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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
500 am akdt Thursday Jul 28 2016

Analysis and upper levels...

The low over the northwestern Gulf is dominating the local weather
pattern over southern Mainland Alaska this morning. While nearly
vertically stacked, the low still has some upper level support
from the left exit region of a modest 75 kt jet. This jet is
pinwheeling shortwaves of upper level energy around the low as
they move from east to west across southcentral. They have been
producing moderate amounts of rain near the coast with lesser
amounts further inland.

East to southeast surface winds are becoming increasingly confined
to the shrinking area between the center of the low and the
coastal chugach. Since squeezing a moving fluid into a smaller
space results in a velocity increase to conserve momentum, the
atmosphere's response to this is the development of a barrier jet
along the northern Gulf Coast in meteorological situations such as
this. Buoy 46061 between Montague and hinchinbrook islands has
been reporting sustained winds of 20-25 mph with higher gusts. It
is very likely that the strongest winds are further offshore of
the coast at this time in an area where there are no observations
to confirm the barrier jet's existence and strength.

Southwest Alaska is in the relatively high pressure area between
an approaching front diving southward across the Bering Strait and
the Gulf low, so much of that region is seeing quiet weather. High
pressure over the Bering is shifting the semi-permanent Bering Sea
stratus and fog around where the winds are strong enough to advect
them. Strong upper level ridging remains in place over the western


Model discussion...

Models are in good agreement through tonight with regards to the
surface low in the Gulf of Alaska as well as the larger scale
synoptic features. Some timing and track differences become
apparent with the next small surface low entering the far
southwestern Gulf on Friday or Friday night. Given the general
weakness of this system however and the fact that it only brushes
the southern Gulf these differences should have minimal impact on
the forecast.

Model agreement has improved significantly with the initial
timing and strength of the trough dropping out of the northwest on
Saturday. Confidence in the feature however decreases rapidly
through the weekend as models morph the trough into a cutoff low
with little agreement on its placement and track. These model
discrepancies magnify through the first half of next week where
the position and progression (or lack there of) of the low to the
east will have significant impacts on the resulting weather for
southern Alaska


Short term forecast southcentral Alaska (days 1 and 2)...

A frontal boundary will continue to push north and west of a
surface low sitting over the northwest Gulf. This will produce
a wet start to the morning from the Kenai Peninsula southward to
Kodiak Island. Meanwhile, the upper level low will lift northward
across the northern Gulf with short-waves tracking inland to the
Copper River basin. The colder air aloft will bring more unstable
conditions and fairly widespread showers from Prince William Sound
to the Copper Basin. One last feature of note is a weak upper
trough which will drop through southwest Alaska today reaching
the western Alaska Range by late this afternoon. Ridging will
set up between this trough and the Gulf low leading to a low
level thermal trough along the Alaska Range, Susitna Valley and
Talkeetna Mountains. Increasing instability combined with some
vertical motion coming from both the east and the west will lead
to widespread showers, especially over the northern Susitna
Valley, Talkeetna Mountains, and northwest Copper Basin. There
will not be much sunshine today, but still expect enough
instability to produce isolated thunderstorms across this area.

The trough approaching from the west will help kick the Gulf low
eastward tonight. This will bring drying conditions to the Gulf
and much of southcentral. As the weak trough moves through there
may be some very light rain on the western sides of mountains in
upslope flow. The best lift will stay up along the Alaska Range,
so expect showers and steady rain at times over the northern tier
of southcentral. A fairly Flat Ridge will build in Friday and
Friday night. This will be enough to push the area of rain/showers
further north to right along the Alaska Range. Most of southcentral
looks dry with some sunshine, though some moisture will likely get
trapped beneath a stabilizing atmosphere aloft. With persistent
south to southwest flow this will likely keep a layer of clouds
banked up against the western slopes of mountain ranges.


Short term forecast southwest Alaska (days 1 and 2)...

Several upper level disturbances will move from the Bering to the
Alaska and Aleutian ranges through Fri night. The first will induce
rain and showers across the region today. There will also be a
slight chance of afternoon and evening thunderstorms, mainly near
the mountain ranges as the impulse interacts with the thermal
trough. The second system will send a front to the western Alaska coast
by Fri night. This will bring another round of rain and showers
that become more widespread by Fri night.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...

Steady northwest flow aloft will keep plenty of fog in the area,
with some sprinkles possible over the central Aleutians through
tonight. A front moving across the northern Bering tonight
through Fri night will spread more light precipitation across the
area, with only a brief period of reduced fog coverage over the
western Aleutians. Wind will slowly increase across the northern
Bering with this system.


Long term forecast (days 3 through 7)...

A zonal pattern starts off the mid term with predominantly
westerly flow and showery precipitation. Moving into Saturday a
trough digs down from the north pushing through southwest Alaska
through Sunday. This trough will bring more rain to southwest
and southcentral Alaska. However, there are some questions on how
much rain for southcentral. Guidance is trending to push more of
the energy south helping to form a closed low near the Alaska
Peninsula or Kodiak Island. Previous solutions kept the trough
more intact with the bulk of the energy staying to the north.
What this means is that rain looks to be widespread across
southcentral Alaska on Sunday. But the position of the low is
still in question and this could lead to varying times and amounts
of rain. Then, moving into next week, high pressure will build in
along the West Coast and help clear out the rain bringing back
fair conditions.


Afc watches/warnings/advisories...
Fire weather...none.



Synopsis and model discussion...jw/jer
southcentral Alaska...seb
southwest Alaska/Bering Sea/Aleutians...ds

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