Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
528 am akdt Wednesday Jun 3 2015
Analysis and upper levels...
The blocking high over the Chukchi Sea continues to shift
westward this morning into Siberia. This is allowing for an
undercutting upper low to dig into southwest Alaska this morning.
Water vapor satellite imagery is depicting the leading edge of the
trough making southward progress into the y-k Delta. Over the
northcentral Pacific Ocean...a broad trough and associated jet maximum
is advecting a warm and moist subtropical air toward the southern
Gulf of Alaska. Over the northern Gulf...an upper level trough
axis is slowly moving toward the Gulf Coast...with widespread rain
moving northward into coastal locations. Finally...a closed upper
level low currently southwest of Kodiak will merge with the
aforementioned area of warm subtropical air as it moves into the
Gulf of Alaska...with a rapidly strengthening low tracking toward
the Gulf Coast this evening and tonight. This low will increase to
maximum gale force to low end storm force as the associated barrier
jet pushes into Prince William Sound.
The wetter and cooler pattern will persist over most of southern
Alaska (with the exception of extreme southwest alaska) into the
weekend as a broad trough persists over the state.
The models are in excellent agreement with the general large scale
pattern heading into the weekend. They are in much better
agreement with a developing low moving toward the northern Gulf
Coast tonight...and remain in good agreement with the general
finer scale details through the short term forecast. There is less
confidence with respect to precipitation across interior portions
of southern Alaska Thursday and Friday as numerous waves of
moisture move across the region. Overall confidence in the
forecast is moderately high.
Short term forecast southcentral Alaska (days 1 and 2)...
A frontal band currently lifting into the northern Gulf will
spread precipitation across coastal portions of southcentral
Alaska this morning while scattered to numerous showers and a few
isolated thunderstorms to the north continue in the cool unstable
air mass inland. A combination of onshore flow increasing behind
the upper level trough rotating onshore today and surface ridging
along the northern Gulf Coast being amplified by the approach of
the next rather strong frontal system lifting north...southeasterly
gap winds will increase substantially this afternoon and evening.
The typically favored locations such as Turnagain Arm...the Knik
river and Copper River channeled areas will see the strongest
winds. However with the support of the front and upper level
trough lifting through as well as long daylight hours think there
is good potential for additional winds to be mixed down and
support breezy conditions over a broader area as well...particularly
on the downslope sides of mountain ranges such as in the Copper
Southeast to easterly winds over the marine waters will increase
rather quickly to gales late this afternoon and evening as the
next frontal system lifts through the Gulf. Conditions look
favorable for barrier jet formation along the northern Gulf Coast
late tonight through early Thursday morning supporting sustained
east winds in the 40 to 50 knots range just offshore of the northern
coast and along the entrance to Prince William Sound. This frontal
system and the associated low will lift inland during the day
Thursday bringing another strong surge of onshore flow Thursday
afternoon and evening.
Short term forecast southwest Alaska (days 1 and 2)...
The wet pattern continues over southwest Alaska as the low complex
approaching Kodiak and the upper level trough over the interior
will keep the atmosphere unstable through Thursday night. Isolated
thunderstorms should develop again this afternoon and this evening
mainly in the interior sections of the lower Kuskokwim valley and
Bristol Bay. The atmosphere remains unstable enough over the
Kuskokwim Delta today to allow for isolated thunderstorms to move
into the Bethel area this afternoon. Tomorrow the best support
for isolated thunderstorms shifts to mainly the lower Kuskokwim
valley as the upper trough merges with the Kodiak low as it moves
Winds will increase along the Kuskokwim coast this afternoon as
high pressure begins to dominate the Bering Sea with strong
northerly flow. As the low approaches Kodiak this afternoon winds
will also increase through the kamishak gap but should decrease
this evening as the winds shift as the front moves into south
Areas of fog are also expected to continue along mainly the
coastal areas although some areas north and east of Bethel will
see fog this morning as clear skies on the northern side of the
trough are allowing radiational cooling.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
Isolated to scattered rain showers will continue mainly from Dutch
Harbor eastward through tomorrow night as the low moving into
southwest Alaska near Kodiak Island will keep the eastern
Aleutians and the Alaska Peninsula unstable. As high pressure
build over the western Bering Sea today...winds will begin to
increase over the northern Bering and increase to small craft
levels over the eastern third of the forecast area. Marine fog and
stratus will remain over the eastern Bering and the Pribilof
Islands through the period.
Long term forecast (days 3 through 7)...
The forecast beginning Thursday afternoon looks much the same on
the large scale. A longwave trough consisting of three merging
systems continues to take shape over the southwest Mainland. The
main difference in guidance from yesterday is the emergence of a
stronger upper level low dropping down from the pole over
northwest Alaska...however a weaker trough over the southwest/Alaska
Peninsula remains. Over the weekend into next week the jet stream
remains south of the Aleutian chain that will send periodic
disturbances into the Gulf.
The forecast and sensible weather impact remain much the same
despite changes in guidance. A front and upper level wave will
push from the Gulf into the interior on Thursday/Friday keeping
the cloudy and showery pattern in place. Behind this
system...lingering cold air aloft on Friday will keep showers along
most higher terrain across the southcentral and southwest. Into
Saturday...the active pattern continues as a weak disturbance/front
moves inland over the Gulf with the coastal areas taking most of
the moisture. Into early next week cold air aloft remains but the
flow slackens a bit which should keep clouds/moisture over the
terrain and possibly some sneaky nice days before a retrograding
polar system reloads the pattern back into an active/progressive
Marine...Storm Warning 119.
Gales 120 125 130 139 351 352.
Synopsis and model discussion...ja
southwest Alaska/Bering Sea/Aleutians...rf