Gale Warning 522, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKDT Thu Oct 12 2017

.ANALYSIS AND UPPER LEVELS...
The Bering Sea has a storm force 959 mb low tracking towards
the Gulf of Anadyr with an associated occluded weather front
pushing into the Southwest Alaska region this morning. This system
is vertically stacked with a closed low at 500 mb with a trough
extending southward into the central Bering before crossing
through the Central Aleutians. The strongest winds are occurring
on the backside of this strong Bering low, and coincides with the
Scatterometer Wind imagery showing gale-storm force winds over
the aforementioned region. While the satellite imagery is
reflecting a large swath of clouds extending from the Eastern
Bering/Aleutians with moisture spreading through the Southwest
Alaska region northward into the central interior. Meanwhile, the
Southcentral Alaska area has an amplified ridge extending from the
Gulf of Alaska, and northward into Barrow. This synoptic feature
is holding off precipitation briefly over the Anchorage Bowl area
before an upper level weak shortwave moves through this morning.
This opens the door for predominantly southwesterly flow
throughout the atmosphere, and combined with an upstream major
shortwave ushers in widespread rain to the Southcentral Alaska
region by late this morning.

&&

.MODEL DISCUSSION...
Models look to be in better agreement with the storm force low
tracking towards the Gulf of Anadyr. There is another wave of
energy developing near the Central Aleutians by Friday. The GFS is
taking the middle ground with the NAM pushing the low farther
north, and the ECMWF depicting the system farther south. Look for
this system to move quickly eastward before exiting into the lower
Gulf of Alaska by Saturday. The forecast confidence starts out
above average, but dips below average as the models struggle with
next synoptic system.

&&

.AVIATION...
PANC...VFR will be the predominant conditions before the ceilings
begin to transition to MVFR by late this afternoon as the bulk of
the rain moves into the Anchorage Bowl. Look for a few hours of
LLWS this afternoon as the Turnagain Arm winds strengthen.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The forecast remains largely on track for a widespread precipitation
event across Southcentral as a frontal system and upper trough
moves in from the west today through tonight. Development of a
surface low over the northwest Gulf today combined with a cold
front moving in from the west will minimize warming over interior
Southcentral. Thus, have trended precipitation-type toward more
snow over the northern Susitna Valley, and the Copper River
Basin. Though temperatures will remain near freezing, could end up
with several inches of slushy wet snow up along the Alaska Range
by the time this storm exits Friday night.

There are some differences in the orientation and amplitude of
an upper short-wave moving through on Friday. However, it is
appearing more, and more likely that the west side of the Chugach
and Talkeetna Mountains will see a period of heavier precipitation
Friday morning. Thus, expect snow levels to steadily drop, with
snow extending lower down the mountains. Precipitation-type near
sea level will remain safely in the form of rain.

Weak cold advection behind this system will produce some gusty
winds Friday through Friday night, primarily in southern Cook
Inlet, Kachemak Bay, and the northern Gulf. More localized gap
winds can also be expected in western Prince William Sound.

Fast flow aloft will bring yet another storm into the Gulf late
Friday night into Saturday. Based on the recent trend toward a
further south track, impacts from this will likely be focused
along coastal areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The front currently moving through Southwest Alaska will keep
steady rain, and gusty southerly winds over much of the region
today as it continues to push eastward towards the Alaska and
Aleutian Ranges. Partial clearing will occur from west to east
during the afternoon in the wake of the front, leaving a
seasonably cool and showery regime in place through Friday as
broad southwest flow once again becomes focused over the region.
This southwest flow will also keep heavier surf along portions of
the Kuskokwim Delta coastline through this evening. Steady rain
may return to parts of Bristol Bay Friday night as the next low to
impact the Bering moves along or near the Alaska Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The sprawling low over the far western Bering has now reached
peak intensity, and will gradually weaken through Friday morning
as it parallels the Russian coast and moves into the Gulf of
Anadyr. The swath of storm force winds over the southwest Bering
will also weaken over the course of the day, however gale force
winds and high seas will continue to impact much of the western
and northern Bering through Friday morning. The associated front
is in the process of clearing the Akpen as it pushes eastward over
Southwest AK. This will leave much of the Bering in a widespread
showery regime in cool westerly flow through Friday morning.
Meanwhile, the next system to impact the Bering will rapidly begin
to take shape this evening as a North Pacific disturbance riding
along the base of the longwave trough helps to spin up a rather
compact surface low in the vicinity of Adak by Friday morning.
This feature will then track rapidly eastward along the Aleutian
Chain, and the Akpen through Friday night, bringing another quick
shot of rain to these areas. A small area of storm force winds
will also be possible on the backside of the low as it moves over
the Eastern Aleutians Friday evening.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Saturday, the models have a low near the Akpen. There have been
some major model discrepancies with the track of the low. The
ECMWF has brought the surface low south of the Akpen while the
GFS has kept it well north of the Akpen. The NAM is trending
slightly more to the south. There is still a lot of uncertainty
with this storm, and it will impact everything from potential
storm surge issues to rainfall amounts. Given the vagaries of the
models, expect more updates associated with this system. Next
week, the pattern stays very active across the Bering Sea. As the
zonal jet continues, expect new systems to be steered across the
Akpen, and into the Gulf. This would result in calmer and clearer
weather across the Mainland. The models are picking up on colder
air pooling up over the interior, and spilling over the Alaska
Range. Increasing confidence that there will be a cold air
outbreak by the middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 411 413.
         Gale Warning 119 120 170 172 173 174 175 176 177 178 179
                      180 181 185 412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...PS