Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
145 PM akdt Monday Sep 22 2014
Analysis and upper levels...
the upper level pattern today has a trough along the alcan border
with an upper level ridge over the eastern Bering Sea and western
Alaska. Farther west there is a cut off low close to Attu. The 250
mb jet stream continues to run south of the area.
A cold upper level trough is moving into northwest Alaska off the
At the surface there is a weak trough along the north Gulf Coast
as well as a weak low in the eastern Copper River basin. These
features are the remnants of the low that was in the eastern Gulf
yesterday. Another interesting surface feature is a mesoscale-low that
has developed just south of Kodiak Island. This feature can be
seen best on the visible satellite as it is too low to be seen
well on the infrared satellite.
High pressure is over the eastern Bering Sea and southwest Alaska
and this ridge extends into the interior of the state as well. A
low complex remains near the western Bering Sea and western
models are in good agreement into Wednesday but then there are
significant problems that show up over both the Gulf of Alaska
and Bering Sea. While all the models keep the high pressure over
the Mainland part of the state through the week the two lows on
the periphery of the high pressure are the issue. On Thursday a
low southeast of the Gulf of Alaska is detected by all models but
there is a discrepancy of over 450 miles between the NAM and Gem
solutions. The European model (ecmwf) and GFS are generally splitting the difference
and they are likely to verify better. In the Bering Sea the model
differences are larger and will have more impact on the forecasts.
While the difference in the low near the Gulf can be blended out
to get a reasonable solution the differences between the models
precludes this. The basic issue revolves around whether the
incoming low will have multiple centers along a broad trough or
whether the low will coalesce into a single strong center. This
second solution is one that the European model (ecmwf) has insisted on the past few
runs. The NAM and GFS and Gem show a solution more like the first
scenario but with very different details. In short, the forecast
in the Bering Sea and Aleutians for the second half of the week
has a greater uncertainty than usual.
Short term forecast...
the remnants of the Gulf low persist over the Copper River basin
with enough dynamics for some rain in the central part of the
basin and snow to the east toward Mentasta Pass. Most of this wet
snow appeared to be melting as it hit the ground.
The rest of south central Alaska has cleared out quite well with
high pressure over the interior and southwest Alaska bringing in
dry air. The exception is the eastern and southern parts of Kodiak
Island are under some cloud cover due to the mesoscale-low just south
of the island. Kodiak normally has clear skies under the northwest
flow aloft but this surface low is enough to bring in the clouds
until it pulls farther south late this evening. The ridge combined
with the trough along the north Gulf Coast has created a pressure
gradient tight enough for gusty outflow winds through channeled
terrain...especially right along the Gulf Coast. These winds will
diminish tomorrow as the gradient weakens but then increase again
on Tuesday night through Wednesday night as the gradient increases
The clear skies will also usher in the coldest overnight
temperatures so far this season the next few nights.
southwest Alaska is under the high pressure and has clear skies
over much of the area. There are a few stratus clouds riding over
the top of the ridge that are being draws toward the upper level
trough in northwest Alaska. This will bring in more cloudiness to
the Kuskokwim valley and Delta tomorrow. This will be short lived
as the ridge will remain over the area until late this week.
the eastern Bering Sea and Alaska Peninsula has large areas of
clear skies today under the ridge.
The western Aleutians have a complex low in the area with areas of
showers in the occlusions around it. The cold air aloft is
significant enough that there were some thunderstorms between
Adak and Amchitka this afternoon. These storms will likely not
last long into the evening but they show how unstable the
atmosphere is in that region with cold air aloft and relatively
warm air over the Bering Sea.
Long term forecast...
after a dry week for most of Alaska...the pattern will begin to
shift as a trough across the Bering and North Pacific shifts east
into the Gulf of Alaska. Much uncertainty riddles the beginning of
the extended forecast...however...as the numerical guidance
struggles mightily with a complex wave merger over the Bering Sea.
The differences likely come down to model physics and
parameterizations...with the European model (ecmwf) favoring a full wave merger
while the Gem...NAM...and GFS are much less bullish. The net
result is the European model (ecmwf) is stronger and faster with the low and
associated front as it progresses into the Gulf of Alaska as early
as Friday. For this forecast...a more modest and slow wave
progression favoring the Gem and GFS was utilized as there is not
high confidence in the outlier European model (ecmwf) solution at this time. With
that said...clouds and precipitation chances will begin to impact
southern Alaska by late week into this weekend...but timing and
the northward extent remains highly uncertain.