Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 181204
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
404 AM AKDT Fri Aug 18 2017

.ANALYSIS AND UPPER LEVELS...

The active longwave pattern continues across the Alaska mainland
this morning with water vapor imagery depicting another jet streak
transiting the Bering Strait and the eastern Bering Sea. With the
active polar jet, continued upper level shortwaves and associated
jet streaks continue to strengthen the arctic trough already in
place across much of Alaska. The trough has been sliding eastward
over the past 36 hours but no change in the overall pattern
currently appears on the horizon. Shower activity has been
observed across southern Alaska in response to the arctic trough
as multiple PV anomalies remain entrenched over the area. Until
this pattern begins to break up, this typical wet and cold August
pattern will undoubtedly continue.

Looking further west toward the Aleutian Islands, the extratropical
remains of Tropical Storm Banyan is now located around 260 NM
south of Adak. The current state of the longwave pattern will keep
this system well south of the Aleutian Islands but a small area
of gale force winds will impact the Pacific side of Alaskan
waters. A triple point low looks to be in the early stages of
development according to the latest scatterometer pass. This low
will also remain south of the Aleutians but could become quite
the rainmaker for Southcentral Alaska in the coming days.

&&

.MODEL DISCUSSION...

Now that the remnants of Tropical Storm Banyan has entered the
upper air network, models are now doing a great job in
initializing the resulting low pressure system. In addition, the
jet streak over the Bering Strait and eastern Bering Sea was
initialized well over the 00Z and 06Z runs which increases
confidence in model output. There is a lot of uncertainty
surrounding shower coverage and rainfall amounts across all of
southern Alaska with the arctic trough remaining in place.
However, the cooler and wetter pattern will continue until the
trough over the Alaska mainland can be displaced. Uncertainty also
surrounds the evolution of Banyan as it moves into the Gulf of
Alaska over the weekend. At this time, it looks as if Sunday and
Monday could be quite rainy for much of Southcentral Alaska as a
moist piece of subtropical air upslopes along the coastal
mountains. Confidence is still a little sketchy at this time but
confidence is increasing in yet another bout of moderate to heavy
rainfall across Southcentral Alaska.

&&

.AVIATION...
PANC...VFR conditions will prevail through mid-afternoon before
the next round of rainfall brings MVFR ceilings and vsby. Conditions
will begin to improve around midnight with VFR ceilings and light
winds expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A fairly cool upper level trough will remain over the area for
the next couple of days. The main upper low just off the northwest
coast of Alaska will slowly slide south and will be centered just
southwest of McGrath Sunday morning. A fairly vigorous short-wave
will push through the area today, with increasing rain/shower
activity over the Kenai peninsula this morning and then over the
Anchorage/Susitna/Matanuska area later this afternoon and evening.
Shower activity will diminish tonight as the short-wave moves
east. Shower activity will increase again Sunday morning as the
main upper low approaches.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
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.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Low to mid level clouds continue to stream toward the Bering from
low pressure over the Arctic and from another 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 into next Thursday time-frame.
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 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 enter the Bering. The Southwest Alaska
area transitions to an offshore drier conditions 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.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...KH



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