Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 170722 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
1022 PM AKST Sun Feb 16 2020

.ANALYSIS AND UPPER LEVELS...
The jet stream is in the process of amplifying and extends
northeastward across the Aleutians into the southwest mainland
before cutting southeastward to over the eastern Gulf and into the
North Pacific. The amplifying jet stream will allow for the
predominant low-track to be across the eastern Bering/Aleutians
and into the southwestern mainland. A complex low pressure system
with its dominant area of circulation located 100 nm east of
Shemya continues to move eastward as this low is being steered by
the aforementioned jet stream. Upper-level ridging currently
extends over the Alaska mainland with the surface high centered in
Yukon Territory. Radar imagery shows a band of light snow
currently moving northeastward along the AKPEN.

&&

.MODEL DISCUSSION...
Models remain in good agreement with the overall synoptic pattern
through Monday afternoon. Models begin to diverge over the North
Pacific Monday night with the next low pressure system that is
expected to move into the central Gulf. Models are similar with
the general track of this low but significantly vary in the
intensity of the low. Tuesday onward, models show significant
differences with the intensity and placement of lows, thus
forecast confidence remains low at this point.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. Conditions could drop to MVFR in light snow later tonight
and continue into the late the morning hours. VFR conditions are
expected beginning late Monday morning, but LLWS should develop as
winds aloft increase.

&&

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

A weak warm front is sliding across the southwest Gulf of AK this
evening. This will bring a quick shot of snow to Kodiak Island
before it transitions to all rain. This front will then make its
way into the southern Kenai Peninsula later tonight before
pushing into the Prince William Sound by Mon morning. As it makes
landfall along the coast, precipitation will initially start as
snow with some light accumulations expected. There could be
pockets of blowing snow through the Portage Valley and Turnagain
Pass on the leading edge of this front early Monday. However, as
the winds push in, warm air will accompany them. This should cause
the snow to mix with rain, especially at lower elevations and
coastal sites. Further inland over the Susitna Valley, the air
will remain cold enough to support all snow.

A strong developing low over the eastern Bering will propagate
another front on a nearly identical track across the Kenai
Peninsula and into Prince William Sound. There will be great
amounts of precipitation for many of the coastal locations due to
the abundant moisture and deep southerly flow. This system will
bring strong southeast winds to the Turnagain Arm, the Anchorage
Hillside and parts of the Matanuska Valley. These winds are
expected to push in during the day Monday and eventually into the
Anchorage Bowl late Monday night. Warmer temperatures will
accompany these winds as they serve to mix down some of the
warmer air aloft. The winds will also serve to downslope many of
these same areas keeping them mostly dry. The western Susitna
Valley will remain cold air, however, as southerly flow will
create impressive snow rates. Several inches of new snow
accumulation are expected there through Tuesday morning.

Following this, a cold front will enter the area from southwest
Alaska late Monday into early Tuesday. Deep southerly flow will
allow for widespread moderate to heavy precipitation to continue
across much of Southcentral, especially in coastal areas. As the
cold air moves in, this should change all of the precipitation to
snow. There could be significant accumulations through Turnagain
Pass, the Susitna Valley and Thompson Pass. Winds will remain
gusty in some locations, so blowing snow will also remain a
threat. This setup will also provide the Anchorage Bowl its best
chance for accumulating snow. For now, winter weather products
were issued for the Susitna Valley and Thompson Pass for
conditions on Tuesday. Stay tuned for forecast updates.

&&

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

A broad upper low over the western Aleutians will be the general
focus in the short term. A surface low in the eastern Bering
associated with a shortwave of the parent low is swinging a warm
front across the Southwest Mainland. Snow has started along the
Bering Sea coast, from Toksook Bay to Egegik. South flow behind
the front will transition precipitation to rain or a rain/snow mix
in the low lying parts of the Greater Bristol Bay area tonight.

Over the Kuskokwim Delta, a Blizzard Warning is in effect starting
at 6 PM today for areas west of Bethel. With deep cold air in place
and steady gradient-driven offshore flow, expect significant
visibility impacts particularly to the Kuskokwim Delta coast north
of Kipnuk. Conditions will briefly improve late tonight. Another
secondary low currently centered roughly 140 miles south of the
central Aleutians will quickly track north into the Bering Sea.
This will bring another healthy round of precipitation Monday to
the Southwest Mainland, as snow or a rain/snow mix in the Bristol
Bay area and snow in the Kuskokwim Valley/Delta. Blizzard
conditions will return to the Delta by mid-morning. With this
second punch, winds will likely be stronger and snow heavier.
Gusts up to 45 mph are expected along the coast, with 4 to 6
inches of snow in the Kuskokwim Delta. Again, the worst conditions
will be along the coast, but there will be potential for blizzard
or near blizzard conditions farther inland (e.g., Bethel) Monday
morning. Meanwhile, as the low continues its track north, cold air
wrapped around the backside will bring a return to snow to the
Bristol Bay area. This changeover should occur Monday
afternoon/evening.

The low will weaken and shift inland Tuesday, and surface winds will
diminish and become more southwesterly. Snow showers will persist
over the entire Southwest Mainland, but precipitation rates will
have a significant drop-off.

&&

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

A broad upper low over the western Aleutians will continue to
bring snow showers and gales to the western Aleutians tonight.
Meanwhile, a secondary low centered over the Pribilof Islands
will continue to bring snow and increased winds to the Pribilof
Islands/eastern Bering. Snow may briefly change over to a
rain/snow mix, but should switch back to snow this evening as the
low tracks north. Another secondary low currently located roughly
140 miles south of the central Aleutians will quickly track north
late tonight. Expect gales along its south side... with an
embedded swath of storms force winds. A High Wind Warning is in
effect for the AKPEN, including Cold Bay and King Cove for late
tonight. This low will bring a healthy shot of precipitation to
the eastern Aleutians and AKPEN. The low will track east of the
Pribilof Islands, triggering another round of precipitation early
Monday morning which looks to be all snow. While much will depend
on the exact track of the low (which has some uncertainty), it`s
looking more and more likely that the heaviest snow and strongest
winds will stay west of the Pribilof Islands. So, while there may
be periods of reduced visibility in blowing snow, significant
impacts look unlikely at this time. However, if the low tracks
farther east than expected this could change. The ensemble spread
indicates that this feature bears watching.

In any case, as the low weakens and tracks northeast towards the
mainland, expect broad north flow over the Bering Tuesday, the
area of gales on the backside of the low diminishing throughout
the day.

&&

.MARINE (Days 3 through 5 Wed through Fri )...
A low over the western Aleutians Wednesday morning will slide
eastward, reaching the Gulf Thursday night and Friday. Gale
force winds are expected with this system. Some storm force winds
are possibly Wednesday and Wednesday night around the Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7 Thu through Sun)...
On Thursday morning the pattern will consist of an upper low over
the Bering Sea, with an associated trough extending southeastward
across southwest Alaska. The trough will very slowly slide
northeast across southern Alaska through Sunday as the main upper
low retreats northward. An upper level Ridge is expected to move
into the Aleutians and southern Bering as the low moves north on
Friday and Saturday. An upper level low just west of the Aleutians
on Friday will move into the western and central Aleutians Friday
night through Sunday, though the exact path and timing of this
feature is somewhat uncertain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155.
         High Wind Warning 181.
         Winter Storm Watch 131.
MARINE...Gale 119 125 130-132 150 160 171 173 174 175 177-185.
         Storm 155 165 170 172.
         Heavy Freezing Spray 139 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...CK/KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...MTL



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