Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
429 am akst sun Dec 21 2014
Analysis and upper levels...
An area of low pressure over the northern Gulf continues to push
to the west/northwest of Middleton Island early this morning.
This low pressure system held together a bit longer than expected
over the past several forecast runs...allowing for a stronger
barrier jet over the northern Gulf and a slightly slower westward
track. Warm air advection on the backside of the low rapidly
warmed the northern Gulf Coast (cordova area) from the middle 30s to
the middle 40s and transitioned precipitation from snow to rain.
Snowfall continues to overspread much of the Gulf Coast and is
pushing further inland over the Copper River basin...Anchorage and
parts of the mat-su valley. The longwave trough extending from the
Kamchatka peninsula to the Pacific northwest coast of the United
States continues to amplify as a potent shortwave and an upper
level jet ranging from 150 to 160 knots helps dig the trough further
south in the North Pacific. Weak surface and upper level based
ridging is developing over the western Bering Sea early this
morning which has helped to shift the remnants of a weak low just
south of the Aleutian chain into the western Gulf of Alaska. An
even more potent jet (possibly approaching 200 kt) has begun to
set up over east Asia and has helped to intensify a low over the
Kamchatka peninsula that has begun to push a front toward the
western Bering Sea.
The 00z and 06z numerical guidance was struggling to initialize
the low in the northern Gulf presently located northwest of
Middleton Island. Based on surface observations and a scatterometer
pass at 0510z...it appears the 00z/06z NAM has initialized the
best...with the European model (ecmwf) being too fast and the Gem being way too
slow. The GFS is a huge outlier with the initialization as it had
more of a complex low feature than an actual closed low.
However...models are in good agreement with snow continuing across
much of southcentral Alaska through the beginning of the upcoming
week...with snowfall becoming more showery in nature with some
elevated instability and a series of weak upper level disturbances
pushing across the area. The NAM was utilized for the most part
over the east domain due to better initialization.
Out west...models remain in very good agreement with a weak
surface and upper level ridge moving from the western Bering Sea
to eastern Bering Sea during the course of the week. Model
agreement begins to falter late Monday into Tuesday as a low
develops to the west of Shemya and pushes into the Bering Sea.
While the models are in somewhat agreement with strength and
position...the Standard deviation of over 5 mb is a bit disconcerting.
Nevertheless...it appears storm force winds will be likely with a
rapidly developing low interacting with a potent (180 knots to 200
kt) jet streak. A mix of the European model (ecmwf) and NAM was utilized over the
Short term forecast southcentral Alaska (days 1 and 2)...
The low in the northern Gulf is bringing in enough moisture from
the south to bring light snow to many locations around southcentral.
However...in some places along the north Gulf Coast it remains
warm enough for rain. As the low tracks west into the Cook Inlet
area today flow will become more southerly resulting in a better
chance for snow showers from the Anchorage bowl area into the mat-
su valley. This higher chance of light snow will last through
tonight before the low weakens and moves south.
Short term forecast southwest Alaska (days 1 and 2)...
The remnants of a dissipating low located over Kodiak will
continue to bring scattered showers along the Aleutian Range and
the Alaska Range through Monday. The winds will remain northerly
resulting in offshore flow resulting in drier conditions for the
rest of southwest Alaska through Tuesday. The northerly winds
will bring in cooler temperatures resulting in below normal
temperatures for the early part of this upcoming week.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
The east siberian high will remain in place resulting in
northerly winds through the Bering Strait southward into the
northern Bering Sea through Sunday before shifting to the
southeast by Monday. This will continue to bring showery
precipitation to the northern Bering Sea with cooler temperatures
through Sunday. By Sunday night a weather front will push through
the western Aleutians into the western to central Bering Sea by
Monday...followed by a North Pacific storm force low tracking
through Shemya by Monday night before moving into the western
Bering Sea by Tuesday. A secondary weather front from the
aforementioned low will be positioned just west of Unalaska by
Tuesday morning. This will bring warmer temperatures with storm
force winds through the western Aleutians into the southern Bering
Sea. The central to eastern Aleutians will receive gale force
winds spilling over into the central Bering Sea by Tuesday. As the
North Pacific low moves into the Bering Sea by Tuesday...this will
produce a large swath of moisture and precipitation through the
Aleutians and the Bering Sea.
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...Gale Warning 177 178
heavy freezing spray 180 181 185
Synopsis and model discussion...mmc
southwest Alaska/Bering Sea/Aleutians...pld