Scientific Forecaster Discussion
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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
453 PM akst Mon Nov 11 2019
Analysis and upper levels...
a large occluded low is lifting northward into the Gulf of Alaska
today, with gale force winds out ahead of the occluded front.
Meanwhile, a strong ridge of high pressure sitting over southcentral
is providing dry conditions with a mix of sun and mostly high
clouds. The exception is the Copper River basin where low clouds
remain firmly entrenched. This ridge will block the Gulf low from
making much more northward progress. Thus, the bulk of rain today
is falling over Kodiak Island and the Gulf, with the northern
extent of the rain shield in the vicinity of the southern Kenai
Out west, a vertically stacked low northwest of the Pribilof
Islands is slowly weakening as it lifts northwestward across the
Bering Sea. A dissipating occluded front along the southwest coast
is producing a few rain showers.
The focus for the upcoming forecast is over the north central
Pacific where a surface low is undergoing rapid intensification.
This is the classic case of cyclogenesis at the left exit region
of a strong upper level jet, with an upstream short-wave trough
digging southward and accompanied by low level cold air advection.
This is all occurring hundreds of miles south of the eastern
Aleutians, so there are no noticeable impacts yet, but stronger
winds will spread northward from this system toward the Aleutians
and Alaska Peninsula. This storm system will also be a key player
in the weather over southcentral on Wednesday.
synoptic differences develop Wednesday among the numerical
guidance with a digging Arctic trough over western Alaska, trough
lifting northward from deep low spinning south of the Alaska
Peninsula, and an associated southerly jet streak between this
low and the downstream ridge. The differences are not large, but
they are important because they will affect the onset of
precipitation across southcentral Alaska, intensity of
precipitation, and precipitation type. With regard to the latter,
the biggest forecast challenge will be precipitation type on
Wednesday from the western Kenai to Anchorage and the Matanuska
Valley. Forecast soundings from all the models indicate
atmospheric conditions will be right on the edge between rain and
snow. If it falls as rain, then there is good potential for
freezing rain at the surface.
panc...VFR conditions and light winds will persist.
Short term forecast southcentral Alaska (days 1 and 2)...
The main focus for the short term forecast surrounds the next
storm moving up from the Gulf on Wednesday, bringing increased
precipitation chances through Thursday. At the onset of this
event...temperatures will be in the lower to mid 30s just inland
with much colder temperatures in the 20s further north. Therefore,
precipitation will be a mix of rain, freezing rain and snow.
Another challenge with this system is snow amounts across the
Copper River basin as this storm system brings a deep fetch of
southerly moisture Wednesday and Wednesday night. Thompson Pass,
is expected to remain below freezing for this event and will
possibly see blowing snow as winds remain gusty out of the
For the start of our forecast tonight, a front and associated low
over the Gulf weakens during this evening with rain diminishing
by midnight. The exception to this will be Kodiak Island and the
western Gulf waters as the the next front moves up from the North
Pacific. This front pushes across to the northern Gulf and
surrounding coasts mid morning on Wednesday. Upper level
shortwaves begin pulling the warmer maritime air mass inland by
Wednesday early afternoon. A much colder air mass holds at the
surface as troughing south of the Mainland supports offshore flow
with ridging over the eastern interior. The highest intensity of
for precipitation is expected Wednesday night and Thursday when
the strong upper level disturbance moves overhead.
Short term forecast southwest Alaska (days 1 through 3: tonight
The dying front moving over the Kuskokwim Delta is producing a
light cold rain this afternoon. The front will dissipate tonight,
ending any precipitation. The next couple days will feature
gradually cooling temperatures as a strengthening northeasterly
wind allows colder air to move into the area. The offshore flow
will have some Success at scouring out the cloud cover over the
area, especially the farther north you go. Meanwhile, a
strong low over the North Pacific, which will be the cause of the
breezy conditions across all of southwest Alaska, will send a
series of dying waves into coastal Bristol Bay Wednesday and
Thursday. For the most part, that will lead to cloudy conditions
over Bristol Bay, with better chances of light rain further south
along the Alaska Peninsula. As that low moves northward into the
Gulf, increasing chances of precipitation are expected across
Bristol Bay and the lower Kuskokwim valley. With cold air much
more entrenched, light snow will be likely on Thursday for
interior Bristol Bay and the lower Kuskokwim valley, with light
rain along the coast.
