Scientific Forecaster Discussion
fxak68 pafc 271301
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
501 am akdt Sat may 27 2017
Analysis and upper levels...
an upper level low centered over the Bering Strait, with a
deepening trough extending over the eastern Bering and southwest
Alaska, will continue to drive the sensible weather for much of
the state. The strong upper level jet just south of the western
and central Aleutians continues to slowly weaken and dive farther
south in the Pacific. At the surface, a low pressure system
south of the Alaska Peninsula continues to bring showery and gusty
conditions to the southern Mainland and Kodiak Island. A weak low
over Norton Sound has helped to keep a few lingering showers
along the Kuskokwim Delta coast during the overnight hours.
Further west, a low pressure system tracking into the western
Bering has brought a frontal system into the western Aleutians,
brining some rain and gusty southwest winds into the area.
models in the short term appear to be in good agreement overall,
with a few differences starting to take shape Sunday morning
through the beginning of next week. Models begin to struggle
during this time frame with the placement of the surface low over
the western Bering, and the timing of precipitation tapering off
over southcentral Alaska Monday. With the system out west, the
NAM and Canadian keep the low nearly stationary over the western
Bering and Aleutians through Tuesday, where the GFS and ec bring
the low south faster before becoming stationary. For this feature
the GFS solution was used mainly for continuity purposes. With
regards to the timing of precipitation tapering off over
southcentral Monday, a NAM/GFS blend was used to show a middle
ground solution between the slightly faster NAM and slightly
panc...VFR conditions should prevail throughout the day today.
There is some uncertainty regarding if and how much of the
Turnagain Arm wind will make it into west Anchorage today.
Chances look marginal that we can get some weak Turnagain Arm
winds into west Anchorage today, with chances decreasing
throughout the day as the down-inlet gradient increases.
Short term forecast southcentral Alaska (days 1 through 3)...
a few showers will linger over interior southcentral Alaska
today, but a more stable air mass is on the way as a warm front
heads northward across the Gulf. Showers along the Gulf Coast will
transition to steady rain later today, then become heavy at times
through Sunday night as an upper low south of Kodiak Island lifts
northward. Strong low level southeast flow will enhance rainfall
amounts over western Prince William Sound where several inches of
rain are expected. These same winds will cause downslope drying
for most inland locations, though cloud cover will remain quite
The forecast for Monday is a bit hazy due to uncertainty in the
exact track of the upper level low. The latest guidance has
trended east with the track, taking it up the west side of Cook
Inlet. If it takes a track close to this would expect a showery
day from the western Kenai Peninsula to Anchorage and the mat-su.
As has been the case over the past several days, surface pressure
gradients will remain favorable for gap winds. The one difference
will be an increasing positive gradient down Cook Inlet as the
warm front reaches the north Gulf Coast later today. This will
cause the turnagain winds to Bend southward. Therefore, expect
much less wind in Anchorage the next couple days.
Short term forecast southwest Alaska (days 1 and 2)...
the Mainland will continue to see cloudy conditions with showery
rain for the weekend. Two upper level systems are combining; one
trough in the northern stream, another in the southern stream. The
end result is continued cold air aloft, which will promote shower
development, and a steady steam of moisture from the south. Most
of the showers will be diurnal in nature, the most coverage during
the daylight hours. However, as the low pressure from the south
gets captured by the northern system tonight, some steadier rain
can be expected from Dillingham westward along the mountains.
Winds will be generally light, except for some gusty easterly
winds coming through the gaps of the Aleutian Range. Expect
temperatures to top out in the low 50s with lows in the upper 30s
to low 40s.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
high pressure will encompass most of the Bering through the
period, promoting widespread low stratus. Two system are on the
periphery of the Bering, a low near Bristol Bay, mainly affecting
the Mainland, and a front toward the western Aleutians. The front
will slowly move eastward and dissipate through the weekend.
Little impacts expected as far as wind/rain with this one.
Long term forecast (days 3 through 7)...
beginning the extended forecast period Sunday evening, there will
be a dynamic shortwave trough lifting north from Kodiak Island
with an extensive upper ridge extending from the southwest United
States Mainland to the Yukon. Another, much lower magnitude
shortwave ridge will be across the Bering Sea. As is typical in
this pattern, there will be a frontal zone moving into the north
Gulf Coast and Prince William Sound along with extensive rain
along the coast. With a negative tilt to the upper trough, there
will be rather efficient cross-barrier downslope drying across the
coastal mountains and the Chugach Mountains. By Monday morning,
this shortwave trough will be lifting into the interior part of
Alaska, with a south to north drying trend through the day as
showers become more scattered.
Uncertainty develops after Monday as all global numerical models
simulate the development of a broad low developing over the North
Pacific then moving into the southern Gulf of Alaska. Here, the
details matter, and the European model (ecmwf) and Canadian gdps are depicting a
slightly farther south track than the more north GFS. The slower
solutions delay the progression of a front moving toward the coast
by an entire day and a half compared to the more bullish GFS.
However, the pattern by mid week is rather similar in all models
with extensive coastal precipitation with extensive clouds and
mountain precipitation inland. For now, the wpc approach was
favored (i.E., A blend of ensemble means which are better at
dealing with uncertainty and will take a more middle ground
approach). This means southern Alaska should anticipate a return
to cloudy weather by Tuesday, although with this downslope
pattern, expect temps to return back to normal if not slightly
above normal. For inland areas, while uncertainty exists, these
patterns tend to be rather dry and strong east to southeast upper
flow results in efficient downslope drying.