Scientific Forecaster Discussion
fxak68 pafc 212352 cca
Southcentral and southwest Alaska forecast discussion...corrected
National Weather Service Anchorage Alaska
352 PM akdt Thu Sep 21 2017
Analysis and upper levels...
steady rainfall has begun pushing into the Cook Inlet region this
afternoon, with Kenai radar showing rain over much of the area
and spreading north up into the Matanuska and Susitna Valley. The
upper level ridge that was over south central Alaska has been pushed
east by the invading upper level low over Norton Sound. The
weakening warm front associated with a slow moving broad low
swirling south of the central Aleutians remains extended across
the eastern half of the Aleutians and into the Bristol Bay area.
This front in conjunction with the upper low slowly tracking
south remains the primary sources of lift for the light rainfall
persisting across parts of the southwest Mainland. The low south
of the Aleutians is keeping light rainfall ongoing across much of
the chain as warm, moist air streams north ahead of the low and
is lifted above the cooler air mass near the surface.
model agreement on the synoptic scale remains very good through
the remainder of the week. The NAM model was preferred for the
first two days with a transition to GFS beyond that. This was due
to the need to continue toward the GFS solution beyond the
panc...periods of rain with predominantly MVFR ceilings are
expected overnight with improvement during the day Friday. Periods
of VFR ceilings may also occur but it looks predominant marginal
conditions until winds become southeast Friday afternoon which
will usher in higher ceilings and less rain. Visibility will
mostly remain VFR with just some occasional MVFR conditions
expected through the night. Some low level wind shear will be in
the area through this evening as winds switch form north at the
surface to easterly winds around 1000 to 2000 ft. The easterly
winds should diminish this evening ending the wind shear.
Short term forecast southcentral Alaska (days 1 and 2)...
the southcentral Alaska region and the Gulf is predominantly
influenced by southwesterly upper level flow. While a weak 1000
mb low is tracking across the Aleutian Range into lower Cook
Inlet with an associated weather front to track from Kodiak
north into the north Gulf Coast by this evening. This synoptic
feature is producing widespread rain through the aforementioned
regions extending into the mat-su valleys, and the Anchorage bowl
as the precipitation continues to spread into the central
interior. Look for the Anchorage bowl to downslope resulting in
drier conditions as the winds begin to shift more southeasterly to
easterly direction resulting in a cross-barrier flow by Friday
morning. While the eastern Kenai Peninsula is being closely
watched with an issuance of a Flood Warning in effect through 1pm
Monday for a Glacier damned lake outburst in the snow river. This
outburst will affect Kenai Lake, and the Kenai River from Cooper
Landing to Skilak Lake. This abundant rainfall along the
aforementioned area will contribute to higher river levels.
Meanwhile, as we head into Saturday a new low pushes over Kodiak
Island with another barrier jet developing along the northern Gulf
by Saturday afternoon. This will bring another round of
widespread rain accompanied by gusty Turnagain Arm winds, and gale
force easterly winds along the north Gulf Coast.
Short term forecast southwest Alaska (days 1 and 2)...
the front that has pushed in over southwest Alaska has stalled
over an area that stretches from eastern parts of Bristol Bay to
the Alaska Range. The front will continue to bring rain to this
area into Friday. However, as the front dissipates Friday, a low
will approach the area from the south Friday night into Saturday
bringing more rain to the same areas. For the Kuskokwim Delta
Region, a low will drop down over the area from the north on
Saturday bringing some rain to the area, primarily along the
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the front over the southern Bering Sea has stalled and will bring
continued rain to the eastern Aleutians into Friday and the
Alaska Peninsula through much of Friday. Meanwhile, high pressure
has built in over the western Bering Sea and will remain dominant
through Saturday keeping northerly flow over the Bering Sea and
Aleutians through the same timeframe.
Long term forecast (days 3 through 7)...
beginning the extended period forecast Saturday evening, with an
upper level low tracking south over the eastern Bering Sea and
bringing waves of North Pacific moisture north toward the Gulf of
Alaska and southern Mainland through the early part of next week.
This pattern will support wet weather across the region into
Sunday afternoon, as a front will move toward the Gulf Coast.
Gusty winds are also expected to increase along the Gulf Coast
Saturday afternoon through Sunday morning, as the front pushes up
along the north Gulf Coast.
Showers will begin tapering off over much of the southern
Mainland by Monday morning, and remain slightly drier through
Tuesday morning when the next system begins tracking into the Gulf
of Alaska. Precipitation will begin to spread north back into the
southwest Mainland and north Gulf Coast Tuesday afternoon, as a
North Pacific low tracks north into the western Gulf of Alaska.
There is some disagreement between the models with regards to the
track and timing of this system, which impacts when the
precipitation spreads further inland. Currently the forecast is
trending toward precipitation moving further inland overnight
Wednesday into Thursday morning, with the low tracking inland over
the Kenai Peninsula.
public...Flood Warning 121 and 125.
Marine...gale 119, . Fire weather...none.