Area Forecast Discussions

000
FXAK68 PAFC 220143
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKST Thu Nov 21 2019

.ANALYSIS AND UPPER LEVELS...
A pair of 968 mb lows are crossing the western half of the area,
with one low along the western coast and another low near Cold
Bay. Several disturbances are seen in water vapor imagery rounding
the base of a Bering trough before ejecting northeast across the
mainland.

&&

.MODEL DISCUSSION...
The models continue to be in good agreement through the weekend,
with model continuity having improved for next week. In general,
they show an active pattern with today`s bering trough progressing
eastward, with a ridge building over the state early next week,
then another Bering trough developing mid to late week.

&&

.AVIATION...
PANC...Surface winds will continue to oscillate between light
northerly winds and southeast winds gusting to 30 knots, as a
pressure trough is seen sitting a few miles southeast of the
field. LLWS will continue through this evening with winds
subsiding after midnight. VFR conditions are generally expected
through this evening, then dropping to MVFR in rain tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Tonight
through Saturday night)...

The first of two surface fronts currently sits over Prince
William Sound with a second over the central Gulf quickly racing
to catch the first. The result is a steady increase in strong
east to southeast winds across the northern Gulf coast through
the favored channeled gaps. As this second front approaches, the
easterly barrier jet along the coast will ramp up to around 60kts
this evening before quickly tapering off as the front moves
onshore and winds turn southerly.

There is still a bit of uncertainty regarding the potential
development of a triple point low over western Prince William
Sound along the front. Specifically, guidance has been flip-
flopping on whether one will spin up in the aforementioned area.
If it does, it should allow for enough of a bend in the wind to
the northeast to keep warning level winds out of Portage and
Whittier. If not, it is still possible for a few stronger gusts in
these areas this evening than is currently in forecast. Given the
uncertainty, however, the afternoon forecast package for these
locations mirrors the overnight package.

The forecast is also still on track for winds gusting to 75 mph
along Turnagain Arm and the Anchorage Hillside. As of this
afternoon, there have already been gusts in the 70 to 75 mph
range. Warm advection out ahead of the first front as well as
subsidence in the lee of the mountains is still expected to keep
the strongest winds out of Anchorage proper. However, the weak
northerly winds near the airport and northern half of town should
give way to gusty southeasterly winds tonight as the front lifts
north. These winds should die down overnight with the flow
becoming more southwesterly behind the front.

Bouts of heavy precipitation (mainly rain with the warm
southeasterly flow) will also continue across coastal
Southcentral ahead of the front. The exception will be Thompson
Pass, portions of the Copper River Basin, and Broad Pass where the
precipitation will fall as snow. Precipitation will become more
showery behind the front, but with a persistent southwesterly flow
and weak pulses of energy moving across southcentral in the mid
and upper levels, the shower activity will continue into Saturday,
especially over the higher terrain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
South flow aloft ahead of a low centered near the AKPEN/Kodiak
Island has maintained precipitation over the Southwest Mainland
today, primarily in the Bristol Bay area and Alaska/Aleutian
Ranges. As the low progresses eastward and fills, prevailing flow
will shift to north or northwest over the mainland. As modified
Arctic air pushes inland, expect a cooling trend over the next few
days. While there`s no great source of moisture, cold air flowing
over warmer waters will bring shower potential inland. With the
cooling trend, by Saturday any precipitation over land should fall
as snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Elevated winds and seas will persist south of the AKPEN into this
evening. Farther west, persistent cold air advection from north
of the Bering Strait will maintain northerly gales and widespread
small craft advisory level winds, with 10 to 15 foot swells over
much of the Bering. The potential for northerly gap winds out of
the Aleutians is being monitored; right now the prevailing winds
on the north side of the Chain look to have too much of a westerly
component to cause enhanced gap winds. Freezing spray is a
concern along and west of the ice edge (Kuskokwim Delta Coast).
Widespread showers are likely over much of the Bering as cold air
flows over warmer water. This pattern will persist through
Saturday, then a low approaching the western Aleutians will
change things up in the rough Sunday to Monday timeframe.

