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FXAK68 PAFC 221244
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT Sat Sep 22 2018

.ANALYSIS AND UPPER LEVELS...

The occluded front associated with a low just north of Nunivak
Island continues to lift northeast through the interior of
Southwest Alaska this morning with rain stretching from north of
Sleetmute and Lime Village across Norton Sound. Broad west to
southwest winds wrapping underneath this low are gusting to near
40 knots along the west coast with gales across the central Bering
north of the Pribilofs. The cyclonic flow around the low is
continuing to draw moisture into the Southwest, keeping clouds and
low-level moisture in place, while shortwaves ejecting from the
upper-level low stacked upon the surface low are helping enhance
shower activity behind the front.

Along the southcentral coast, an eastward advancing warm front is
pushing low stratus and rain across the region. Light to moderate
rain has been falling all night from the northern Susitna Valley
south and east through the Cook Inlet, Kenai Peninsula, and Prince
William Sound. Farther west, the precipitation is tapering off
along the AKPEN and over Kodiak Island as the trailing cold front
advances east. Southerly gap winds have also continued overnight,
most notably through the Copper River Basin were gusts over 20
knots have been reported. Gusty southeasterly winds along
Turnagain Arm are also continuing, and as of early this morning
have once again moved over the western half of Anchorage with
gusts over 20 knots reported at the airport.

For Anchorage, two notable stretches of weather have come to an
end with this change in the pattern. First, the 0.27" of rain
measured as of 1:00am AKDT at the airport ends the 25 day stretch
of no measurable precipitation for the city (previous measurable
rainfall occurred Aug. 26th). Second, the high temperature of 59
degrees ends the 28 day stretch of daytime highs at 60 degrees or
warmer (previous high temperature below 60 degrees occurred Aug.
23rd).

&&

.MODEL DISCUSSION...

Overall, the models remain in good synoptic agreement regarding
the evolution of the Bering low and the development of a secondary
surface low in the western Gulf through Sunday. Beyond that,
there is still a bit of uncertainty in the timing and placement of
the secondary low as it tracks along the AKPEN and toward the
southern coast. Throughout the day, the NAM had been the outlier
when compared to the GFS and EC, with a more easterly track of the
low into Prince William Sound. However, the latest 06z NAM run
has fallen in line with the GFS and EC and the westerly track
solution. This has marginally increased forecast confidence,
especially with rainfall and increasing southerly gap winds Sunday
and Monday, but there are still some timing issues that need to
be resolved. Most notably, the EC solution continues to be the
slowest in the forward progression of the low.

&&

.AVIATION...
PANC...Generally, VFR conditions will persist through today.
However, ceilings may drop below 5,000 feet for a time this
morning during the heaviest of the rainfall as the front moves
through Anchorage. Gusty southeasterly Turnagain Arm winds have
moved over the airport complex and will continue throughout the
day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Precipitation will diminish from west to east today as a front
over the area progresses east, allowing somewhat drier air to
filter in behind the system. Some showers will remain over the
area through Sunday, mainly north and east of Anchorage as the
flow aloft remains from the southwest and some moisture remains.
Sunday night a strong system will move rapidly north over the
western Gulf. The models are in fairly good agreement that the
surface low will slide from just southwest of Kodiak Island Sunday
evening, up Cook Inlet Monday morning, and then north of the
Susitna valley Monday afternoon. This will bring another round of
rain to the area Sunday night and Monday. There will also be
fairly strong Turnagain Arm/upper hillside winds with this system
beginning Sunday evening. As the low moves north, breezy southerly
winds will push up Cook Inlet Monday afternoon with the rapid
pressure rises behind that low.

&&

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

A surface low over the eastern Bering slowly tracks inland tonight
and then remains nearly stationary through Monday night as the
upper level low remains along the west coast. This keeps a
persistent pattern across the region with ongoing showers and
gusty south to west winds. The one notable difference during this
timeframe will be a trend towards cooler conditions as the low
taps into the cold arctic air. The next system to monitor moves up
to Bristol Bay on Sunday night, spreading moderate rain along
with easterly winds as it races north.

&&

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

Low pressure across the eastern Bering keeps gusty winds across
the northern waters today as it advects a cold air mass toward the
coast. Winds are expected to taper down tonight as the low moves
inland and fills. South to west winds change to the northwest
direction along the western waters late tonight with a ridge
moving up from the North Pacific. The next front to monitor races
up to the Alaska Peninsula early Sunday. This system rapidly
increases winds to gale strength with moderate rainfall along its
boundary.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday Night through Friday)...

Monday night continues to feature a low centered in the Norton
Sound region that will be lifting northward away from the area.
Clearing looks to be gradual behind the low however, with showers
and light rain expected to continue along the area mountains
overnight. Mostly cloudy skies are expected to persist for much of
the day Tuesday, though much of southern mainland Alaska will be
dry for the day. High pressure briefly moves over the area Tuesday
afternoon and night, which will help with some clearing for
inland locations, but the forecast models disagree on how cold the
air mass gets, with the foreign models suggesting warmer
conditions than previous days.

The models have also come in worse agreement on the track and
speed of the next low approaching southern Alaska from the south
Tuesday night. The foreign models are in somewhat better agreement
with each other than the GFS. To favor consistency, the EC and
Canadian solution was chosen, indicating rain at the leading edge
of another warm air mass beginning at the coast early Wednesday
morning, and gradually spreading northward through the day. On
this point, the EC and Canadian solutions begin to diverge, with
the Canadian's more eastward track of the center of the low along
the coast Wednesday night into Thursday suggesting a very wet
period for nearly all of Southern Alaska. Meanwhile the EC now
takes the low west of the Pribilofs, which would mean very little
rain for much of Southcentral. With such disagreement in the
models past Tuesday night, forecast confidence is low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 121.
MARINE...Gale Warning 180 181 414.

&&

$$

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