Small Craft Advisory 129, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
534 AM AKST Sat Feb 8 2020

.ANALYSIS AND UPPER LEVELS...

The main weather focus is a hurricane-force 963 mb low north of
Nikolski. This feature is currently at peak intensity but is
beginning to show signs of a transition to an occluded system as
colder and drier air wraps in and the upper-level low catches up
with the surface low. This is evident in the significant dry slot
developing across the AKPEN overnight and wrapping back into the
center of circulation along the occluded front. That said, this
is still quite a powerful storm system with 60 kt sustained winds
wrapping underneath the low and hurricane-force wind gusts, as
indicated on the latest ASCAT pass and surface observations from
Nikolski and Akun, respectively. An occluded front sweeping toward
the Southwest Alaska coast and bending back across the Pribilofs
to the center of the low is bringing snow and gusty easterly winds
across interior Bristol Bay, rain with southerly winds along the
immediate Bristol Bay coast, and blizzard conditions with wind
gusts in excess of 60 mph for the Pribilof Islands. Blizzard
conditions with wind chills of 30 to 50 below zero are also
occurring across the Kuskokwim Delta, mainly west of Bethel, due
in part to the arctic airmass in place and strong northeasterly
winds out ahead of the front. Over the AKPEN and eastern
Aleutians, southerly winds are increasing above 40 kts as the low
approaches from the west. Elsewhere out west, a front extends
across the western Aleutians with easterly gales and a mix of rain
and snow.

For Southcentral, a warm front associated with the aforementioned
low is lifting north across southern Cook Inlet this morning with
areas of mixed rain and snow from Kenai south. Southeasterly gap
winds, specifically through Turnagain Arm, are also increasing
ahead of this front as the pressure gradient the low a ridge
across the northern Gulf coast tightens. This has prompted
issuance of a High Wind Warning for Turnagain Arm and higher
elevations through the morning hours. Kodiak Island currently sits
south of the warm front, in the warm sector out ahead of the cold
front moving into the western Gulf. Light snow has also developed
overnight across the northern Susitna Valley as a moist,
southerly flow is overrunning the colder air at the surface.
Scattered rain and snow showers across portions of Prince William
Sound and the Copper River Delta are giving way to the larger
precipitation shield ahead of the front as seen on by the
Middleton Island radar.

Numerous advisories and warnings have been issued across the
Southern Mainland and Bering/Aleutians covering various weather
hazards associated with this storm system. Please see the forecast
discussions and Watches/Warning/Advisory list further down this
page for specific details.

&&

.MODEL DISCUSSION...

Model agreement in the short term remains consistent with all
models continuing to handle the evolution of the upper-level low
as it moves over Bristol Bay today and lifts north, phasing with a
second low near the Seward Peninsula. As for the surface low
currently north of Nikolski, guidance has been faster with the
timing of the system as it lifts onshore this afternoon. The 00z
EC was the fastest, with the NAM and GFS slower, and the GEM
slowest. However, the 06z NAM sped up its timing with the 06z GFS
following suit, closely matching the EC. In addition, guidance is
in generally good agreement with a second storm-force Pacific low
and associated front moving across the Aleutians, although there
are differences in strength and timing. Thus, forecast confidence
is average through the weekend.

&&

.AVIATION...

PANC...VFR conditions are expected through late morning. Low level
wind shear is a concern through the first half of the morning as
a strong southeasterly low-level jet will provide significantly
higher winds at 2,000ft than at the surface. Through the morning,
the surface southeasterly Turnagain Arm wind will increase and
bend over the airport complex ahead of a frontal system. As the
front pushes over Anchorage late morning, the southeasterly winds
will increase with gusts to 40 kts over the aerodrome. This will
potentially negate any speed shear concerns; however, MVFR
ceilings and visibilities are expected as some precipitation moves
through. Directional low-level wind shear is then expected this
afternoon as gusty southeasterly surface winds continue but winds
just above the surface turn southwesterly behind the front. Gusty
southerly winds with VFR conditions are then expected to persist
through the remainder of the TAF package.

&&

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

Fairly unsettled weather is expected across Southcentral over
the next 24 hours. The culprits, a warm occluded front moving
rapidly northeast across Southcentral today and the trailing
hurricane force low moving onto the Southwest Mainland. The front
spreads rain and snow across the coastal areas to as far north as
the Matanuska Valley and southern Susitna Valley today. Moist
southerly flow will sets up in the low to mid levels today which
will produce widespread snow to much of the Susitna Valley and
Talkeetna Mountains(including Hatchers Pass). The Susitna Valley
north of Talkeetna is expected to get 10 to 20 inches of snow
before it tapers off Sunday. Areas along the Talkeetna Mountains
and Hatcher Pass could see 2 ft or more. Broad southwesterly flow
continues aloft through Sunday, so upslope areas along the
mountains across Southcentral could see some continued snow
shower actively.

