Area Forecast Discussion
Issued by NWS Anchorage, AK

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296
FXAK68 PAFC 240106
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKDT Sun Sep 23 2018

.ANALYSIS AND UPPER LEVELS...

The upper level low driving our current pattern is now over the
Yukon Delta with the main trough east of the date line. Showers
are still observed on radar this afternoon across Southwest Alaska
with surface observations indicating low level stratus and fog
along the northern AKPEN. An embedded shortwave is nearing 160W
with very isolated lightning strikes still showing up. This will
be the kicker that stirs up the next round of wind and rain for
the AKPEN and Southcentral later tonight and Monday. For the
moment, this vort max is still disconnected from the main corridor
of Pacific jet stream.

Southwest flow continues over the mainland and the AKPEN ahead of
this system. Flat ridging is seen over the western chain and an
upper level ridge of high pressure remains in place over the
Gulf. As the embedded shortwave nears the southern Mainland, more fall-
like conditions will arrive to kick off the last week of September.

&&

.MODEL DISCUSSION...

Models remain in fairly good agreement through the next 24 hours.
As the low moves northward over the AKPEN Mon morning, models show
minor disagreement regarding exact location. Currently, the
EC/GFS solution positions the surface low further east, while the
NAM keeps the surface low over Bristol Bay. The EC/GFS solution
was preferred.

&&

.AVIATION...

Expect wind shear to start Monday morning with northerly surface
winds under stronger southeasterly flow. This should ease by
midday as surface winds return from a southerly direction.
VFR conditions will last through early Monday morning before
lower ceilings and visibility reductions arrive midday.

Frontal passage is expected midday and the afternoon goes on,
winds will become more southerly with gusts to 35 kt or possibly 40
kt.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Sunday
night through Wednesday night)...

Mostly clear skies this afternoon will give way to increasing
cloud cover this evening as a rapidly intensifying low currently
south of Cold Bay gets swept north by a strong 110 kt upper level
jet. Rain has already pushed into Kodiak and will continue to
spread north across the Kenai Peninsula, North Gulf Coast, and
eventually into the Anchorage area and Mat-Su Valleys over night.
The advancing front out ahead of the low will tighten the pressure
gradient across the Gulf Coast leading to strong terrain enhanced
winds, most notably along the Turnagain Arm and Upper Hillside
Monday morning. Winds are expected to peak by mid to late morning
with gusts up to 65 mph. The strong cross barrier flow will dry
out the typical lee side locations in the morning with heavy rain
expected along the Gulf Coast.

As the low center lifts north, winds will abruptly switch to a
southerly direction as rapid pressure rises combined with decent
cold air advection will cause winds to accelerate up Cook Inlet in
the afternoon. Recent model runs have strengthened the low even
more, thus there is good confidence that locations along Cook
Inlet and up into the Anchorage Bowl will see wind gusts reaching
40-50 mph in the afternoon/evening hours. A brief period of rain
will move up Cook Inlet at the same time before drier air moves
in behind the front. Strong SW flow will keep showers over the
higher terrain in the evening through Tuesday morning with good
orographic lift along SW facing slopes. Cold air aloft will also
drop snow levels to near 3500 ft so local peaks could see their
first snowfall. In fact, snow lovers will delight in the fact that
the Talkeetna Mountains (near Hatcher Pass) could see their first
accumulating snowfall Monday night into Tuesday Morning, with 2-4
inches expected above 4000 ft. The higher peaks in the Talkeetnas
(above 6000 ft) could even see up to 10 inches by Tuesday morning!

Skies will begin to clear out by Tuesday afternoon as the low
pushes north out of the area and high pressure moves in behind it.
Clearing skies Tuesday night could lead to the first freezing
temperatures for many locations in Southcentral Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Wednesday)

Showers associated with a weakening area of low pressure, will
continue to develop this evening north of the Nushagak Hills.
Meanwhile, a strengthening area of low pressure will reach the
AKPEN by sunset, as it crosses the central interior. An associated
warm front will bring a surge of warm and moist air to the east of
its track as a warm front crosses this area, fueled by a 60 knot
low-level jet at 925 mb. Easterly winds will increase as the front
approaches, with a cold front also crossing this region on the
heels of the initial warm front. Widespread rainfall associated
with this storm will affect virtually all of the interior (mainly
east of a line from Cape Newenham to the Kilbuck Mountains). Some
of which may be moderate to locally heavy at times, with lighter
showery type rainfall further west. Colder air will advect
eastward, as its drawn down from the north in the cyclonic flow.
This may allow some of the interior valleys to drops below
freezing for Wednesday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Wednesday)

A deepening area of low pressure will cross the AKPEN tonight,
then trek northward during the day tomorrow, as it rides the spine
of the Kilbuck Mountains. A pair of fronts (an initial warm front
followed by a cold front a few hours later) will bring varying
wind directions with a rapid increase in wind speeds to the
Peninsula and the adjoining coastal waters. Visibilities may also
be reduced in the heavier periods of rainfall.

For the remainder of the Bering Sea and Aleutians, look for
isolated to widely scattered showers to continue through Monday
night with light winds expected. A transient ridge of high
pressure will build across the region for Tuesday, as it quickly
shifts from west to east. As it does so, an approaching cold front
will bring gale force sustained winds with an increasing sea state
and steady rainfall to western portions of the area.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday Night through
Saturday)...
Wednesday night will start out with a front associated with a
North Pacific low moving northwestward, spreading rain across
Kodiak and the Alaska Peninsula. The models are trending a bit
further northeast with the northern edge of the precipitation
shield as compared with yesterday`s runs. As such, they`re now
suggesting the rain will spread across much of Bristol Bay, and
eventually northwestward across the Kuskokwim Delta. However, the
EC and GFS are about 18 hours apart as to when that rain makes it
into the Kuskokwim Delta. Thus, the differences between the
models still leave significant uncertainty as to the specifics of
when and how much rain falls in any one place in Southwest Alaska.
On Friday through Sunday, the trailing front behind the parent
low lingers along or off the coast, gradually drifting westward
away from mainland Alaska by Sunday. The presence of the front
should mean a sustained period of gusty southeast winds across
Southwest Alaska Friday and Saturday.

For Southcentral, the front and associated precipitation over
Kodiak will continue there into Friday, though the heaviest rain
should be winding down by Friday morning. During the day Thursday,
the rain will overspread the southern half of the Kenai Peninsula
and up the coast to about the Seward area before beginning to
diminish and retreat southwestward Thursday night. High pressure
over the interior will hold strong and keep Anchorage and points
north and east dry throughout the entire long-term forecast
period, though Thursday now looks to be a cloudier day than it
looked like in the models yesterday.

Temperature-wise, the entire long-term forecast will feature a
warming trend. Behind the front, much warmer air will move into
Southern Alaska Friday through Sunday, which will help support
well above average afternoon highs, though the longer nights and
mainly clear skies will still support some chilly nights for
lower elevation locations as inversions likely set up each night.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130 131 132 138-141 150-160.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...KO/DS
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JW



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