Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 120018
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 PM AKDT Wed Sep 11 2019

.ANALYSIS AND UPPER LEVELS...
The large vertically stacked low in the central Bering Sea is the
remnants of former typhoon Lingling. The moisture and energy of
former typhoon Faxai is near the Alaska Peninsula and is getting
entrained into this system. The area of greatest dynamics is
associated with the `Faxai` part of the storm (Alaska Peninsula
and Bristol Bay regions) where a triple-point low should develop
near Shelikof Strait Thursday morning.

A short wave trough extends from Prince William Sound through the
Copper River Basin and is moving to the northeast. This wave has
brought some rainfall to both the north Gulf coast and Copper
River Basin today.

&&

.MODEL DISCUSSION...
The overall synoptic features are being handled well by the
models. However, there is enough of a difference with the
formation and tracking of the triple-point low near Shelikof
Strait Thursday morning to cause some forecast uncertainty,
especially in Cook Inlet. The NAM and Canadian solutions have
been quicker with a track up Cook Inlet which would bring in more
winds and rain earlier than the GFS and ECMWF solutions. Those
models track the triple-point more toward Middleton Island. This
second solution is a more typical pattern for a triple-point that
develops near Shelikof Strait so that solution was preferred.

&&

.AVIATION...
PANC...VFR conditions will persist. The biggest question is
whether the Turnagain Arm wind will make it over the airport or
remain to the south. At this time it looks like it should mainly
stay to the south but a period or two of gusty southeast winds
cannot be ruled out for tonight into tomorrow. There should also
be some low-level wind shear developing overnight with the winds
aloft increasing.

&&

.FIRE WEATHER...
Most of Southwest and Southcentral Alaska will see wetting rains
over the next few days. The one area that will be a little drier
will be the Copper River Basin where there may be sufficient
downslope to greatly diminish the amount of the rain over the
Tokaina fire. The one big wrench in all this is that could give a
little more rain than expected over Tokaina is that this moisture
is from a former tropical system. That may go a long way toward
overriding the downslope due to the sheer amount of moisture and
vertical depth of the moisture. Gusty southerly winds will move
over the fire area, but the influence of Tazlina Lake may make
for rather erratic winds over the fire.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

A shortwave trough lifting across the Copper River Basin will
continue to produce clouds and areas of showers from Prince
William Sound on north through the overnight hours. As this
feature departs, a second wave currently over the Barren Islands
and ahead of the main front will lift northeast over the
Southcentral Mainland. A southwesterly flow aloft with this
feature will bring a chance for rain along upslope areas of the
eastern Kenai Peninsula and Western Susitna Valley. A brief
period of light light rain is also possible across the Anchorage
Bowl and Mat-Su Valleys, but a quick transition to a downsloping
wind as the main front advances will limit precipitation amounts
in these areas.

The focus then shifts to the main weather event: an approaching
frontal system attached to a Bering low. Rain and strong
southeasterly winds will develop over Kodiak Island tonight as a
strong upper-level shortwave lifts into the western Gulf in
association with the front. The entrainment of tropical moisture
from the remnants of Faxai will lead to heavy rain from Kodiak to
Prince William Sound as this front progresses north and east
Thursday and Thursday night. In addition to the rain, strong winds
will develop ahead of and along the front, with gales over the
coastal waters and gusty, southeasterly gaps winds across the
mainland. The strongest winds are expected through terrain gaps of
the Kenai and Chugach Mountains, as well as, the Copper River
Valley. Downsloping with limited precipitation along the lee of
the mountains is expected due to the cross-barrier flow. However,
given the strong forcing with this system and copious tropical
moisture, there may be enough energy to overcome the effects of
downsloping at times, leading to a bit more rain along the lee
than is usual with these events.

The front will begin to slow late Thursday and creep closer to
the coast by Friday as the upper-level wave becomes negatively-
tilted and a secondary surface low spins up near the Barren
Islands. The development of this low will pull the Turnagain Arm
winds down inlet, keeping the strongest winds out of Anchorage.
This feature, along with the weakening front, will also allow
gusty winds to diminish over the Anchorage Hillside by late
Thursday.

Clouds and showers linger across Southcentral as the weakening
Bering Low drifts over the AKPEN and into the Gulf. A drying trend
is then on tap Friday night into the weekend as the low tracks
farther east.

&&

.FIRE WEATHER...

The focus for fire weather will be on the strong front on
Thursday. This will bring gusty east to southeast winds to the
Interior Kenai Peninsula as well as the Copper River Basin. The
Kenai Peninsula is more likely to see periods of rain while the
Copper River basin remains mostly dry.

&&

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

The strong storm has moved into the Southwest coast this
afternoon, already bringing heavy rain and strong winds into the
50+ MPH range. Southeast winds have also brought some minor
flooding to parts of the Kuskokwim Bay coast. The front moves
inland tonight, with the rain moving inland as well. Expect 1 to 2
inches of inland rain as the front moves eastward through
Thursday. The low itself, as it weakens, will skirt the coast then
move across the AKPEN Friday, with showers slowly ending. Coastal
erosion across parts of the southern AKPEN remain possible,
although would likely be very localized.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The seasonably strong gale force low has now wrapped up and
covers the entire Bering Sea, with the low center across the north
Bering and the front moving inland along the Southwest AK coast.
Moderate to heavy rain across the Eastern Aleutians will continue
through the evening before the front clears the region.
Thereafter, the low will drift east with cold air advection around
the backside of low keeping widespread gale force winds over most
of the Bering Sea. Given the fetch length, there will likely be
high seas topping out at 24-27 ft across the open waters between
the Pribs and the Aleutians. Higher seas will move into Bristol
Bay and the Eastern Aleutians as the storm moves across the AKPEN
Friday.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Confidence is high with the weakening and exit of our current
storm early Saturday. We are also confident another low will be
moving through the Bering Sea Saturday and Sunday. This low looks
to be weaker than the current system, with small craft level winds
currently forecasted. We will be taking another look tonight to
see about increasing wind speeds to gale force, but this system is
still not looking very strong. The Gulf of Alaska looks quiet
after the exit of the storm Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7/Saturday through Wednesday)...

The long term starts with a weak shortwave ridge in the Gulf of
Alaska and another round of low pressure for the Bering Sea. The
large scale pattern is pretty quick moving, so high pressure in
the Gulf and Southcentral Alaska will only last for day, while the
low pressure system in the Bering Sea moves east quickly. It will
move into Southwest and Southcentral Alaska on Sunday. Troughing
behind this low will keep the Bering and Aleutians under low
pressure as well. This means with the exception of a nice day in
Southcentral AK on Saturday, it will be showery and mostly cloudy.
We are confident in this forecast trend of returning to a fall
pattern through the weekend and into early next week, as guidance
has indicated this occurring for a few days now.

However, approaching the middle of next week, longer range models
are a mess quite frankly. They disagree completely with each other
on even just the basics of placement of major high and low
pressure systems, so there is really no telling what the weather
will be past the middle of next week.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 120 125 130-132 136 137 150 155 160 165
170-179. FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...BJB


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