Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
542 am akst Wednesday Nov 25 2015
Analysis and upper levels...
The upper levels continue to have a strong 120 knot southwesterly
jet positioned over the Alaska Peninsula extending through the
lower Kuskokwim valley into the upper Yukon valley region. The
satellite imagery is showing abundant moisture streaming into the
southwest Alaska area and the southcentral region. The radar
imagery continues to show numerous showers over the Bristol Bay
region and through the Cook Inlet into the mat-su valleys. The
main concern will be with the temperatures through the Susitna
Valley with the 850 mb temperatures trending toward warmer
temperatures except along the western valleys where colder
temperatures remain in place. The warm southerly flow continues to
erode the cold dome in place in the Susitna Valley and the surface
temperatures have finally risen above freezing near Talkeetna while
the temperatures throughout the Anchorage bowl are in the 40s and
the Matanuska Valley is reflecting the warm air advection into
the region. There is closed upper level low over the western
Chukchi Sea with an associated trough extending southward through
the Bering Sea and across the central Aleutians into the North
Pacific Ocean. The 300 mb level is showing a strong 110 knot
northwesterly jet stream digging into the base of the longwave
trough. The satellite imagery is showing the aforementioned
synoptic feature with a cyclonic-wave forming near 49n 156w this
morning. Farther upstream the western Aleutians have a swath of
moisture streaming from the North Pacific side and the rest of the
Bering Sea is under a showery weather regime.
The models are in good agreement with the storm force low
tracking along the western Aleutians this afternoon as it
continues to move parallel along the chain through the end of
this week. The other synoptic feature that will come into play
for the southcentral region is the development of the North
Pacific low located just south of Kodiak Island by this afternoon.
The models start out in good agreement with this system but begin
to struggle with placement of this system as it moves northward.
The Canadian-reg and the European model (ecmwf) models start out in agreement
as the low tracks through the Shelikof Strait but the European model (ecmwf)
tracks the low over the Alaska Range and the Canadian-reg take
the low up through the Cook Inlet. Where the NAM/GFS models track
this system over the eastern side of Kodiak Island by Wednesday
evening before having the low dissipate over the Homer area by
Thursday morning. The forecast confidence starts out above average
but begins to waver due to the uncertainty of the storm track for
the system entering the western Gulf into the central Alaska
Short term forecast southcentral Alaska (days 1 and 2)...
An impressive front continues to push moisture northwards to the
southcentral region. While today snow will be lighter north of
Talkeetna than it was yesterday...heavy snow is expected to occur
again this afternoon through tomorrow morning as a low pressure
system that is currently near Kodiak surges northwards towards
the inlet. South of Talkeetna...temperatures remain just above
freezing...and rain is falling instead of snow...along with
isolated pockets of freezing rain. Starting this evening...the
southerly winds that have been transporting warmer air into the
southern Matanuska Valley are expected to weaken and switch to
northerly. This should allow temperatures to drop just enough for
snow to occur across the entire Matanuska Valley by tonight.
Heavy rain mixed with snow is also possible across much of
southcentral beginning this evening and into tomorrow morning.
Before the low pressure system reaches the inlet...pressure
gradients will be less favorable for the type of downsloping that
limited rainfall across Anchorage yesterday. As a result...steady
rain is expected across Anchorage from this evening through
Short term forecast southwest Alaska (days 1 and 2)...
The main forecast challenges through Thursday will continue to
revolve around snow potential over eastern portions of Bristol
Bay and the Kuskokwim valley as precipitation is enhanced by a
potent North Pacific wave riding northward along the frontal
boundary stalled over the area. The highest snow amounts are
expected to be largely confined to the Alaska and Aleutian
ranges...with storm totals generally on the order of 3-5 inches
through Thursday afternoon across the lower elevations to the west
of the mountain ranges. Precipitation looks to become more
widespread across the southwest Mainland on Thursday night as a
front associated with the next low tracking across the Aleutian
chain reaches the coast.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
The Bering will be dominated in the short term by the strong low
tracking along the Aleutian chain as it moves from near the
western Aleutians today to the vicinity of Dutch Harbor on
Thursday night. This system will spread heavy precipitation over
much of the southern Bering...with precipitation beginning as snow before
quickly changing over to rain as warm Pacific air surges northward
behind the front. The other main impact will be strong
winds...with gale force winds spreading eastward along the chain
through Thursday night and a smaller embedded area of storm force
winds impacting the central Aleutians through this evening.
Long term forecast (days 3 through 7)...
Models are in agreement that the large and slowly moving low
pressure center over the central Aleutians Thursday evening will reach
the eastern Bering on Sat and then drift. The second low reaching
the western Aleutians on Sat will remain south of the chain as it
moves to Kodiak Island early Tuesday where it will stall. The
forecast utilizes this general solution...with wpc ensembles
helping to fine tune the trajectory of the second low.
In the east...warm advection on southerly flow will bring warmer
temperatures to much of southcentral Thursday evening through Sat.
Rain will be locally heavy along the coast and upslope sides of
mountain ranges. This will continue with slightly cooler
conditions into early next week.
In the west rain through Sat will be substantial across the chain
and then into SW Alaska as the low transits and then moves over the
eastern Bering...gale and storm force winds will also be
associated with the low. Rain and snow showers...along with
slightly lighter winds...will then spread across the chain and
into SW Alaska as the second low moves just south of the island into
early next week.
public...Snow Advisory 145
Marine...storms 173 174 175 176 177
Gales 119 120 125 131 132 150 155 165 170 172 178 195
351 352 411 412 413 414
Synopsis and model discussion...period
southwest Alaska/Bering Sea/Aleutians...cumulonimbus