Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion...corrected
National Weather Service Anchorage Alaska
145 PM akst Tuesday Dec 10 2013
Analysis and upper levels...the upper level ridge over the Gulf of
Alaska is rapidly weakening and moving to the east as an upper level
low south of the Alaska Peninsula moves into the Gulf of Alaska.
A weak short wave is moving out of the interior of the state and into
south central Alaska and the Copper River basin...with the bulk of
the associated lift in the Copper River basin.
At the surface...high pressure dominates the majority of the state. A
moderate low south of Kodiak Island is tracking northeast into the
Gulf of Alaska as a weak low in the western Bering Sea heads toward
the central Aleutians.
Model discussion...models are doing well with the first low that
will be moving into the Gulf of Alaska and are good with the synoptic
pattern with the second one. The second one is giving all the models
trouble as it moves into the Gulf of Alaska Thursday. There is
disagreement as to whether the low will split into two distinct
centers or whether it will remain consolidated into one center. The
movement of the warm front toward the Gulf Coast is also a Point of
contention. The GFS and European model (ecmwf) are close enough to each other to
give a reasonable solution at this time but the NAM is a bit of an
outlier with its structure...which does not seem as reasonable at
this time. In short the NAM is good for the first two days of the
forecast period but a blend of the European model (ecmwf) and GFS looks much better as
we move into Thursday night and Friday.
Short term forecast...
southcentral...the widespread fog is dissipating as the upper level
ridge moves out of the area and the surface gradient increases. This
will allow better mixing of the lower atmosphere as well as an influx
of drier air into the area. The upper level disturbance moving into
the area this afternoon and evening is producing some light snowfall
in the Copper River basin and a few places in the Susitna Valley.
The increasing pressure gradient will begin to increase the winds
through channeled terrain tonight and persist into Thursday. The
Matanuska Valley is likely to see the strongest of these winds over
the next day or so.
Southwest...the increasing pressure gradient will continue to
increase winds over the area as well as bring in drier air and some
Long term forecast...the longer term forecast is tricky due to the
amount the details will dictate what will happen. What does look
certain is that a large high will build over the central Bering Sea
with the low moving into the northern Gulf of Alaska. This will keep
the Bering Sea region into southwest Alaska mostly dry under the
ridge. Southcentral Alaska is another story. There looks to be snow
along the Gulf Coast by Thursday with it possibly mixing with or
turning to rain as the warm front moves over the area. Inland areas
of southcentral will be highly dependent on the exact track the low
takes as well as the associated warm front. There looks like there
will be a shot of snow due to overrunning as this system moves into
southcentral and then the down-sloping will shut it off over lower
elevations...including the Anchorage area. By Saturday the storm
should start to bring colder air back over the southcenteral area as
well as cut off the down-sloping winds. This increases the chances of
snowfall at that time. It is Worth watching this storm
develop as there are many different ways this event may play out.
Marine...gale 126 127 128 130 132 138 139 150.
Heavy freezing spray 126 127 180 181.