Scientific Forecaster Discussion
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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
405 PM akdt sun Sep 24 2017
Analysis and upper levels...
there is an upper level low centered just south of Cape Newenham
this afternoon. An associated surface low is located just to the
east of the upper low center. A trough extends east-northeast from
the upper low into southcentral Alaska. Showers associated with
the low extend from the Canadian border westward to the pribilofs.
High pressure is over the western Bering Sea, but this is quite a
"dirty" ridge with plenty of cloud cover and even some showers
across the Bering and Aleutians.
the numerical models are in good agreement through Monday evening.
After that, they begin to show some differences with exactly how
the low pushing in from the south develops. The European model (ecmwf) is quicker
with the eastern side of the front (off the coast of the
panhandle), with the Canadian Gem a little slower and the GFS and
NAM the slowest. The models are also a little different in how
they handle the main surface low south of Kodiak Island on
Tuesday. This creates some uncertainty in the forecast, and
confidence is thus a bit below normal today.
panc...VFR conditions and light winds will likely persist. There
is a risk of some fog tonight. The chances of this developing
depends on cloud cover. If there is some clearing, then fog could
Short term forecast southcentral Alaska (days 1 and 2)...
the remnants of a low over the Gulf and Prince William Sound will
continue to usher moisture into southcentral Alaska which will be
most visible in the form of cloud coverage and shallow pockets of
fog. As time elapses, the cold pool of air aloft will continue to
modify and move inland resulting in a drying trend over the
Anchorage bowl and the mat-su area. All the while, enhanced
values of precipitable water will continue to advect into the
Cordova and Valdez region. The robust ridge that is off the coast
of California will continue to amplify with the ridge axis nosing
its way deeper into the Gulf of Alaska on Monday. This should
bring stable conditions as far north as Prince William Sound and
the Kenai Peninsula. Reduced pops over the Gulf this forecast
package for Monday. Additionally with weak ridging influencing the
region fog may develop in some areas. By Tuesday the ridge axis
will shift eastward with southerly flow setting up over the Gulf
Short term forecast southwest Alaska (days 1 and 2...Mon and tue)...
one low will pull away from Bristol Bay, allowing for a brief
period of drying, before another moves in to take its place. The
first low is currently drifting from north to south just outside
of Bristol Bay. This will allow some wrap around rain to continue
to fall Monday along portions of the SW coast and in a band from
the Alaska Range to the yk-Delta coast. Late Mon into early Tue, a
drying trend will ensue as some induced ridging builds aloft. The
second low will then creep up from the North Pacific towards
Kodiak Island on Tue. Rain will start once again over the Alaska
Peninsula around daybreak. The more widespread impact will be the
increasingly strong northeast winds. Gusts could reach 20 to 30
mph along the Bristol Bay coastline by afternoon, with even higher
winds expected in Shelikof Strait. This dry, offshore flow will
also help to keep the rain in check and should actually allow for
most inland places to stay dry on Tue.
Short term forecast Bering Sea/Aleutians (days 1 and 2...Mon and
northerly flow will be the rule across the Bering. This will keep
gusty winds and the potential for showers in the forecast,
especially along the north side of the chain. There is limited
cold air near the Bering Strait at this time, so not expecting
anything overly strong with near neutral temperature advection. A
ridge with an associated closed high pressure center will drift
towards the western Aleutians on Mon. This will bring some warmer
temperatures to Shemya and Attu, and could lead to foggy conditions
developing once again.
Long term forecast (days 3 through 7)...
Models have come into better agreement with an area of low
pressure moving across Kodiak Island on Wednesday. The recent
forecast reflects the increase in model agreement with gale force
winds likely across the northwestern Gulf of Alaska and Shelikof
Strait Tuesday night and Wednesday. In addition, it looks like a
piece of the subtropical jet will bring an influx of moisture to
allow for some decent rainfall over Kodiak Island and the Gulf
Coast. Interior locations will remain showery as the low doesn't
take a favorable track for significant downsloping conditions.
Otherwise, the forecast hasn't deviated much from yesterday in
thinking that the upper level trough digging out across Alaska
late this week will quickly be shunted eastward by a transitory
shortwave ridge. Behind this ridge will be yet another upper level
trough but this time it's the Arctic jet stream that is
strengthening rather than any subtropical influence. With colder
air expected to move over the Bering Sea and much of Alaska, a
cooler and showery period of weather is expected through the
public...Flood Warning 121 125.