Temperatures across the Kuskokwim Delta are expected to fall 10 to
15 degrees per day the next couple days with the intrusion of the
polar air mass. The lower Kuskokwim valley, already fairly cold
near Sleetmute, will feature temperatures falling 5 to 10 degrees
per day, while that cold air takes a bit longer to move into
Bristol Bay. This is because the core of the coldest air will stay
to the west and largely miss Bristol Bay until later in the week.
Short term forecast Bering Sea/Aleutians (days 1 through 3:
tonight through thursday)...
A North Pacific low that is rapidly strengthening well south of
Dutch Harbor will approach the Alaska Peninsula and eastern
Aleutians tonight into Tuesday. Rain and strong winds along the
northern periphery of the low will impact that region Tuesday and
Wednesday. The low will then drift northeastward into the Gulf on
Thursday. Behind this low, an outbreak of polar air will race
southward across the Bering starting late in the day Wednesday,
reaching the Aleutians on Thursday. This much colder air is likely
to produce snow squalls over a large portion of the Bering Sea,
including the pribilofs, though accumulations with any one squall will
be minimal. A secondary low will develop near Cape Newenham
Thursday afternoon, helping draw some of the cold air eastward
into the Alaska Peninsula, along with producing snow of its own.
The primary hazard from this low, in addition to the cold air and
strong winds, will be heavy freezing spray, especially along the
coast of the Kuskokwim Delta and Nunivak Island. Over the open
waters in the area between Nunivak Island, St. Matthew island, and
the pribilofs, localized blizzard conditions will be possible in
any snow squalls Thursday afternoon and evening.
Marine (days 3 through 5)...
from Wednesday night through Saturday, winds in the western and
central Gulf will generally remain 30 knots or less. Localized
gales are possible at time from southern Cook Inlet, across the
Barren Islands through shelikof straits as troughs and fronts
rotate through with brief periods of gales also possible along the
barrier jet region of the northern Gulf Coast. Northeasterly
gales are expected to persist over the northern Bering near Saint
Matthew Island Wednesday night through Friday and along portions
of the Kuskokwim Delta coast through Thursday. Winds across the
remainder of the Bering, Aleutians and near the Alaska Peninsula
are expected to generally remain 30 knots or less into Saturday.
A shift to a significantly stormier pattern is likely Sunday
through the middle of next week.
Long term forecast (days 3 through 7)...
by Thursday afternoon, the upper level pattern will have
progressed to a large double centered upper level low stretched
across much of the area. The colder upper level low center off of
the Kuskokwim Delta coast will rotate to the south then east and
into Bristol Bay through Friday, while the warmer upper level low
to the southeast of Kodiak Island lifts northeast into the
central Gulf as it continues to weaken. By Friday afternoon the
Gulf low will dissipate into an open trough and frontal system
stretched across the northern Gulf Coast with the low over
Bristol Bay becoming the dominant circulation.
The next North Pacific frontal system will track into the far
southern Gulf Friday afternoon and evening and then continue to
the northeast into the eastern Gulf Friday night through Saturday
to be followed by an additional frontal wave following a similar
track Saturday night and Sunday. Meanwhile, the low over Bristol
Bay will move slowly to the northeast sliding onshore over the
Kuskokwim Delta Saturday through Saturday night with several
associated frontal bands and troughs rotating through southwest
Alaska late Thursday night through Sunday morning. The upper low
will open to a trough and lift northeast into interior Alaska
Sunday and Sunday night swinging a shortwave trough through
A strong westerly jet stream will push in south of the Aleutians
Sunday and Monday then strengthen through the middle of next week.
This pattern shift will open the door for a series of strong, fast
moving systems tracking in from the west along the Aleutian chain
and pushing progressively further east into southern Alaska and
Marine...gale 131 132 138 150 155 165 170 173 174 176.