Scientific Forecaster Discussion
fxak68 pafc 191251
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
451 am akdt Tue Sep 19 2017
Analysis and upper levels...
The upper levels across southern Alaska feature a split jet stream
over much of the Bering, merging into a single jet further east as
the broad flow rapidly increases its speed over the southern Gulf.
The center of the upper low is rapidly moving out of the Gulf with
the jet. South of the jet, longwave ridging is building across the
Aleutians, associated with a tongue of warmer air aloft.
Across southcentral, the upper level flow is generally weak, as
the upper low moves away and a col moves over the area. As a
result, winds are generally very light, allowing all of the
moisture that arrived with yesterday's rainfall to persist. Thus,
widespread low stratus remains over much of the area, and with
temperature/dewpoint spreads relatively minimal with weak winds,
the threat for fog is in place over much of the area. The one
inhibiting factor is the cloud cover, which is precluding much
cooling, so the cooler, sheltered spots stand the best chance of
seeing fog overnight, while elsewhere some patchy spots are
The models are all in good agreement through the short-term. One
area of difference comes late Thursday night into Friday with the
handling of whether the rain ends for a time or if the persistent
southwesterly flow causes steady rain to continue throughout
southcentral. Forecast confidence is high.
Panc...light winds with VFR conditions will persist through the
Short term forecast southcentral Alaska (days 1 and 2)...
as a low exits across the eastern Gulf this morning, weak impulses
on the northern side of the low will track westward into southcentral.
Meanwhile, a very weak trough will move in from the west. The
opposing and confused flow will provide just enough lift in a
still moist atmosphere to produce isolated showers and hold in
some mid to high level clouds, especially early in the day. A
fairly flat and progressive upstream ridge will ultimately win out
later today into tonight, so generally expect decreasing clouds
with some breaks of sun. Tight pressure gradients between the
exiting low and a Stout surface ridge over southwest Alaska will
continue to produce strong gusty winds along south side of the
Alaska Peninsula today, though there will be a gradual downward
trend. Weaker and more isolated gap winds will also be felt near
Seward and Whittier.
A fast moving upper level short-wave trough will cross the
northern tier of southcentral Wednesday bringing a quick shot of
light rain to inland areas. Otherwise, it looks like a fair fall
day across the remainder of the area. A bit of a pattern change
will quickly follow, as flow over the North Pacific amplifies and
sends a warm front northward toward Mainland Alaska. There is some
uncertainty in timing, but it looks favorable for clouds and rain
to move back into the region Wednesday night through Thursday.
Short term forecast southwest Alaska (days 1 and 2)...
the forecast period commences with weak ridging and northwest
flow over southwest Alaska with a trough upstream near St.
Lawrence Island. With ridging in place expect drier conditions
across the Bristol Bay region through today, however the break in
precipitation will be more short lived as a fast moving system
tracks from the northern Bering into the Yukon Delta. Wednesday
the trough will transition from positive to negatively tilted as
it moves inland resulting in widespread precipitation.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a deep low is churning south of Adak and there is a ridge axis
near Unalaska. As the low pushes northeast expect the mild
conditions to deteriorate and be replaced with gusty winds and
moisture advection. Sensible weather wise expect rain and gale
force winds, with these conditions spreading east to the Alaska
Peninsula tonight and then north to the Pribilof Islands on
Long term forecast (days 3 through 7)...
beginning the extended period forecast Wednesday night, there will
be an elongated Arctic trough over the northern part of Alaska
with a secondary North Pacific trough centered over the eastern
Aleutians. A very strong northeast Pacific subtropical ridge will
be south of the Gulf of Alaska. This will set in motion a broad
region of southwesterly flow aloft over southern Alaska with rain
impacting the entire region. This pattern will continue through
Saturday with wet and cloudy weather, although no significant
heavy rainfall. By Saturday evening, the pattern becomes a bit
more complicated as the most recent simulations of the GFS and
European model (ecmwf) deterministic models have trended toward a strong secondary
subtropical warm front merging with the Arctic trough. The
associated low would track into the Gulf of Alaska and toward
southcentral sometime Saturday evening into Sunday, bringing a
swatch of possibly gale force winds to the coast and another batch
of rain, locally heavy along the coast. Either way, even with that
uncertainty, there is high confidence that the overall pattern
will keep a trough aloft and moist flow directed at southern
Alaska. This will keep wetter conditions in place into the next
Marine...gales 130 131 170 172 173 174 175 176 177 178 179 411 412