Area Forecast Discussion
Issued by NWS Anchorage, AK

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730
FXAK68 PAFC 121257
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Thu Sep 12 2019

.ANALYSIS AND UPPER LEVELS...

A strong upper level jet is currently situated nearly parallel to
the Aleutian chain. Its associated 500 mb low centered west of
Saint Matthew Island encompasses the entire Bering. The large
surface low is becoming vertically stacked and gradually
weakening, although its front is still advecting tropical moisture
northward and presently bringing periods of heavy rain and gusty
southeasterly winds to the AKPEN and Bristol Bay coast.

A series of shortwave troughs are moving over Southcentral Alaska,
currently leading to persistent rain over the Kenai Peninsula and
Prince William Sound. Scattered showers are indicated near
Anchorage and over the Mat-Su.

&&

.MODEL DISCUSSION...

Latest model runs are in fairly good agreement in the near-term.
Though there are subtle timing differences with an approaching
front, confidence is high regarding widespread precipitation
across Southcentral today and Friday.

&&

.AVIATION...

PANC...VFR conditions will persist. Although gusty Turnagain Arm
winds are expected to develop this morning, confidence is high
that they will remain light at the airport itself.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Friday night)...

Conditions will deteriorate across Southcentral Alaska this
morning as the storm system over the Bering Sea approaches. As
has been advertised in previous discussions, the dynamics with
this system are strong and moisture content is high. Thus, expect
widespread rain for all except the Copper River Basin as we head
through the next 24 to 36 hours.

A gale force front will track across the Southwest Gulf and
Kodiak Island this morning, then begin to slow down this afternoon
as the upper trough supporting it becomes negatively tilted and a
triple point low develops in the vicinity of southern Cook Inlet.
Southeast winds will pick up ahead of the front, particularly
through mountain gaps in the Kenai and Chugach Mountains (most
notably Turnagain Arm). It looks like these gap winds will peak
late this morning. The front will then begin to shift from a more
north-south orientation to a an east-west orientation. This will
cause the east-west pressure gradients to slacken and winds to
diminish a bit despite the fact that front will continue to get
closer. By the time the front finally crosses the Kenai Peninsula
and Anchorage tonight it will be in a much weakened state.

Meanwhile, out over the northern Gulf a surface trough will
develop out ahead of the front thanks to some interaction with the
exiting low over the southeast Gulf. A barrier jet will form along
the north Gulf coast and into Prince William Sound as this trough
lifts toward the coast. Winds will then diminish as the trough
moves onshore and likely won`t get any stronger as the actual
front arrives tonight.

Despite the low level southeast flow, still expect periods of rain
in the often downsloped lee of the Kenai and Chugach Mountains.
This includes the western Kenai, Anchorage, and Matanuska Valley.
The flow will however ensure that rainfall in these areas remains
light. The heaviest rain will fall along the Gulf coast and Prince
William Sound, with 1 to 2 inches of rain for most areas and
isolated higher amounts. As the front finally pushes onshore
tonight, the upper level short-wave trough will be oriented east-
west, meaning there will be little wind to help push the front
inland. Thus, expect rain to linger from eastern Prince William
Sound westward to the northern Kenai Peninsula and Anchorage.
Inland areas may see some light rain, but it won`t amount to
much.

The Bering Sea low will make its way into the Gulf on Friday,
bringing showers to Kodiak and the Gulf. Clouds and some shower
activity will linger over Southcentral as well. The low will then
quickly exit eastward as we head into the weekend, leading to a
drying trend and increasing sunshine.

&&

.FIRE WEATHER...
Most of the active fires will see some rain and wind out of this
storm system. While the Copper River Basin as a whole will remain
drier, it increasingly looks like the Tokaina fire will see some
rain as the front pushes onshore tonight and downslope winds
weaken.

&&

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

A front associated with the seasonably strong storm across
Southwest Alaska will continue to progresses farther inland and
weaken through the evening hours. This system has already brought
over an inch of precipitation to areas along the coast, as well
as wind gusts of 45+ MPH, which will continue to move eastward
with the front this afternoon.

Strong onshore flow will persist through this afternoon and will
continue the threat of flooding along the coast. Areas around
Kuskokwim Bay currently receiving coastal flooding can expect this
to continue today. Minor coastal erosion will also be possible
with flooding along the coast.

A showery pattern will keep things wet through Friday across the
Southwest even after this front and its low center have moved
eastward towards Southcentral. Saturday will see a brief period
of dry conditions before a second front approaches the Southwest
coast later this weekend, along with another round of rainfall
and strong onshore winds.

&&

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

Confidence remains high with the strong low and the associated
gale force winds stretched across the Bering sea. The low center
will track eastward and into the Southwest coast through the over
night hours as the systems begins to weaken. By Friday morning,
winds begin to shift into the eastern Bering and AKPEN, weakening
to small craft advisory. Widespread rainfall is expected to
continue with this system before clearing out on Friday. High seas
up to 25 ft across the waters between the Pribilofs and Aleutians
are still expected due to the fetch length.

A second low will impact the Bering on Friday as a low approaches
the western Aleutians. This low will be weaker than the current
exiting one, but winds of small craft advisory criteria are still
expected. However, there is less confidence in the exact low
position and strength of this system.

&&

.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 Warning: 119 120 125 130-132 136 137 139 141 155 165
170-176 179.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...BJB



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