&&

.MARINE (Days 3 through 5)...
The period begins Sunday evening with weak ridging over the
eastern Bering and a 1006 mb surface low in the vicinity of Prince
William Sound. Further out west in the western Aleutians a gale
force front associated with a 990 mb Kamchatka low is entering the
forecast area. Model agreement is okay for day 3, as GFS and Nam
are in line with some gales in the gaps along the Northwest Gulf
Coast, NAM has a bit weaker solution to winds, but it also puts
the surface low a little further to the east. By Monday afternoon,
gales have pushed offshore into the central Gulf and the low in
the Bering has deepened to 965 mb with a wide area of gale force
to sub-storm force winds in the western half of the Bering and
near gales in the remaining half of the eastern Bering. Models are
pushing a strong high into the southern Gulf Monday into Tuesday
afternoon with values of 1040 mb, this feature is also acting as a
block on the Bering low which appears to track northeast into
Siberia. Its front will still produce storm force winds in the
northern Bering Tuesday morning with strong gales in the coastal
waters of southwest Alaska. By Tuesday evening winds subside in
the eastern Bering and we`re left with an area of high pressure
over much of southcentral Alaska. In the beyond it will be of
interest how much influence the high pressure in the north Pacific
exerts and whether we see a blocking pattern set up like we had
in previous weeks.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The upper level trough over Southwest Alaska will shift east into
Southcentral Alaska Sunday through Sunday night spreading a
significantly colder air mass over southern Alaska. In response to
this and a leeside trough forming in the northern Gulf offshore
flow will increase rapidly over Southcentral Alaska Sunday night
and continue through Monday. Winds will be strongest through the
typical favored terrain gaps with northerly winds through Seward
and Resurrection Bay, westerly winds through Whittier and Passage
Canal and north to northerly winds through Thompson Pass, the
Valdez area and across the Copper River Delta.

The next frontal system associated with a Kamchatka low will push
into the western Aleutians Sunday morning followed quickly by a
second stronger storm force front Sunday night. This second very
large front will progress across the western and central Bering
Monday as the initial front moves through the eastern Bering to
reach the Southwest Alaska coastline by Monday afternoon as it
merges with the warm front of the larger system. The warm front
will move onshore spreading abundant precipitation across
Southwest Alaska Monday night through Tuesday while the cold
frontal portion of the storm tracks from the central Bering into
the eastern Bering. With a warm, rather moist air mass overrunning
the low level cold air in place in Southwest Alaska this is going
to be another system to watch closely for significant winter
weather impacts and a wide variety of precipitation types
possible.

A north Pacific frontal wave approaching the eastern Aleutians
and Alaska Peninsula from the south on Tuesday will phase with the
upper level trough associated with the cold front in the Bering.
This interaction will spin up a new closed low south of the Alaska
Peninsula on Wednesday as the leading edge of the front pushes
pushes across Kodiak Island and the western Gulf. As the frontal
system spreads abundant precipitation across Southcentral Alaska
Wednesday afternoon through Wednesday night, the surface low will
continue to the northeast to reach Prince William Sound by early
Thursday morning. While model confidence remains low with today`s
runs tracking this surface low significantly further to the north
than yesterday`s, if more northerly track holds this storm will
bear very close watching for potential mixed wintery impacts to
Southcentral Alaska. An initial weaker overrunning front Tuesday
night or Wednesday following the rather cold temperatures Monday
and Tuesday is a possible freezing rain concern with the stronger
and moister front arriving Wednesday or Wednesday night bringing
the possibility of heavy rain to the coast and heavy snow inland.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101.
Winter Weather Advisory 145.
MARINE...Storm Warning 119 125.
Gale Warning 131 132 138 150 155 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...LB/JR