Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 200138
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
538 PM AKDT Sat Jun 19 2021

.ANALYSIS AND UPPER LEVELS...

A long-wave trough is positioned across most of Alaska, with a
weakening ridge over the northern mainland. A cut-off low remains
centered near the south end of Kodiak Island, with a short-wave
trough extending northeastward into Southcentral Alaska. This
trough is weakening as some of the energy drives northward toward
Interior Alaska while some rotates westward into Southwest Alaska.
This is leading to considerable cloud cover and areas of rain and
showers across the southern mainland of Alaska. Conditions are
trending drier over Southcentral Alaska as the trough exits north
and west.

Meanwhile, a vertically stacked low over the north-central Pacific
is tracking northward toward Kodiak Island and the Gulf of Alaska.
This will be the big weather maker for Southcentral on Sunday. Out
west, a large and deep low is reaching its strongest point as it
passes south of the western Aleutian islands. This is producing
rain and small craft to gale force winds from about Adak westward
to Attu. In between the Aleutians and mainland Alaska weather is
quiet, with just some areas of low clouds and fog.

&&

.MODEL DISCUSSION...
Models are in good agreement with large scale features. The
continuing challenge is the weaker and smaller scale short-waves
moving around the upper level lows. This creates some level of
uncertainty in the precipitation forecast over southern Alaska.

&&

.AVIATION...
PANC...VFR conditions will persist. However, as a surface occluded
front and upper level short-wave approach from the Gulf on Sunday,
ceilings will likely drop to 5000 foot level or lower as steady
light rain moves in after 18Z.

Conditions look favorable for gusty southeast winds this afternoon
and evening, with lower pressure inland as the upper trough exits
to the north and west. While Turnagain Arm winds will re-develop
on Sunday, strong N-S pressure gradients will cause winds exiting
Turnagain Arm to turn strongly southward and remain far from the
terminal. Thus, once southeast winds die down tonight, winds will
remain light all day Sunday.

&&

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

Cool and wet weather will continue through early Monday and then
drier and warmer as we head into Monday afternoon and Tuesday. An
upper level shortwave will continue to move west tonight with rain
for the Susitna Valley ending this evening. Showers and a few
thunderstorms are still possible over the Wrangell Mountains and
north of Gulkana closer to the Alaska Range. This activity will
diminish this evening as the airmass stabilizes. Behind this wave
expect a brief period of drier weather for inland locations
tonight. Our focus then turns to a low pressure system moving into
the Gulf on Sunday. The front associated with this low will bring
rain to coastal locations while a shortwave rotates across
Southcentral on Sunday, which will increase precipitation
chances. This means that many valley areas will see measurable
rainfall on Sunday.

By Monday, the remnants of this system will linger over the Gulf
Coast and slowly shift off to the east. Clouds will persist for
interior locations with cool temperatures, and rain will continue
for eastern Prince William Sound and the Copper River Basin.

Tuesday expect a dry and warmer day as clouds depart with
afternoon showers possible mainly over higher terrain. There is
some uncertainty with the forecast on Tuesday as the GFS wants to
keep the area cloudy with widespread showers as it pushes an
upper level low inland while all the other models bring a ridge of
high pressure over the area. For now it looks like the GFS is the
outlier and we prefer the warmer and drier solutions.

&&

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

The latest in a series of easterly waves is moving across
Southwest Alaska today. It is producing showers as it goes with
isolated thunderstorms still possible this afternoon. Plenty of
cloud cover today will inhibit convective development, but there
are enough breaks in the clouds that we can still expect some
storms to fire. Going forward the next few days, weather will
remain unsettled the next few days with low pressure in the area.
Rain showers are the result, with no thunderstorms expected.
Models have been consistent in forecasting this pattern so
confidence is high and little change was made to the forecast
today.

&&

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

Little change in the forecast today. The forecast is panning out
as expected and we have high confidence in model guidance going
forward. Fog/low stratus will continue for much of the Bering and
and Aleutians. Low pressure is moving into the Western Aleutians
as anticipated today and will track very slowly east across the
Aleutians the next couple of days. A weak and quick moving low
will also be present in Bristol Bay tomorrow into tomorrow night.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...

An active weather pattern is expected as a Western Aleutian low
merges with a North Pacific low south of the Aleutians on
Wednesday. This low then moves to the Southern Bering for
Thursday. The storm to keep an eye on is the one moving up from
the Pacific during the day Wednesday. There remains high
uncertainty in the track of this low. It could end up anywhere
from the western Gulf/Kodiak to the southern Alaska Peninsula. On
the other hand, everything is pointing toward an unseasonably
deep low. Thus, widespread gale force winds are likely along the
path of this storm. At this point, the most likely areas to see
gales are the northern to western Gulf coast and Kodiak Island
waters as well as along the Alaska Peninsula. There is an outside
chance of some localized storm force winds through channeled
areas. Gale force northerlies are also possible on the back side
of the merging lows on Thursday, primarily over the central Bering
down to the Aleutians. More likely though, most areas will max
out at small craft advisory level winds.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through Saturday)...

A strong upper level jet stretching across the North Pacific will
lead to a broad 500 mb circulation centered south of the Aleutians
on Wednesday. The deterministic GFS shows the strongest associated
surface low at approximately 970 mb tracking into the western
Gulf by Wednesday afternoon. Although other models/ensembles are
slightly weaker, the overall outcome would be the same given a
large high pressure system in the east-central Pacific leading to
a long southerly fetch of moisture advecting into the Gulf between
these two features. As such, locally heavy rain is possible
Wednesday into Thursday across Kodiak Island and the southern
Gulf. Given the position of the aforementioned high pressure,
winds look to be oriented more easterly (would be more southerly
if the high were centered farther north and slightly west). Thus
Seward and areas over the western Kenai would be downsloped and
see lower total rainfall. Still, conditions will be quite wet
along the Gulf coast/Prince William Sound, with some light rain
occasionally spreading inland across southern Alaska. Overall,
this is a cloudy and cool pattern for much of the area.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...

PUBLIC...NONE.
MARINE...Gale: 119 120 131 138 175-178 411.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BJB
MARINE/LONG TERM...BL/KZ


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