Scientific Forecaster Discussion
fxak68 pafc 160144
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
444 PM akst sun Jan 15 2017
Analysis and upper levels...
an upper level trough covers most of Mainland Alaska and the eastern
half of the Bering this afternoon, with a ridge over British
Columbia and the Yukon. A small cut-off low is forming over the
western Gulf as a lobe of the ridge builds from the Panhandle to
the north Gulf Coast. A branch of the jet extends from Bristol Bay
northeast across the Yukon. This places the right entrance region
of the jet Max, the area with the highest upward vertical
velocity, over southcentral Alaska. This is also interacting with an
area of abundant moisture.
At the surface, a deep low moving north is approaching the Gulf
of AK, with a front moving inland along the north Gulf Coast.
Another low is well south of the western Aleutians. A surface
ridge extends over the Panhandle and ahead of the front along the
northeast Gulf Coast, while weak high pressure is over the
Bering. A precipitation shield with rain along the north Gulf
Coast and snow inland is building to the west. A fairly strong
demarcation exists between cold air over the Anchorage bowl and to
the north and west, compared with above freezing values along the
coast and spreading toward Cordova and Prince William Sound.
models are in good agreement with the Gulf low, but lose
consistency with the low south of the Aleutians. The forecast
follows the Gulf low with some NAM/GFS, but because of run to
run consistency, uses the ec out west. Confidence continues to
increase with the Gulf low, but remains moderate in the west.
panc...snow will persist through Monday afternoon producing
primarily MVFR vis/ceiling. There may be short periods of IFR
conditions, most likely Monday morning between 12z and 18z when
the heaviest snow is expected. Cold air advection and lingering
light snow may cause lower conditions to linger through Monday
afternoon or perhaps even longer.
Short term forecast southcentral Alaska (days 1 and 2)...
another fast moving storm system from the North Pacific brings
wind and snow challenges to the forecast through Monday. Winter
weather products were issued with the afternoon package for much
of the region as confidence increased today with this storm
system. Models struggled with the dynamics of this system between
model runs, however they trended toward general consensus of a
pattern that supports a strong low moving into the Prince William
Sound late tonight along with upper level support for snow. The
upper low currently moving into the Gulf absorbs into the eastern
side of the Arctic low diving across the West Coast. This energy
forces the Arctic trough into a negative tilt late tonight as a
strong short wave rounds the base of the system. The tilt is a
good orientation to advect abundant moisture inland from the Gulf.
This combined with a strong shortwave propagating up to the coast
and a 962 mb surface low in the sound will bring widespread snow
and gusty winds through channeled coastal terrain through Monday.
Strong gusty winds are expected behind the frontal passage across
the Gulf waters along with cold air advection with winds tapering
down on Tuesday as the surface low weakens on it's eastward track.
Short term forecast southwest Alaska...
a very cold Arctic trough will settle over the region through
Tuesday. Temperatures will continue to be well below average, with
the coldest temps in favored valleys. Wind chill values will creep
toward -35 across the Kuskokwim Delta, but the coldest interior
temperatures will remain displaced from the strongest winds near
the coast, so no Wind Chill Advisory will be issued. A few
isolated snow showers will be possible across the Delta tonight,
otherwise expect dry conditions through the period for the
remainder of southwest Alaska.
Short term forecast Bering Sea/Aleutians...
a cold Arctic air mass will remain over the Bering Sea, supporting
areas of snow showers owing to steep low level lapse rates. Some
of these snow showers may be briefly heavy with minor accumulations
possible. With offshore flow off a well established ice edge off
the southwest Alaska coast, expect extensive areas of heavy
freezing spray into the eastern Bering Sea.
Long term forecast (days 3 through 7)...
the southcentral/southwest Alaska will be influenced by northerly
to northwesterly flow through Saturday. This will allow cooler
air to filter in to the aforementioned regions. The coldest
temperatures look to occur over sheltered inland areas of the
lower Kuskokwim valley, Bristol Bay, Susitna Valley, and the
Copper River basin. Temperatures will be quite cold everywhere,
though persistent tight pressure gradients along the coast will
produce gusty gap winds and maintain marginally warmer
Meanwhile, the models continue with discrepancies with the upper
level closed low over the Bering. While the GFS has a 962 mb low
just west of Saint Paul Island, and the NAM/European model (ecmwf) depicts this as
a 983 mb low just south of the central Aleutians by Wednesday. By
Saturday models are in better agreement, and place this feature in
the North Pacific Ocean south of Kodiak Island near 52n 153w. By
Saturday afternoon a complex low pressure system approaches the
western Aleutians. The associated front begins to swing through
the aforementioned region bringing abundant moisture with warmer
temperatures accompanied by gale force southerly winds. The models
struggle with timing and placement of this storm force low as it
tracks through the western Aleutians in the Bering heading into
Monday. Therefore, the forecast confidence is below average for
the long term forecast period.
public...Blizzard Warning 131. Winter Weather Advisory 101 111 125 141.
Marine...heavy freezing spray warning 130 139 160 180 181 185.
Storm Warning 130 131.
Gale Warning 119 120 121 127 132 136 137 138 150.