In addition to precipitation, many marine and land areas will
experience fairly gusty winds. East to west pressure gradients
will peak this morning across Turnagain Arm just prior to frontal
passage and produce a short period of warning level southeasterly
winds this morning. As the Turnagain winds weaken, Gusty
southerly winds will spread across Anchorage Bowl. There will also
be fairly gusty gap flows out of the Knik River Valley and along
the Copper River. Winds region wide will slowly diminish overnight
and through Sunday. Temperatures in windy areas will tend to be
on the mild side, but will cool off as winds drop and cooler air
spreads back in from the west tonight and Sunday.

&&

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

A powerful storm over the Bering will keep gusty winds, snow and
rain challenges in the forecast through early Sunday. The leading
front associated with this system stalls through today keeping
ongoing snow development over the Kuskokwim Delta and western
areas of Bristol Bay. Strong easterly flow just off the surface
will cause downslope drying across the western Alaska and
Aleutian Ranges. Precipitation is expected to become widespread
across the southwest late this afternoon or early evening as the
storm system over the Bering begins to advance northeast.
Downsloped areas today will see snow spread across from east to
west tonight as flow aloft becomes southerly. Locations along the
Aleutian and Alaska Ranges may see much higher snow amounts than
currently forecast and should check back for the updated forecast
this afternoon as models are starting to show higher snow amounts.
Public products were updated this morning with an extension for
the blizzard warning across the Kuskokwim Delta which also
provides added details. The winter advisories for the Lower Kuskokwim
Valley and Bristol Bay were kept intact. Weather conditions are
expected to improve on Sunday as the storm system weakens on a
northeasterly track across the interior. Cold air advects across
the Southwest with clearing skies and light northerly flow Sunday
night into Monday.

&&

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

The strong storm over the southern Bering will begin to track
northeast during the afternoon. Gusty winds were increased across
the Alaska Peninsula area north into the eastern Bering. Hurricane
force winds were added to marine areas for today on the north and
south side of the Alaska Peninsula. Seas are expected to rapidly
build in response with the highest heights ranging between 25 to
50 ft. The high wind warnings were updated to reflect slightly
higher speeds and the warning for the eastern Aleutians was
adjusted to end the product a little sooner. The storm is expected
to move inland overnight with a short reprieve in the weather for
much of the Bering on Sunday. The exception to this will be
across the Western Aleutians as the next front moves up from the
North Pacific with the boundary moving to Shemya by mid morning.
Precipitation may start as snow, however it should change over to
rain by early afternoon. Storm force easterly winds are expected
with this storm which moves to the western Bering Sunday
afternoon. There is still some model uncertainty on the strength
of this storm and may need some adjustments.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

A high end gale force storm will move through the central and
western Aleutians into the western Bering from late Sunday into
Tuesday. Storm force winds are possible along the frontal
boundaries. There is still considerable uncertainty for the timing
and strength of this system.

For the Gulf, Northwest gale force winds will increase across the
Barren Islands starting Monday afternoon and will continue through
Tuesday. Winds and seas should abate Wednesday and beyond.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through
Friday)...

We start the long term period Sunday with high pressure over the
western part of the state, high pressure south of the Gulf of
Alaska and another storm on tap moving toward the Aleutians and
Bering Sea. Models are in generally good agreement with the
location of the low moving into the Aleutians and the frontal
placement on Monday. However, they differ significantly with the
intensity of this storm. The front is then expected to slowly
track east across the bering and Aleutians through the middle of
the week, before dissipating as it crosses the southwest mainland
Thursday.

This pattern and scenario support the Arctic air pushing back
across the southern mainland, persisting through Thursday, thus
returning the mainland to the colder air, similar to the past few
weeks. The Bering Sea and Aleutians will be in a highly active
weather pattern, with frequent periods of strong winds and
precipitation through the extended period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155 195.
	     High Wind Warning 181 185.
         Winter Weather Advisory 145 152 161.
         High Surf Advisory 161.
MARINE...Hurricane Force Wind Warning 155 165 170 172 174.
         Storm Warning 138 150 160 171 179 180 181 412 414.
         Gale Warning 119 120 125 128 130 131 132 139 140
                      176 177 178 185 351 352 411 413.
         Heavy Freezing Spray Warning 179 180 185 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...SS