Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
412 PM akst Monday Jan 26 2015
Analysis and upper levels...
a trough extended from a closed low over the Arctic Ocean south
and west across the central interior. A weak high pressure ridge
was evident across the Gulf with a low complex over the Aleutians
and eastern Bering Sea. Water vapor imagery showed cirrus clouds
being entrained in the westerly flow and spreading across the
high pressure continued cold temperatures across the
Mainland. Cold ambient temperatures combined with gusty winds
produced wide spread wind chills of 40 to 50 below across the
Delta. Local wind chills of around 40 below were observed with
brisk gap flows in Thompson Pass.
An 988 mb low center was evident north of Unalaska with the
trailing occluded front extending across the Alaska Peninsula to just west
of Kodiak Island. Snow was evident in advance of the this front
over Bristol Bay and extended east to near Kodiak. Rain showers
were observed south of the front over the Alaska Peninsula. Brisk winds in
advance of the front will combine with snowfall to produce
blowing snow across Bristol Bay through tonight.
the 12z European model (ecmwf)/GFS model runs are in good agreement and reasonable.
The 12z NAM was the outlier...however the 18z NAM has come into
agreement with the hemispheric models.
Short term forecast southcentral Alaska (days 1 and 2)...
Tricky low temperature forecasts in store the next couple of
nights/mornings as high level clouds and patchy fog formation will
be the main determinants as to how cold it gets. For now...decided
to go with a slightly warmer to middle-of-the-Road forecast for
most areas...but any brief clearing and/or decoupling of winds in
exposed areas will result in a rapid plunge of several degrees.
As for precipitation...flurries will be possible with fog
formation around Anchorage and the Copper River basin tonight and
also again Tuesday evening. A weak disturbance near the north Gulf
Coast will bring some light snow showers to eastern Prince William
Sound through tonight.
Kodiak and the southern Kenai Peninsula will begin to experience
increasing probabilities of precipitation over the next couple of
days as easterly flow increases ahead of a weather front that will
move north toward the area. Kodiak will continue to see periods of
light snow but will gradually mix with and then change to rain as
warmer air moves north with the approaching weather front.
Short term forecast southwest Alaska (tonight through Thursday night)...
Wind chills remain a significant concern across the Kuskokwim
Delta this evening as wind chills have remained in the 35 below to
45 below threshold through the majority of the day. However...there
is some good news as temperatures are expected to gradually warm
overnight...which should lead to wind chill values in the range of
15 to 30 below come Tuesday morning. Overnight wind chills will
remain a concern across the Kuskokwim Delta as northeasterly winds
at the surface continue to usher in the bitterly cold Continental
Arctic air mass that has built over the interior. The lower Kuskokwim
valley will also continue to see some bitterly cold weather but
luckily have light winds which is preventing wind chills from
becoming an issue. Temperatures across the lower Kuskokwim valley
will gradually warm through the remainder of the week...although
temperatures are still no expected to warm above zero.
Over the Bristol Bay area...a Winter Weather Advisory for snow and
blowing snow has been issued as radar reflectivities are already
showing decent snow moving from south to north. Areas north and
west of King Salmon (including Dillingham and koliganek) will see
anywhere from 5 to 8 inches of snow accumulation while the
remainder of the zone will be on the dry side...largely from
downsloping as a warm front pushes north. However a few isolated
snow showers near King Salmon cannot be ruled out. Strong winds
will accompany the snowfall north and west of King Salmon which
will allow blowing snow conditions to develop that will reduce
visibilities to one half mile or less at times through Tuesday
morning. As snow tapers off Tuesday morning...skies will
gradually clear through the remainder of the week with a brief
warm up observed Tuesday and Wednesday.
Short term forecast Bering Sea/Aleutians (tonight through Thursday
A broad complex low will continue to skirt just south of the
Aleutian Islands through Wednesday afternoon before taking a
southern turn into the North Pacific as a strong ridge of high
pressure begins to situate itself across the Bering Sea. A warm
front associated with the complex low has moved north of the
Aleutian Islands and has stalled just south of the Pribilof
Islands. This is evident in the surface temperature trends over
the past 24 hours. The Pribilof Islands are in a "sweet spot" as
they continue to see cold northeasterly flow as precipitation with
the decaying warm front shifts north. This will allow the period
of snow that has been observed throughout the day to continue.
With the building high pressure over the western Bering...a strong
pressure gradient has developed and created gusty winds across the
Pribilof Islands leading to blowing snow conditions. The core of
the strongest winds will shift westward Tuesday afternoon and
bring strong winds to the central Aleutians Tuesday afternoon
through the remainder of the week.
Long term forecast (days 3 through 7)...
a weakening front/inverted trough will bring precipitation to
Kodiak Island and areas along the north Gulf Coast towards the
end of the week...but otherwise much of the southern Mainland will
remain in a dry offshore flow pattern through Friday as a Stout
upper ridge builds over the western Bering and temperatures slowly
moderate to near normal values.
Forecast confidence begins to diminish this weekend into next week
as the upper ridge migrates eastward and evolves into a closed
upper high over the eastern Bering/western Mainland. Where this
feature ultimately sets up will have a large impact on the
northward progression of a North Pacific low moving towards the
southern Gulf...and thus have a large impact on precipitation
chances over the Mainland. At this point it appears that any
precipitation will remain far enough south to mainly impact areas from
the Alaska Peninsula to the Gulf Coast...with much of the Mainland
and Bering Sea remaining dry as persistent offshore flow keeps
gusty winds along the north Gulf Coast.
public...Wind Chill Advisory 155. Snow/blowing Snow Advisory 161
blowing Snow Advisory 195.
Marine...Gale Warning 120 127 130 131 132 138 139 150 155 165 170
172 173 175 176 177 179 185.
Heavy freeze spray 121 126 127 128 130 139 140 141 185.
Synopsis and model discussion...British Columbia
southwest Alaska/Bering Sea/Aleutians...mc