Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion...corrected
National Weather Service Anchorage Alaska
422 PM akst Sat Dec 20 2014
Correction to add marine warning
Analysis and upper levels...
as the 49th state anticipates the psychologically important
turning Point of gaining daylight tomorrow...the weather map
across the state is starting to look a little more interesting.
A broad area of low pressure remains in the Gulf...with numerous
shortwave troughs pinwheeling around the common center...and the
stronger waves also have a reflection of lower pressures at the
surface. One such shortwave is moving across the northern Gulf
today...with an attendant surface low (estimated around 969 mb)
due south of Cape Suckling. Showers across the northern Gulf are
giving way to a more stratiform-looking precipitation shield as
the low moves into radar range. This low...combined with cold air
in place over interior Alaska...continues to produce moderate dry
northerly offshore winds from Prince William Sound to the Kenai
Peninsula. As rain/snow move in from the Gulf...this dry air will
need to be moistened up in order for precipitation to reach the ground.
To the west...dry offshore flow continues across the southwest
Mainland...with a partly cloudy and cold day unfolding for a good
portion of the area. A weakening and vertically stacked low
pressure system south of the Dutch Harbor area is slowly moving
out into the Pacific...with the main impact of this feature
currently being some gusty northerly winds over the western half
of the Aleutians. Meanwhile...Arctic air is spilling down from
the Bering Strait across the northern and western Bering
Sea...kicking off some snow shower activity as it expands
the overwhelming focus throughout the short term (first 2 days)
forecast period is the evolution currently unfolding in the Gulf
of Alaska and southcentral Mainland. As mentioned in previous
discussions...the model consensus over the last day or two has
shifted toward a more northwesterly track of the disturbance
currently east of Middleton Island. Confidence continues to
increase that this feature will weaken and track inland tonight somewhere
over the northern Kenai Peninsula. As this happens...cross-barrier
flow will weaken and precipitation is likely to spread across
much of the Cook Inlet region. The GFS and Gem-regional both
depart from the consensus track over the northern Kenai (gem-
regional to the north and GFS to the south) and were not
considered in forecast preparation today. That leaves basically
the European model (ecmwf) and the NAM...though even they have a timing issue
remaining. The European model (ecmwf) is about 6 hours faster with landfall than
the NAM...although the 18z NAM has closed that gap slightly. A
midday satellite scatterometer pass has fortunately sampled the
center of circulation...and lends support largely to the faster
European model (ecmwf) solution. Of note also is the fact that satellite imagery
clearly suggests two discrete centers of circulation in the
north/northwest Gulf...rather than a single elongated pressure
center. The European model (ecmwf) is the only model to depict a two-low solution.
As you might guess...the European model (ecmwf) is preferred across the entire
forecast domain today...largely due to Superior performance across
Short term forecast southcentral Alaska (days 1 and 2)...
a nice warm conveyor belt showing up on radar satellite today
looks to bring light snow to much of the region. The
responsible low pressure system will move into western Prince
William Sound tonight then inland as it rapidly weakens. Snow will
begin to move into Cordova and Valdez early this evening then
westward over whitter and Seward. On the west side of the chugach
snow will begin around midnight and continue into Sunday. This
looks like a high probability/low accumulation event. The highest
amounts look to fall somewhere between Anchorage to Kenai. Once
the system passes tomorrow morning the southcentral will be under
the influence of an unstable regime aloft for the next couple
days. The unstable flow combined with weak disturbances within it
make for a low confidence forecast. The probability for snow
showers is quiet high within this regime but where exactly they
set up is less certain. Expect snow and rain showers along the
north Gulf Coast tomorrow and Monday with scattered to numerous
snow showers inland. Temperatures will remain around normal with
generally light winds.
Short term forecast southwest Alaska (days 1 and 2)...
the remnants of a weakening low positioned over Kodiak will
continue to bring scattered showers along the Aleutian Range and
parts of the Alaska Peninsula through Monday. The rest of the
southwest Mainland will remain under drier offshore flow through
Monday. By Monday morning an upper level ridge will push into the
eastern Bering while steadily amplifying. This synoptic feature
combined with the cold air filtering into the region from the
north will bring cooler temperatures resulting in below normal
temperatures through Monday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the weakening low just south of the central Aleutians continues to
track to the southeast this evening...slowly weakening winds over
the western and central Aleutians/Bering. Colder air behind this
system continues to filter down over parts of the western and
central Aleutians/Bering through tomorrow morning...keeping
showery conditions and gusty winds in the forecast for that area.
A frontal system will begin to push into the western Aleutians
from the southwest tomorrow afternoon...bringing another round of
gale force southerly winds and a mix of rain and snow over the
western Aleutians heading into Monday. With this system bringing
in some warmer temperatures from the North Pacific...the
predominant precipitation type will start off as snow and steadily
change to rain by Monday afternoon.
Long term forecast (days 3 through 7)...
low pressure remains the dominant weather pattern over the Bering
through early next weekend. Models have not come into better
agreement on the upper low placement in the western Bering on
Tuesday which leads to uncertainty through the extended forecast.
The general focus will be along the front that moves from the
central Bering to the southwest coast Tuesday into Wednesday.
Gusty gale winds and mixed precipitation accompany the front
across the Aleutians and Bering with storm force winds expected
across the western Aleutians as the center of the low moves north
of the Aleutian chain. The front slows in progression on Thursday
as is pushes into the Mainland and elongates its boundary eastward
into the Gulf. A triple point low is likely to develop in the
western Gulf middle week with an eastward propagation which keeps most
of the precipitation along the Gulf water and surrounding coast.
Marine...gales 127 177 178
heavy freezing spray 180 181 185.
Synopsis and model discussion...ad
southwest Alaska/Bering Sea/Aleutians...tp