Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 260035
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 PM AKDT Tue Apr 25 2017
.ANALYSIS AND UPPER LEVELS...
The overall upper level pattern over the state has changed little
since yesterday with the stacked low near the central Aleutians
and troughs extending out of this large low into the interior of
the state and over the Gulf of Alaska. Weak ridging aloft extends
into the Copper River Basin from the ridge that is over British
Columbia. A series of weak upper level waves are traveling through
the Gulf of Alaska and this should be the case the next few days.
The jet stream is far to the south and not feeding any significant
upper level support into the surface low that is just a little
northeast of Adak.
There is a little bit of upper level support feeding into a low
that is west of Vancouver Island. This low will move into the
eastern Gulf of Alaska and help keep the southeasterly flow over
the Gulf of Alaska the next few days.
Models are doing better today than yesterday...especially in the
Gulf of Alaska. The model solutions all show the main low going
into the eastern Gulf of Alaska with a trough trailing westward
across the rest of the Gulf. Yesterday`s models had a more
balanced strength of the lows with the GFS (wrongly) consolidating
all the energy halfway between the centers. Today the GFS, NAM and
the rest of the bunch all have the eastern low solution and only
differ with the timing of some of the upper level waves moving
across the northern Gulf from the southeast. NAM was the preferred
model for these features.
In the Bering Sea, the models all keep the low in the central
Aleutians nearly stationary through Wednesday before moving it
eastward Wednesday night through Friday.
The GFS model was preferred with the track that moves to low just
north of the Aleutians/Alaska Peninsula before dissipating it
There is also a significant difference in the track and strength
of a low moving into the Western Bering Sea Thursday night and
Friday with the ECMWF stronger and farther southeast than any
other model. This will need to be watched over the next few model
runs to see if a common solution appears. At this time the strong
front associated with the low will be the main weather feature
for that area and models are all fairly close with their track so
current forecasts should not be too far off despite the model
PANC...VFR conditions should prevail over the airport through
Wednesday. Wind direction is the most challenging aspect for the
PANC TAF due to the presence of the Turnagain Arm wind blowing
just south of the airport. This wind is expected to remain south
of PANC but could turn it southeast during the next two afternoons
and evenings. Even if it does wind speeds are expected to remain
under 10 KT. Winds should revert to light northerly overnight.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A few rain showers have been seen across interior portions of
Southcentral Alaska but places in the Matanuska Valley and the
Anchorage Bowl have seen very little in rainfall as downslope
winds continue to the keep the low levels dry. Rainfall looks to
continue along the northern Gulf coast through tomorrow morning
as a frontal boundary continues to slowly dissipate over the
northern gulf. Gusty winds through Turnagain Arm, the Knik River
Valley and the Copper River will continue through late this
evening before decoupling as the atmosphere stabilizes. This
should be an end to the gusty winds as the pressure gradient
begins to relax with the frontal boundary in the gulf on it`s way
out. Cloudy skies will continue to be the norm on Wednesday but
guidance is hinting at clouds breaking a little bit by late this
week. As the clouds break, temperatures are expected to warm into
the 50s for a good chunk of Southcentral Alaska by late this week.
It isn`t out of the question that a few convective showers could
be seen toward higher elevations by Friday with colder air now in
place in the upper levels (See change in the freezing level and
-20 degrees C line on the PANC soundings from yesterday and today)
but we are still a bit out from any thunderstorm activity.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Low pressure across the southern Bering will send several short
waves across the Southwest through Friday. This persistent pattern
brings higher chances for precipitation through the end of the
week along with continued southerly flow.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The low pressure system over the southern Bering weakens into an
open wave on Thursday and lifts slowly north. The next front
moves into the western Bering Thursday night. Gale winds are
expected along the front which slowly moves to the central Bering
and Aleutians Friday afternoon.
.LONG TERM FORECAST (Days 3 through 7)...
The long-term forecast (Weekend and early next week) looks like a
continuation of the rather cloudy pattern that has spread across
much of mainland Alaska. The current stationary low in the central
Bering will weaken and move to the southeast over the next few
days, but by the weekend it will be replaced by a renewed low
pressure system moving through the Bering and parking near the
AKPEN or Kodiak Island. The models have been struggling with where
this 2nd low will set up shop...and the final position will be the
difference between continued cloudy with occasional showers at
inland locations, or thick clouds with steady rain for many areas.
For now, the more showery solution is preferred, as it shows a bit
more run-to-run consistency and also is supported heavily by
climatology. This should mean near or slightly above normal
temperatures continuing over southcentral Alaska, and above-normal
over Southwest Alaska.
SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH