Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion...corrected
National Weather Service Anchorage Alaska
416 am akst Thursday Dec 18 2014
Analysis and upper levels...
satellite imagery shows two large storm systems in the
region...both centered over the North Pacific where progressive
upper level flow dominates. A very large and mature storm force
low is centered south of the Gulf of Alaska with the occluded
front slowly lifting northward bringing gale force winds and rain
to the Gulf and Kodiak Island. Out head of this a short-wave ridge
is moving over the southern Mainland resulting in quiet
conditions. There some areas of fog over southcentral...but as
pressure gradients tighten with the approaching front and offshore
flow strengthens expect most of this to dissipate. Meanwhile a
200+ knots jet is driving eastward across the northwest Pacific and
into the north-central Pacific causing rapid cyclogenesis with a
series of lows strung out just to the south of the Bering Sea and
Aleutians. The low closest to the western Aleutians and associated
occluded front are lifting northward toward the Aleutians...so
gale force winds and rain are on the way.
Model discussion...models seem to be converging on a common
solution with triple point low forming over the far northeast
Pacific Thursday morning and rotating westward across the
southern Gulf Thursday afternoon through Thursday night. All
solutions except the NAM are grouped fairly close together...so
will updated forecast to reflect this low (which it does not
presently). The entire system now looks like it will remain
farther south than prior guidance indicated...meaning winds over
the northern Gulf/Prince William Sound area will be a little
lighter than currently advertised.
Numerical guidance continues to struggle with handling of multiple
lows south of the Aleutians. Current forecast seems a fair
representation of the general winds and weather expected.
However...there may be some directional issues depending on where
the current low along the Aleutians stalls out. With multiple weak
lows in place south of the Aleutians/Alaska Peninsula/Kodiak
Island through the weekend forecast confidence in this area is
below average. Forecast confidence farther north is generally
Short term forecast southcentral Alaska (days 1 and 2)...
impacts of the large northeast Pacific low will initially remain
down over the Gulf and Kodiak Island. When the triple point low
crosses the Gulf this afternoon and tonight the upper ridge will
be pushed northward into interior Alaska allowing some precipitation to
make it up to the north Gulf Coast. Some component of offshore
flow will persist in the low levels so precipitation will be light.
Meanwhile localized outflow winds will set up through major gaps
today as pressure gradients tighten and low level flow becomes
more uniformly offshore. The air mass in the Copper River basin is
colder than the rest of southcentral...but there really isn't much
in the way of advection of this air. Thus expect this to be a weak
Short term forecast southwest Alaska (days 1 and 2)...
a couple weak upper level waves moving over top of the large low
over the northeast Pacific will cross southwest Alaska the next
couple days. This will bring a couple quick shots of precipitation...but
whatever does fall will be light and brief. With the exception of
the Alaska Peninsula...the air mass in place will remain cold
enough to support snow.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the bulk of active weather will be confined to the Aleutians and
Alaska Peninsula the next couple days as lows pass just to the
south. A generally unstable air mass over the Bering Sea will
continue to produce some showers...but nothing very well
Long term forecast (days 3 through 7)...
confidence in the general evolution of the large scale pattern as
we head into the early to middle part of next week is high...with
very good run-to-run continuity in numerical guidance. Confidence
in some of the specific features is not quite as high...but
latest solutions are not terribly different...so confidence is
As has been advertised in previous discussions the series of lows
strung out across the North Pacific and Gulf of Alaska this
weekend will all consolidate into one very large cyclonic
circulation centered somewhere over the western Gulf or near the
Alaska Peninsula by early next week. As the systems progress a
little further north this weekend warmer low level air will move
back into southcentral as well as the Bristol Bay area. Therefore
the threat of freezing rain will return once again...although the
bulk of precipitation should remain along the coast.
A transition in pattern will commence next week as a deep trough
moves out of Asia and into the Bering Sea. A building ridge out
ahead of this trough will help provide a quick shot of cold
advection downstream of the trough...over the Bering Sea this
weekend spreading to Mainland Alaska early next week. There could
even be a quick shot of snow over southcentral depending on what
happens to the remnants of the consolidated low mentioned above.
The GFS/European model (ecmwf) are both depicting development of a deep surface low
on the front edge of the trough moving out of Asia. Its hard to
say exactly how deep it will be...but with a strong upper level
jet in amplified flow conditions look favorable for rapid deepening.
With the downstream ridge continuing to build the storm will be
forced northward across the Bering Sea. This track is very
important to the forecast over Mainland Alaska because it means
the bulk of warm Pacific air will remain over the Bering Sea.
Therefore...as the upper trough continues to progress eastward to
Mainland Alaska the middle of next week the air mass will likely
remain cold enough to support all snow over interior southwest
Alaska and over much of southcentral Alaska. It is way too early
to have much confidence in this since dynamic patterns often
produce some surprises...but there is definitely potential for
some fresh snow across the southern Mainland of Alaska in time
Marine...Gale Warning...119 120 127 130 131 132 136 137 138 139
150 176 177 178.
Heavy freezing spray warning...185.
Synopsis and model discussion...seb
southwest Alaska/Bering Sea/Aleutians...seb