Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
453 PM akdt Thursday may 28 2015
Analysis and upper levels...
satellite imagery shows a strong vertically stacked low crossing
the western Aleutians with a frontal system tracking north and
east across the Bering Sea. Central pressure bottomed out this
morning somewhere in the lower to middle 970s supported by a 150 knots
upper level jet...quite impressive for this time of year.
Observations across the region along with satellite scatterometer
wind data indicates widespread gale force winds with more
localized storm force gusts with this system.
Meanwhile...a ridge of high pressure is building out ahead of this
storm over the eastern Bering and into southwest Alaska. Stable
conditions under the ridge have led to areas of low clouds and
fog..primarily along the Kuskokwim Delta. This ridge will be the
big story over the next few days as it strengthens over Mainland
Alaska bringing hot and dry conditions to portions of southcentral
and and interior southwest Alaska.
Lastly...a short-wave trough is exiting to interior Alaska. The
tail end of this trough may just clip the northeast Copper River
basin with a few showers or thunderstorms this afternoon into
evening. Otherwise...with the ridge building in sunny skies will
dominate most of southcentral Alaska today.
although there are few observations out in the western Aleutians...
based on the surface pressure at Shemya it appears all the models
are a bit too weak with the central pressure. Thus...numerical
guidance is a little on the low end for winds early on. Boundary
layer winds are much more representative than surface winds.
Otherwise...models have a good handle on this system and the
larger scale synoptic pattern across the Alaska region over the
next few days.
There is a notable change in the position of the upper ridge over
Mainland Alaska. Model runs from Wednesday morning indicated the
main center would be over the Arctic. All solutions now indicate
the center over southcentral or the Gulf of Alaska with weaker
ridging over northern Alaska. Equally important to the temperature
and relative humidity forecasts is the position of the ridge
axis. The latest guidance indicates this axis will be centered in
the Cook Inlet region...which means northerly offshore flow will
prevail across southcentral. This will limit sea breezes and
result in very warm temperature both inland and along the coast. A
small shift in the ridge axis to the east would alter the
forecast. Even so...forecast confidence is high for above average
temperatures with potential for record or near record highs in
select locations of southcentral.
Short term forecast southcentral Alaska (days 1 and 2)...
a strong area of high pressure will settle over southcentral
Alaska Friday through Sunday bringing hot and dry weather to
most of the region. Offshore flow from the ridge axis over Cook
Inlet will dominate through much of the period helping to keep sea
breezes at Bay along the north Gulf Coast and Prince William
Sound. This will result in periods of gusty winds in
Seward...Whittier...and Valdez which will aid in warming
temperatures even further. The warmest day will be Saturday where
several locations will approach record high temperatures.
with the high temperatures and offshore flow humidities will be low with
many dropping down to around 30 percent in the southcentral. The
lowest values will be in the Copper River basin and Susitna Valley
on Saturday where relative humidity values will drop into the upper teens.
However...they should not reach critical levels unless winds
increase to around 20 miles per hour. At this time there is high confidence
that winds will not increase to that level.
Short term forecast southwest Alaska (days 1 and 2)...
a fairly quiet weather pattern is expected through Saturday over
most of the southwest Mainland as the high over the Gulf of Alaska
builds over Mainland Alaska. The front associated with the strong
Bering Sea low will begin to impact the Kuskokwim coast and
portions of the Alaska Peninsula tonight and will stall over the
coastline tomorrow. This will bring areas of rain and gusty winds
overnight tonight before decreasing tomorrow. Rainfall amounts are
expected to be fairly light as this system does not have as
strong of a moisture tap as recent systems that have impacted the
Temperatures inland will warm and dry out as the ridge builds into
the area with warm southerly flow moving into the region.
High temperatures into the low 70s are expected in the lower
Kuskokwim valley and interior Bristol Bay region. Temperatures
will be more moderate along the coast where cloud cover and
precipitation from the stalled front will keep temperatures to
a more seasonal range.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
active weather will continue over the Bering Sea as the strong low
and its associated front over the southwestern Bering Sea will
push northward through tomorrow. Widespread middle to upper gale
force winds will are expected tonight over much of the Bering Sea
and the Aleutian/Pribilof Islands before weakening tomorrow...limiting
gales to the northern Bering Sea tomorrow night. Seas will be the
highest tonight in the central Bering Sea as the low moves
northward. Winds and seas will diminish through the day tomorrow
as the low moves back towards Russia.
Widespread rainfall is expected along the front overnight before
the front stalls over the southwestern Alaska coast tomorrow. The
heaviest rainfall amounts are expected over the western Alaska
Peninsula as the front stalls...but as this system does not have
the deep moisture tap that other recent systems have
had...rainfall amounts will be more moderate.
Long term forecast (days 3 through 7)...
hot and dry conditions will persist through the weekend across
much of the southern Mainland beneath a high amplitude ridge
extending from the Chukchi Sea over Mainland Alaska and into the
northeast Pacific. The pattern will then begin to change by early
next week as the ridge is bisected by a potent shortwave trough
dropping down its eastern periphery along with a broad upper low
moving eastward across the North Pacific. This will usher in a
moderating trend as temperatures return to near or slightly above
normal values by the middle of next week...with chances for
diurnally driven convection beginning to increase as colder air
moves in aloft. Meanwhile...another low moving off of the
Kamchatka peninsula this weekend looks to stay south of the
Aleutian chain as it pushes eastward across the North Pacific next
week. This will result in benign conditions over much of the
Bering beneath weak upper level flow.
Marine...gale 170 171 172 175 176 177 178 179 181 185 .
Synopsis and model discussion...seb
southwest Alaska/Bering Sea/Aleutians...rf