast started with a
12Z operational model blend that removed the CMC by day 5 Tuesday.
...Weather/Hazards Highlights...
The wave likely to be over the Gulf of Alaska at the start of the
period Monday should focus a brief period of enhanced
precipitation over the southeastern coast and Panhandle early in
the week. For the rest of the week confidence decreases for
specifics of precipitation coverage and amounts from the Panhandle
westward as well as for the surface/wind pattern. The preferred
intermediate forecast would depict a trend toward lighter and more
scattered precipitation within a full envelope ranging between dry
conditions mid-late week and one or more episodes of possibly
meaningful/focused precipitation Tuesday onward. The surface
pattern would feature strongest low pressure staying well
southward over the Pacific with only weak features rotating around
the northern side of the overall circulation. The remainder of
the mainland should be fairly dry though a weakening front during
the latter half of the week could bring some precipitation. The
Aleutians will see a decrease in precipitation amounts/coverage
through the first half of the week followed by an increase in
moisture/winds over at least the western Aleutians as a front
approaches later in the week.
Expect above normal min/max temperatures to be the most common
over the North Slope through most of next week. Elsewhere there
should be greater coverage of below normal readings for highs
while anomalies for morning lows will likely be more balanced
between above/below normal if not tilted a bit more toward the
warmer side of climatology.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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
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