Scientific Forecaster Discussion
fxak68 pafc 151655 cca
Southcentral and southwest Alaska forecast discussion...corrected
National Weather Service Anchorage Alaska
855 am akdt sun Sep 15 2019
Analysis and upper levels...
An embedded trough axis within an upper level low in the Bering
is currently centered over Bristol Bay. The associated positive
vorticity advection combined with the left exit region of a
strong low level jet is leading to significant divergence aloft
which will help in enhancing precipitation. Ample moisture is
present with pwats exceeding one inch, however total rain amounts
may be limited as the strong upper level westerly flow will cause
the system to quickly move across southwest Alaska. Meanwhile,
much quieter conditions are seen farther east with a weak upper
level ridge in place between the aforementioned Bering low and
another upper low centered over the British Columbia coast. This is leading to
mostly clear skies over southcentral, though as such, patchy fog
While there are subtle timing differences, models remain in good
agreement regarding the progressive pattern over the next few
days. Confidence is high for widespread accumulating rainfall
today, with downsloping not being much of a factor in generally
Panc...VFR conditions will generally persist through this morning,
although patchy fog may bring brief periods of lower ceilings,
particularly before sunrise. Rain is expected to move into
Anchorage this afternoon, with MVFR conditions likely through this
evening. Gusty southeasterly winds are likewise expected this
afternoon, but should only persist for 3-4 hours. Otherwise, light
winds will prevail.
Short term forecast southcentral Alaska (days 1 and 2/today
through Monday night)...
A strong upper level short-wave trough and surface front will
approach southcentral today, with rain spreading from Kodiak
early this morning to the Kenai Peninsula, Anchorage and the
mat-su by midday. Rain intensity will be moderate to heavy at
times, especially in upslope areas along the eastern Kenai
Peninsula as well as Cook Inlet and the Susitna Valley. There
will be a period of southeasterly low level flow across the
Kenai and Chugach Mountains which will lead to lighter rainfall
amounts right in the Lee of the mountains, including for Anchorage
and the Matanuska Valley. In particular, the Palmer area tends to
be quite a bit drier in this flow pattern and they may see very
little measurable rain out of this.
The upper level short-wave will weaken as it wraps north and then
west around the upper level low moving in from the Bering Sea
this evening. Thus, the best dynamics will never make it over to
the Copper River basin, though weaker short-waves rotating around
the low center will produce some rain.
Meanwhile, a triple point surface low will form along the front
east-northeast of Kodiak Island this afternoon. This low will
strengthen in response to another strong short-wave south of the
upper low center. This low looks a little stronger than previous
forecasts, so have expanded the area of gale force winds over the
Gulf waters tonight into Monday. The low will quickly become
vertically stacked as the upper center arrives late this evening.
At this point in time, rain will begin to break up into showers
across southcentral. Conditions will quickly dry out on Monday as
the low exits across the southeastern Gulf.
Winds will be strong and gusty across the western Gulf and Kodiak
Island behind the front beginning this afternoon. The primary
cause will be tight pressure gradients between the eastern Bering
Sea and Gulf, with a building ridge over the eastern Bering and
the deepening low over the Gulf. Pressure rises will accelerate
winds through the gaps of the Alaska Peninsula, leading to strong
gusts on the south side of the Alaska Peninsula and into the Gulf.
These winds will linger all the way through Monday before flow
finally begins to shift Monday night.
Gusty winds will also develop along the north Gulf Coast and
Prince William Sound Monday afternoon through Monday night as
the Gulf low exits. As is typical, Seward/Resurrection Bay and
Whittier/Passage Canal will see some of the strongest winds.
Lastly, a short-wave trough diving out of the Arctic will swing
through interior southcentral Monday night. Based on the latest
trends in the track and strength of this trough, it looks
favorable over the northern Susitna Valley and northern Copper
River basin. Depending on the track of this short-wave, it could
also reinforce/strengthen offshore winds along the Gulf Coast.
Most of southcentral will see wetting rains with the system
moving in on Sunday. Gusty southeast winds will also develop
through the typical gaps, to include turnagain and Knik arms
and along the Copper River. The overall fall pattern, with
wetting rains every 2 or 3 days looks like it will persist
for at least the next week.
Short term forecast southwest Alaska (days 1-3: sun-tue)...
A strong front associated with a vertically stacked low in the
Bering will continue to bring widespread rain across southwest
this afternoon along with gusty conditions. This system is moving
more quickly than the one that occurred earlier this week, thus
won't have quite as much rainfall totals associated in this round.
Rainfall accumulations between one half inch to an inch are
possible stretching from Bristol Bay to the Kuskokwim coast and
Delta. The heaviest rainfall should taper off this afternoon, with
a few showers lingering across the region through the overnight
Monday will generally be a quiet day in terms of winds and
precipitation once this first system has moved into southcentral
Alaska. Stronger winds and rainfall returns on Tuesday morning as
a new frontal boundary approaches the southwest coast. This
system will continue to push inland through Tuesday afternoon,
brining another round of widespread rain once again to southwest
Short term forecast Bering Sea/Aleutians (days 1-2: sun-mon)...
A swath of Small Craft Advisory level winds will continue across
the central and eastern Bering and Aleutians through tomorrow
morning as the low center pushes inland over southwest Alaska.
Gale force winds can be expected near the akpen as westerly flow
continues this morning through the afternoon. As the wind shifts
to a northwesterly direction Monday morning, there will be gale
force winds out of bays and passes along the akpen, which will
weaken by Monday afternoon.
A new system will enter the western Bering on Monday. Models have
come into better agreement on the strength and position of this
storm track, thus increasing the winds to gale force beginning on
Monday morning. As this low progresses eastward, Small Craft
Advisory winds, will continue across the central Bering and south
of the Aleutian chain through Tuesday as the low weakens.
Marine (days 3 through 5)...
There is high confidence of a low moving through the central
Bering on Tuesday that will bring gale force winds to the area. As
the low tracks to the east and weakens winds will diminish for
the Bering and Gulf. There is good confidence that another low
will approach the Aleutians Wednesday into Thursday and that it
will bring small craft winds to the islands.
Long term forecast (days 3 through 7)...
Models are starting to fall in line with the system tracking
across the Bering Tuesday and there is now good agreement of
widespread rain for southern Alaska Tuesday and Wednesday. On
Wednesday another low approaches the western Aleutians from the
North Pacific with it expected to stay south of the Aleutians
through Thursday while it tracks to the east. Thursday night the
models struggle with what to do with this system, lowering
confidence heading towards the weekend. Because of this the
ensembles were preferred by the end of the week.
Marine...gale warning: 120 131 132 138 150 155 160 177 178