Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
410 PM akdt Tuesday Aug 30 2016
Analysis and upper levels...
An Omega blocking pattern encompasses all of Alaska, with ridging
over the Mainland sandwiched between a low spinning over the
Aleutians and another low over the southeast Panhandle. This is
allowing for persistence of the sunshine and warm temperatures
over the southern Mainland amid offshore northerly flow. The
orientation of the pressure gradient continues to support gusty
winds in favored channeled terrain including the Matanuska and
Susitna valleys and along the northern Gulf Coast. The broad low
pressure system to the west is strengthening as it drifts
northward in the far northern Pacific and approaches Adak. This
system is sending a frontal boundary northward through the
Aleutians and southern Bering. Ahead of the front, weak flow under
a col has allowed for fog formation in the early morning hours
over Bristol Bay and the Alaska Peninsula. This fog retreated
throughout the morning and dissipated by mid afternoon.
Models are in good agreement synoptically in the short term. The
consensus is that the blocking pattern will start to break down
towards the end of the week, with a steady moderation of
temperatures over the Mainland through Friday. There are some
minor model discrepancies in the development of a weak triple
point low south of the Alaska Peninsula Wednesday night through
Thursday. However, impacts to the forecast of these minor
differences are small as the low dissipates before reaching the
southwest Gulf of Alaska Friday morning. Forecast confidence
remains above average through the end of the week.
short term forecast southcentral Alaska (days 1 and 2)...
Windy conditions out of the Matanuska Valley will gradually
subside through tomorrow morning across southcentral Alaska,
though those winds will likely remain through at least Thursday
toward the northeast Prince William Sound region as a surface low
reforms along the northern Gulf Coast and an upper low moves south
across the area. Otherwise sunny, warm, and dry weather will
continue. In wind-sheltered areas away from large bodies of
waters, radiational cooling will allow for temperatures to
possibly drop into the 30s during the early morning hours, with
even some 20s for high-elevation valleys and basins well inland.
Temperatures will continue a very gradual cooling trend as high
pressure relinquishes its hold to our northwest.
Short term forecast southwest Alaska (days 1 and 2)...
A building and rather anomalously strong upper level ridge will
continue to build over southwest Alaska through Thursday, keeping
warm and dry weather over the region. Weak inland winds will
allow for the development of more valley and lake fog, especially
across interior portions of the lower Kuskokwim valley and Bristol
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
A North Pacific low and and associated warm front are now in the
process of lifting into the western Bering Sea and across the
western and central Aleutians. This low will spread rain and
southeast winds into the 20 to 30 knot range across the entire
Aleutian chain/Pribilof Islands by Wednesday afternoon as the low
reaches maximum intensity. The low will then weaken and occlude
Thursday, with periodic rain lingering over most of the Bering
Long term forecast (days 3 through 7)...
A little bit has changed in the extended forecast over the past
couple of models runs. The area of low pressure south of the
central Aleutians will push a front through the southern Bering
Sea and northern Pacific on Friday. However, it's looking more
probable that a triple point low develops south of the Alaska
Peninsula Friday afternoon and should slow the momentum of the
front as it approaches the Alaska coastline. This has led to a 12
to 18 hour delay for when the frontal boundary was supposed to
begin moving inland. This will bring the first chances of rainfall
to southwest Alaska Friday afternoon/evening instead of late
Thursday. This front, as expected before, will continue to push
eastward toward southcentral Alaska and increase rain chances by
Sunday. The front will fall apart quickly once inland so it is
quite difficult to judge the likelihood of rainfall at this time.
It should be noted that as the upper level ridge continues to
erode that a return of some southwesterly flow aloft is likely.
However, it will not be nearly as steep as before therefor no
significant moisture plume is expected to advect northward toward
Models are still in overall very good agreement through early
next week with the main synoptic features but do struggle with a
few shortwaves in the Arctic. Low pressure looks to encompass
much of the state by midweek next week with a showery regime
returning to the state. However, the extent and coverage of these
showers will depend greatly on an Arctic trough that enters the
state on Tuesday.
Synopsis and model discussion...Rd
southwest Alaska/Bering Sea/Aleutians...ja