Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
437 am akst Monday Nov 24 2014
Analysis and upper levels...
a 997 mb low currently 110 miles southwest of Sitkinak Island is
slowly moving in the general direction of the Gulf of
Alaska...weakening rapidly as strong high pressure ridging
continues to move rapidly north and east toward the Mainland from
the central Bering Sea. This ridging is getting amplified in part
due to a seasonably strong and quasi-stationary low pressure
system sitting approximately halfway between Attu and the
Kamchatka peninsula of Russia. Numerous vigorous disturbances are
rotating around the broad cyclonic circulation with a very active
upper level jet stream.
Model discussion ...
models are handling the large-scale synoptic environment well in
developing significant upper level ridging over the Mainland of
Alaska Tuesday through Thursday. Most of the significant
discrepancies in the next 2 to 3 days lies with the active jet
streak pattern that will continue to evolve to the south of the
broad low over the far western Bering as it continues to sit and
spin through the early to middle-week period. Models depict a pair of
lows moving north and east across the Aleutians and into the
Bering Sea Tuesday into Thursday before merging into the parent
low center. The European model was preferred slightly over the GFS
and Canadian models for this forecast package.
Short term forecast southcentral Alaska (days 1 through 2)...
the stagnant pattern of the past few days will begin to change
today as amplifying upstream flow over the north central Pacific
and Bering Sea helps kick the low along the Alaska Peninsula
eastward across the Gulf. As a result the mean flow in the low
to middle levels over southcentral Alaska will gradually veer from
south-southeast to southwest over the next 12 hours. This change
in flow will start to break up the persistent shower activity in
the Prince William Sound region.
Meanwhile the development of upslope flow on the west side of the
chugach and Talkeetna Mountains combined with weak vertical motion
from upper level disturbances embedded in the flow will deepen
moisture from Anchorage up to the Matanuska Valley and eastern
Susitna Valley...places which have remained quite dry under the
current flow. Aside from an increase in clouds in these
locations...this will set the stage for a possible light snow
event Tuesday/Tuesday night as an upper trough moves through. The
trough will be weakening due to increased anticyclonic flow with
a building upstream ridge. There is a model consensus that the
best vertical motion will be over the northern tier of the Susitna
Valley. Since numerical guidance has just recently locked in to
this feature and it is on the weak side will generally stick with
chance probability of precipitation for now.
A strong ridge will build in from the west Tuesday night bringing
ideal conditions for radiational cooling...so have trended
downward with night-time temperatures. Stable conditions and
light winds at the surface also favors formation of fog in Cook
Inlet. This always makes for challenging sky and temperature
forecasts. Farther east weak cold advection in the wake of the
upper trough combined with tightening pressure gradients as a
surface high builds over the interior will lead to weak outflow
winds (for this time of year) along the north Gulf Coast/Prince
Short term forecast southwest Alaska (tonight through Tuesday
a few snow showers will continue to develop across southwest
Alaska along the backside of low pressure system as it moves east.
Otherwise dry cool conditions persist through Tuesday night. A
warm front approaches the West Coast along the Kuskokwim Delta
Tuesday night bringing a chance for snow.
Short term forecast Bering Sea/Aleutians (tonight through Tuesday
a front has pushed into the western Aleutians bringing gusty gale
winds and rain. The frontal boundary pushes slowly into the
central Aleutians and northern Bering tonight. Cold air advection
behind the front will bring mixed showery precipitation back into
the western Aleutians tonight. Gusty winds will briefly diminish
behind the front as the parent low steers the next two low centers
toward the western and eastern Aleutians Tuesday night.
Long term forecast (days 3 through 7)...
Models are in good agreement that a high amplitude blocking upper
ridge will develop by Wednesday over Mainland Alaska and the Gulf...with a
cut-off low embedded in a trough over the Bering. The ridge will
remain in place into Friday when a major pattern change occurs. The
Bering cut-off low cuts through the ridge...transforming the high
amplitude pattern into one with shorter waves moving progressively
across the forecast area. As is typical with waves moving through
relatively zonal flow...timing is the main issue Sat through Monday.
There is fairly high confidence that southcentral will remain dry
beneath the ridge into the weekend...although rain and snow may
affect the coast by sun. The forecast for the Bering is of lower
confidence due to timing uncertainty. It utilizes a blend of the
GFS...ec..and wpc guidance...and has the surface reflection of the
upper Bering low slowly weakening as it moves through the Bering
Wednesday through Friday. This low will bring rain with gales and then
small craft winds to the Bering and Aleutians before it crosses
the eastern Aleutians and dissipates in the North Pacific on Sat.
A new low from Japan will approach the western Aleutians Sat and
sun and will bring more rain and wind to the chain for the
Marine...gales 173 174 175 176 177 178 185.
Synopsis and model discussion...cirrocumulus
southwest Alaska/Bering Sea/Aleutians...kh