Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
546 am akdt Friday Oct 21 2016
Analysis and upper levels...
As expected, the deformation band developed overnight over the
Cook Inlet region with some embedded convective activity. While
the Kenai nexrad was showing pretty high reflectivities overnight,
it appears these were enhanced due to bright banding as snow was
melting as it fell through the melting layer. Rain has changed
over to snow this morning but a tad later than expected as colder
air has been slower to filter through the Alaska Range. Talkeetna
on the other hand has been snow from last evening onwards with
rough estimates from ASOS liquid totals showing 2 to 3 inches of
wet snow. The transition from rain to snow will continue from
west to east through the early morning hours but with the later
timing of the cold air, snow totals may be a little lower across
Anchorage and parts of the Matanuska Valley.
Looking back at the big picture, a hodgepodge of troughs and
ridges encompass Alaska and the coastal waters. The feature that
will impact the Mainland the most is an Arctic trough extending
from the Arctic all the way into the northwestern Gulf of Alaska.
This trough is responsible for the cold air that has been filtering
in across the state. The trough will provide extra lift across
southcentral Alaska through the morning hours which will keep snow
(and possibly some rain) common across the Cook Inlet region
through the morning. A very nice upper level low is developing off
the eastern coast of the Kamchatka peninsula and is in the
process of pushing a gale force front across the western Bering
Sea. The front will quickly stall out over the western Bering Sea
today as downstream ridging from the aforementioned low amplifies
in response to the deepening low.
Models continue to remain in good agreement with the evolution of
the synoptic pattern. The biggest question, however, still remains
as cold air has been slow to filter into southcentral Alaska.
Depending on the model of choice, precipitation tapers off at
different times depending on how quickly the upper level trough
pushes through. As of now, the Gem is verifying the best based on
convective precipitation observed prior to midnight over the Kenai
Peninsula. The Gem solution would favor a longer lasting precip
event which would be helpful to squeezing out a little extra snow
given how slow the cold air has been displacing the warmer air.
Ceilings are expected to remain below drop below 1000 ft through
the early evening morning hours as snow continues to fall over the
area. IFR conditions will then rise back to VFR conditions by late
morning as the weather system pushes to the east. Gusty northerly
winds will develop by this afternoon.
Short term forecast southcentral Alaska (days 1 and 2)...
snow will continue early this morning, followed by a colder air
mass and windy conditions in most areas the next couple days. Snow
will taper off in most areas by mid-morning today, leaving a total
storm accumulation of 1-3 inches around the area. Snow will linger
along the east side of Anchorage and the hillside the longest,
possibly through mid-day.
A disturbance getting caught up in the incoming trough will
spread moisture from Cordova this morning through Valdez and into
the Copper River basin bringing rain early to the coast, mixing
with snow toward Valdez, and all snow inland. Snow will linger in
the Copper River basin and Prince William Sound into this evening
before tapering off.
Behind this system, cold air filters in from the Alaska Range, and
will drive gusty winds across the southcentral, even more so
through favored gaps of the chugach into Prince William Sound,
which will linger through Sunday afternoon.
Short term forecast southwest Alaska (days 1 and 2)...
an upper level disturbance diving across the southwest will bring
snow showers through the afternoon, then cooler and drier
conditions ensue through Saturday night. Northerly flow is
expected to be gusty through Saturday afternoon and then taper off
in the evening as ridges tracks from the eastern Bering. Patchy
fog during the night time hours may develop as temperature
inversions steepen overnight across the Kuskokwim Delta.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a strong storm system along the western Aleutians and Bering will
bring rain and gusty winds further north and east by this
afternoon. The eastern Bering and Alaska Peninsula remains under
the influence of dry northerly flow until the front pushes through
on Saturday. The low center pushing the front moves quickly north
along the western Bering on Saturday. This system re-enforces
strong southerly gusts into Sunday morning.
Long term forecast (days 3 through 7)...
the mid term forecast for much of the southern Mainland is
characterized by cold air advection and a ridge building in from
the west that will bring outflow winds through the beginning of
next week. As the ridge moves off to the east, winds will die
down, but it is still unclear as to how long the fair conditions
will remain over the area. This is due to continued uncertainty
between the models as the system moves from the North Pacific into
the Bering by Tuesday. Once the ridge gives way to the Bering Sea
system, the overall pattern becomes more progressive in increasing
zonal flow with multiple systems tracking into the Bering and Gulf
through next weekend.
The ec remains the most bullish and fastest solution bringing a
much stronger system into the western and central Aleutians Monday
night, and quickly tracking it to the northern Bering Tuesday
morning. Where on the other hand, the GFS and Canadian are both
carrying a much weaker and slightly slower solution, but have
trended slightly stronger with the most recent model runs. Overall
there is slightly better agreement than yesterday, but still
enough uncertainty to keep confidence low for specific impacts.
Marine...gales 121 126 127 128 130 131 132 138 150 173 177 178 185.
Synopsis and model discussion...mc
southwest Alaska/Bering Sea/Aleutians...kh