Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
434 PM akdt Thursday Mar 26 2015
Analysis and upper levels...
A look at the morning weather map continues to show a broad
longwave trough across the North Pacific...with approximate trough
axis through the eastern Aleutians. The storm that had moved up
toward Kodiak Island yesterday has now weakened...and sensible
weather across the region is now dictated primarily by the
numerous embedded shortwave features moving around a mean low
position south of the Alaska Peninsula. Cross-barrier flow across
the Kenai and Chugach Mountains continues to keep the Cook Inlet
population centers almost entirely dry. Meanwhile...between one-
half and one inch of rain has fallen across the eastern Kenai
Peninsula...with Portage area reporting closer to 2 inches in the
last 24 hours. An area of pressure rises (isallobaric maximum)
behind yesterdays storm is moving northwestward toward lower Cook
Inlet. This feature is beginning to weaken the down-inlet pressure
gradient and will eventually reverse this gradient resulting in
the development of southerly winds around Cook Inlet by late this
West of the Alaska Range...weakening cross-barrier flow is
allowing some of the embedded shortwaves to have a little more
Success bringing precipitation all the way to the ground. Portions of
Bristol Bay as well as the Delta received a brief period of
rain/snow earlier from one of these shortwaves. This
showery/intermittent type of precipitation pattern is continuing
this afternoon and should persist overnight.
Out west...high pressure remains in charge across the western
Bering/Aleutians. A stationary and weakening frontal boundary over
the central/eastern Aleutians continues to bring steady rain west
of Cold Bay and east of Atka...including Dutch Harbor...however
early afternoon satellite imagery is beginning to show warming
cloud tops as this feature begins to diminish.
The models are in good synoptic agreement...but beyond a day or
two really start to struggle with the details of the pattern. This
is not a huge surprise as the models perform much better when
there are coherent and strong synoptic features...as opposed to
weak embedded waves moving in from the data-sparse ocean regions.
In the first few days of the forecast...the largest concern is
where and how strong a surface low moving up into the Gulf on
Saturday morning will be. As is often the case..the European model (ecmwf) has been
the most consistent...and is pretty close to the position of the
Gem-regional this morning. The afternoon forecast will reflect a
blend of these two. The NAM and especially the GFS have been quite
inconsistent...jumping hundreds of miles from run-to-run as the
models struggle to resolve where the upper waves will be let alone
when and where the attendant surface lows will develop. The main
impact of the low track will be the strength of a barrier jet
along the eastern Kenai Peninsula extending down into Shelikof
Strait...and at this time will continue with a forecast of gale-
force strength based on the European model (ecmwf) low position.
Short term forecast southcentral Alaska (days 1 and 2)...
A weather front will move inland tonight and dissipate. It will be
accompanied by short bursts of wind during the evening...and winds
will then diminish overnight. After frontal passage moist flow
from the south will keep showers in the region through Friday...with
rain mixing with snow at higher elevations and north of the
Chugach Mountains tonight and Friday night. Up to an inch of
accumulation is expected through Thompson Pass tonight and Friday
night..and Turnagain Pass tonight.
The next North Pacific front will cross the Gulf Friday night into
Sat and bring rain to the coast with some snow at higher
elevations...and another round of wind through channeled terrain.
Interior areas will be sufficiently downsloped to prevent
Short term forecast southwest Alaska (days 1 and 2...Friday and sat)...a
complex and broad area of low pressure centers will continue to
spin through the region for the next couple of days. Each wave
will bring a brief shot of precipitation to the area with some
gustier winds...but none of them is expected to be too strong.
Most of the precipitation should stay all rain for Bristol Bay
until you get up into the higher elevations of the Kilbuck
Mountains. Temperatures will remain cold enough to keep things all
snow in the Kuskokwim Delta and lower kusko valley. The gustiest
winds will be around 40 miles per hour confined primarily to the channeled
higher terrain of Bristol Bay and the Delta coast.
Short term forecast Bering Sea/Aleutians (days 1 and 2...Friday and
sat)...the upper level trough will remain anchored over the
eastern Aleutians and into the Alaska Peninsula through Friday. With
low pressure nearly directly overhead...these areas should
experience relatively light winds but plenty of on and off rain
showers. On the western flanks of the low...cold air continues to
be pulled down through the Bering. This is making for a tighter
gradient and some gustier northernly winds from Atka to Dutch
Harbor and to the pribilofs. Any blowing snow concerns for the
pribs should be limited as they will remain only on the fringe of
the heavier quantitative precipitation forecast. High pressure keeps things quite quiet over the
western Bering on Friday before a developing area of low pressure
slides off the Kamchatka peninsula on Sat. This low will move to
the east through the weekend and should help to bump the upper
level trough east of this area.
Long term forecast (sunday through thursday)...the next gale
force North Pacific low will be located on the east side of Kodiak
Island by Sunday. The associated weather front will extend along
the eastern Kenai Peninsula eastward through Cape Suckling area.
This will bring widespread rain along the coastal areas. The matsu
valley and the Anchorage bowl will downslope out resulting in
drier conditions through the weekend. The low pressure system
will slowly move to the northern Gulf by Monday and become
stationary before dissipating by late Tuesday. This system will
continue to bring warm temperatures and precipitation all in the
form of rain along the coastal region except mixing with snow at
times at higher elevations in the inland areas. The surface flow
will remain southerly through Thursday with continued warm
temperatures to the southcentral region.
The Bering Sea and the Aleutians will remain in persistent
northerly flow through the weekend. The North Pacific low will
remain near the Kodiak Island area before moving into the northern
Gulf by Monday. This will continue to bring warmer temperatures
and rain along the eastern aleutains...the Bristol Bay area
through the Monday. The western Aleutians will have a weak wave
move into the area by Monday and move along the chain to the
eastern Aleutians by Wednesday. This will bring warmer
temperatures with rain...which will spill over into the lower
Bering Sea as the low tracks eastward through the middle of next
week. On the backside of the low pressure as it moves eastward the
western Aleutians will see a mixture of rain/snow with the
slightly cooler air behind the system. Farther north in the Bering
they will see a wintry mix due to the cooler temperatures. By
Thursday the low over the eastern Aleutians will have moved near
Kodiak Island. The western Aleutians/Bering Sea will be in a
southerly flow with warmer temperatures with the next North
Pacific low approaching the western Aleutians.
Marine...gales...130 131 132 150 179.
Heavy freezing spray 179.
Synopsis and model discussion...ad
southwest Alaska/Bering Sea/Aleutians...MO