Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
345 am akst Thursday Dec 12 2013
***long duration snow event expected for parts of southcentral with
locally heavy accumulations across parts of the coast and coastal
Analysis and upper levels...
The upper air pattern shift is complete as the high latitude blocking
which dominated Alaska the past week and a half has retrograded west
to eastern Siberia. A ridge of high pressure extending into the
Beaufort Sea is allowing for the development of a polar vortex over
the Alaska and Aleutian ranges...with associated dry and cold air
dominating the Mainland of Alaska. A rapidly deepening North Pacific
low associated with an anomalously deep trough near 165w 40n will
track northeast...weakening into a series of low amplitude upper
level shortwave troughs. An associated warm front will lift into the
Gulf of Alaska late Thursday into Friday. A weak shortwave
trough...currently tracking into the Gulf of Alaska...will favorably
interact with the polar vortex...allowing the vortex and cold air
behind it to shift the overall position east over Cook Inlet.
Eventually...by Friday...the warm front will begin to interact with
the closed vortex...with the warm front forming east of Kodiak
Island...and a surface low tracking into then stalling in Prince
The exact details of the formation of the deformation/frontal
band...track of the low...strength of the northerly winds down
Susitna Valley...the warm push along the coast...the degree of cross
barrier flow...et cetera are all slightly uncertain at this time...which
will have dramatic impacts on local snow accumulations (see short
The suite of dynamical numerical models are in excellent agreement
with the large scale synoptic pattern through the weekend...and are
in good agreement with finer scale details. There continues to remain
small detailed discrepancies in individual models...and given the
complexity of the pattern...will likely continue even as snow is
falling. With that said...there is high confidence light to moderate
snow will overspread southcentral by late Friday regardless of model
solution given the pattern. The European model (ecmwf) remains the favored synoptic
model as it has shown Superior consistency the past two and a half
days...with all guidance trending towards its solution. The 06z
Canadian regional dps was also utilized.
Short term forecast...
middle level clouds are already streaming into much of southcentral
Alaska...with light snow showers pushing into Prince William Sound
and the eastern Copper Basin. Moderately strong gap winds are
continuing this morning across all favored terrain locations...with
the coldest air and wind chills pushing through the eastern Alaska
Range into the Copper Basin. The pressure gradient will relax
slightly today...but gap winds will likely continue into the weekend
as long as polar air continues to filter in from the north.
By this afternoon and evening...light snow will continue to expand
and overspread the northern Gulf Coast into Prince William Sound with
light accumulations. Flurries and light snow will also move into
Turnagain Arm and possibly Anchorage this evening. A much more
substantial push of moisture will lift into the Gulf Friday...with
moderate to heavy snow overspreading the coastal regions east of
Kodiak...with snow spreading inland Friday afternoon and evening.
Snow will increase across most locations into the overnight...with
the possibility of a brief changeover to rain along the coast east of
Prince William Sound. However...a strong cold front will rapidly
sweep through the Gulf early Saturday...with more moderate to heavy
snow along the coast. For Thompson Pass...the pressure gradient will
remain tight...with cold air continuing to accelerate through the
pass...which will allow for both very heavy snow and strong winds
with blizzard conditions by late this evening.
Snow accumulations will be trickier inland...with large differences
developing middle day Saturday and beyond...but it does seem that the
bulk of accumulating snow comes Friday and early Saturday...with
lighter snow thereafter lingering into Sunday. Storm total
accumulations for Anchorage and vicinity will generally range from 4
to 8 inches...but there is the potential for locally higher values.
Those details will be ironed out in later forecasts.
the main story across the region will be continued windy and cold
conditions...with wind chills dropping into the -15 to -35 degree
range through Friday. Light snow will clip the Alaska and Aleutian
ranges Saturday into Sunday...but the remainder of the Mainland will
cold air will sweep through the Bering by late this afternoon...with
increasing rain and snow showers as the cold air overrides the warm
water surfaces. Friday into Saturday...northerly flow will
increase...which will favor areas of snow and possibly reduced
visibility from the Pribilof Islands down to Dutch Harbor and east to
the Alaska Peninsula. These snow showers will last well into the
weekend with continued strong northerly winds.
Long term forecast...
Monday and beyond...generally quiescent conditions will be ongoing
across much of the region as surface high pressure builds in behind
the departing storm system. Mainland Alaska will generally be colder
and drier likely through middle to late next week...with a strong
warm front possibly pushing into the Bering Sea by midweek.
public...Blizzard Warning 131.
Winter Weather Advisory 141.
Marine...gale 119 126 127 170 173 179 180 181.
Heavy freezing spray 126 127 130 138 139 140 141 150 160 165
180 181 185.
Ahsenmacher Dec 13