Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
413 PM akdt Friday Jul 3 2015
a fairly compact upper level low centered near Norton Sound is
keeping all areas from Bethel north socked in clouds and limiting
instability. South and east of there...ridging is keeping skies
mainly clear...allowing instability to increase as indicated by
the developing cumulus field from north of Dillingham up into the
southern Kuskokwim valley. The low digging southward may provide
just enough forcing to produce a few afternoon thunderstorms in
this region. A much larger upper level low is centered over the
central Aleutians. A digging trough on the southeast side of this
low is tapping tropical moisture....which is streaming northward
toward southern Alaska. This amplification of the upper level
pattern will lead to a cooler and wetter pattern for much of the
eastern Bering and southern Mainland over the coming days.
the most notable feature at the surface is a 998 mb low south of
Dutch Harbor that is feeding off the influx of tropical moisture
bringing very wet weather to the eastern Aleutians and the
southern Alaska Peninsula. An associated warm front marking the
leading edge of the tropical moisture is moving northward toward
Mainland Alaska. Over southcentral...the air mass is moist and
only slightly unstable (as indicated by the 12z Anchorage sounding).
Thus would expect any showers which develop this afternoon and
evening to be confined primarily to the higher terrain.
models remain in generally good agreement with large scale features
through the weekend. All solutions have trended faster and a bit
stronger with the warm front over the northern Pacific as it tracks
northward to the southern Mainland tonight through Saturday night.
Generally prefer the NAM solution for good handling of the details.
It is the fastest with arrival of rain in the Kuskokwim Delta
Saturday...so will go with slightly slower timing than the NAM.
southwest Alaska will be on the receiving end of much of tropical
moisture streaming northward across the Pacific as the long-wave
trough lifts northward over the next few days. Expect fairly
widespread wetting rains as upper level waves and surface fronts
move through. The surface fronts will be fairly strong for this
time of year...so although it will generally be wet there will be
periods of strong gusty winds which could affect behavior of
existing fires. The prevailing flow with each of these will be
east to southeast.
The front moving up from the Pacific will rapidly weaken as it
heads inland across southcentral Alaska Saturday afternoon and
evening so expect wetting rains to be confined to the coastal
zone. This front will push the thermal trough all the way north
of the Alaska Range ushering in much more stable conditions for
the entire region. A ridge will build back in on Sunday leading
to a return to warmer and drier conditions before another front
approaches early next week. A surface high will persist over the
next few days from the eastern Gulf to the north Gulf Coast...
undergoing fluctuations in strength as the various fronts move
through. This ridge will be at its strongest as the surface fronts
near...which will be Saturday and then again on Monday. Expect
strong gusty gap winds during the afternoon and evening hours
through the usual places (turnagain arm/Knik River Valley/copper
Short term forecast southcentral Alaska (days 1 and 2)...
Daytime heating has been sufficient to trigger isolated showers
over most of southcentral this evening...with isolated
thunderstorms possible over the Alaska Range and Talkeetna
Mountains. On Saturday the approaching front will spread rain over
coastal areas...persisting into Sunday...while only a few showers
will slip across the Kenai and Chugach Mountains. Clouds will be
slow to spread over the Copper River basin and Susitna Valley...so
daytime heating will provide the mechanism to trigger a few
afternoon and evening showers in those areas this weekend. As the
front moves inland...gusty channeled flow will increase to strong
wind criteria through Turnagain Arm on Saturday...with slightly
weaker gusts through the Copper River and out of Knik Arm. These
winds will diminish significantly on Sunday.
Short term forecast southwest Alaska (days 1 and 2)...
a nice evening is in store for much of the Mainland as the area
lies between a system to the north and an approaching front from
the south. The only chance for precipitation this evening is
northern areas of the Kuskokwim Delta and valley where isolated
showers and thunderstorms remain possible during the evening.
Tonight a front will begin to impact the Alaska
Peninsula...spreading rain over much of the Mainland on Saturday.
Some gusty easterly winds will accompany the front tomorrow
morning with the strongest being through the gaps of the Alaska
Peninsula/Aleutian Range. A short break happens on late
Saturday/early Sunday before another front approaches from the
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the current frontal boundary draped from west to east will
progress northward tonight stalling over the Pribilof Islands. Yet
another disturbance and front will move south of the eastern
Aleutians on Sunday continuing the wet and active pattern. Expect
areas of dense fog in the northeastern Bering Sea with visibility
reduced to one mile or less.
Long term forecast (days 3 through 7)...
A large vertically stacked low tracking across the Aleutians and
into the Bering late Sunday will remain the dominant feature in
that area through much of next week. The upper level ridge
extending across south central Alaska will continue to sharpen
through Sunday night then shift gradually east Monday before
breaking down Monday night and Tuesday. Southeasterly gap winds
could get fairly strong Monday afternoon and evening as the
strengthening of the surface ridging and onshore flow from the
northern Gulf coincides with diurnal heating inland and a shifting
of the thermal trough further north in interior Alaska. The larger
channeled areas such as Turnagain Arm...the Knik River Valley and
Copper River valley will likely see the strongest winds however
breezy conditions are also possible in many of the smaller
southeast to northwest running valleys as well.
Initially the high amplitude upper level ridge will keep the
incoming Bering frontal systems confined to southwest Alaska.
However...as the ridge shifts east and then flattens...precipitation
and cooler weather will be able to spread further east into south
central Alaska Monday night and Tuesday. Additional shortwave
troughs rotating around the large low over the Bering will
continue to push enough moisture across southwest and south
central Alaska to bring chances of rain to much of the area
through the end of the week.
Marine...Gale Warning 120 130 131 138.
Synopsis and model discussion...jw
southwest Alaska/Bering Sea/Aleutians...ml