Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 180054

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKDT Thu Aug 17 2017


A broad upper level trough associated with relatively cold air
aloft is supporting widespread showers across much of the
Southcentral region. The sounding this morning indicated steep
lapse rates throughout the depth of much of the atmosphere and
near saturated conditions. This instability was confirmed by a
few lightning strikes near the Kenai Peninsula this morning.
Meanwhile high pressure maintains hold across the central and
western Bering.



Models remain in good agreement in the short term with the strong
upper level forcing from the PV anomaly that now stretches across
much of the state. However, once more there is uncertainty in
where rainfall will develop this afternoon and evening across
Southcentral. While most models keep the majority of showers along
the mountains. However, high resolution models hint at the
possibility of a strengthening Turnagain Arm jet again this
evening, which would produce a convergence boundary with the
southwest low level jet over Anchorage.



Cold air aloft underneath an upper level low will continue to move
in over southcentral leading to a relatively unstable airmass. A
shortwave trough will push through the area this evening and will
act as a forcing mechanism to help initiate widespread rain
shower activity with periods of locally heavy rain across the
region. Rain showers will die down overnight into tomorrow morning
before another more vigorous shortwave trough digs down and
across the Southern Kenai Peninsula on Friday. Rain will spread
north and east up the Kenai and into the Prince William Sound as
well as into the Anchorage and Mat-Su areas by the late
afternoon/early evening hours.

Zonal flow sets-up behind the departing shortwave leading to
drier conditions for Saturday across most of the region, though
showers will develop again over the mountains in the afternoon.


A wet and cloudy pattern persists through the upcoming weekend as
an upper level trough remains over the state. Periods of rain and
patchy fog are the biggest challenges for the upcoming weekend.
Showers are expected to be the prevailing precipitation pattern
as broad surface troughing lacks any notable focus. The airmass
across the southwest mainland is conditionally stable and will
likely have weak surface inversions set up during the late night
and early morning hours allowing patchy fog to develop.

The next potent shortwave develops across the Bering Strait on
Saturday and dives across the Kuskokwim Delta Saturday night with
rain developing along this dynamic set up. This wave moves
southeast through Sunday spreading rain across the valleys to the
western Alaska and Aleutian Ranges by Sunday afternoon.


Low to mid level clouds continue to stream toward the Bering from
low pressure over the Arctic and from another low just south of
the central Aleutians. The ridge over the central Bering blocks
either low from tracking across and holds its position through
this weekend. Areas of fog develops across the stable air mass
associated with the ridge. The lowest visibilities are expected
during the late night and early morning hours, improving slightly
during the afternoon and evenings. Several upper level
disturbances dive south from the Arctic low along the eastern half
of the Bering which will bring light rain to the Pribilof Islands
and the Alaska Peninsula Sunday.

The low pressure currently south of the central Aleutians moves on
a slow easterly track through Saturday bringing rain to the Chain.
This system shoves southeast Saturday night with a building ridge
from the North Pacific amplifying across the western Bering
through Sunday.


.LONG TERM FORECAST (Days 3 through 7)...
The active wet pattern remains intact through the long term
forecast period as we head through the middle of next week. There
in a North Pacific low tracking eastward parallel along the
Eastern Aleutians before it moves into the Gulf by Sunday
afternoon. This system remains almost stationary in the northern
Gulf before exiting into the lower eastern Gulf by Tuesday, and
then tracks into the North Pacific Ocean by Wednesday. The models
struggle with the timing as this low tracks along the Aleutian Chain
before entering the Gulf. Look for the bulk of the moisture to
remain along the northern Gulf coastal communities. Meanwhile, the
Bering Sea has a ridge of high pressure building into the region
before a North Pacific 997 mb low tracks near 45N 167E with an
associated front entering the Western Aleutians by Monday morning.
By Tuesday evening the high pressure moves southward, and the
aforementioned low moves into the western Bering. The models have
difficulty as this system tracks northward. The WPC keeps this
system as a broad area of low pressure, while the ECMWF has a more
consolidated system near Saint Matthew Island by Wednesday
evening. As we go into Thursday the WPC remains broad brushing
this system, but the ECMWF keeps it consolidated as it tracks into
the eastern Bering. Therefore, look for marine layer stratus/fog
with the high pressure ridge before transitioning to widespread
showery regime as the low pressure system enters the Bering. The
Southwest Alaska area will transition to an offshore drier flow
briefly before transitioning back to onshore flow by Tuesday. This
will advect marine layer stratus into the region with showery
regime setting up by Wednesday. Since there continues to be model
discrepancy with the timing/placement of the different synoptic
features, the forecast confidence will be barely average through
the long term forecast.





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