Heavy Freezing Spray Warning 158, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
637 AM AKDT Fri Oct 29 2021

Correction made to include WWAs.

.ANALYSIS AND UPPER LEVELS...

An upper level shortwave is moving across the northern Susitna
Valley and the Copper River Basin with a few lingering snow
showers that should end this morning. Another low is sitting near
the Bering Strait with a shortwave rotating into western Alaska
however this feature will not affect Southwest Alaska. A deepening
low pressure system is producing storm force winds south of the
AKPEN, and producing rain from Unalaska towards Cold Bay and Sand
Point. Radar shows a few rain and snow showers across Prince
William Sound as a weakening surface low moves across the area. A
few showers are seen over Cook Inlet west of the Kenai Peninsula,
otherwise quiet elsewhere. Temperatures are in the teens across
Southwest Alaska with 20s across the Copper River Basin and upper
20s to low 30s from the Kenai Peninsula up to Anchorage and the
Mat-Su valleys. And mid to upper 30s across the Pribilofs and
Aleutians, with 20s and 30s across the Alaska Peninsula.

&&

.MODEL DISCUSSION...

Models are in good agreement. However, there are some minor
differences in the evolution of the low pressure system south of
the Alaska Peninsula and the large scale trough extending from
the southern Gulf through the eastern Bering and Southwest Alaska.
The biggest forecast challenges will be timing of the change from
snow to freezing rain and rain across Bristol Bay and the Lower
Kuskokwim Valley tonight through Saturday and when the strong
winds will move across the Anchorage Bowl Saturday afternoon and
evening.

&&

.AVIATION...
PANC...Light winds with VFR conditions will persist through this
morning with a few lingering low clouds around the terminal,
mainly over Cook Inlet. Northerly winds will increase around noon
today as a down inlet flow develops in response to a strengthening
storm system near Kodiak Island. The flow aloft will quickly turn
southeast and increase as a front approaches the Southcentral
coast. The combination of northerly surface winds and
southeasterly winds aloft will introduce wind shear over the
terminal by mid afternoon. Wind shear is expected to increase
through the late afternoon and evening hours as winds aloft
increase. Wind shear will persist through early Saturday afternoon
before gusty southeasterly Turnagain Arm winds are expected to
bend over the terminal. Periods of sprinkles or light rain are
possible by late Friday evening. Ceilings should remain at or
above 5,000 ft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Sunday morning)...

An upper-level wave draped across the Southcentral coast this
morning will continue to lift north toward the interior. An
associated surface low that spun up in Cook Inlet yesterday
evening has now weakened as it moved north over the southern
Susitna Valley overnight. A few lingering snow showers persist
near Kenai this morning, but will diminish as the upper-level
wave moves north.

The upper-level pattern will amplify in the wake of the
aforementioned wave as an upper level low south of the Aleutian
Chain moves east toward the western Gulf. The upper-level ridge
downstream will build from the Pacific Northwest to the Yukon,
causing the upper-level low to stall and eventually move north
toward the AKPen by Saturday. This feature will weaken Sunday as
it elongates. However, the overall pattern will remain stagnant
with Southcentral Alaska remaining under the influence of a moist,
southerly flow ahead of the upper-level trough.

At the surface, an associated area of deepening low pressure will
move to an area well south of the AKPen. Its front will move from
the western Gulf with rain and gusty east winds developing over
Kodiak Island Friday morning. The front will move to the northern
Gulf by late Friday with a deep, tropical fetch of moisture
lifting toward the Kenai Peninsula. This atmospheric river will
bring copious amounts of rain and high elevation (above 3,000 ft)
snow to the eastern half of the Peninsula and Prince William
Sound through Sunday as the low continues to strengthen as it
slowly drifts north toward the AKPen. In addition to the
precipitation, the combination of the building coastal ridge and
the deepening low will induce strong southeasterly gap and
mountaintop winds. These winds are expected to increase through
Friday night and peak Saturday morning before slowly winding down
late Saturday evening into Sunday morning. Winds may gust to as
high as 90 mph across Turnagain Arm and the Upper Hillside of
Anchorage and as high as 80 mph through Portage Valley. Anchorage
and Palmer could also see a period of gusty winds by late Saturday
afternoon as a trough extending across Southwest Alaska helps to
reorient the pressure gradient in a way to draw the gusty winds
across the Anchorage Bowl and Knik River Valley. The strong
southeasterly flow aloft will help limit precipitation for the lee
of the mountains; however, upper-level waves embedded within the
southerly flow along with the amount of moisture being drawn
northward with both work to negate the downsloping from time to
time, especially by Saturday afternoon.

The other main impact from this system will be the potential for
flooding, especially for areas around Seward and along the Anchor
River due to prolonged heavy rain and runoff from snowmelt.
Seward could see upwards of 5 to 8 inches of rain with higher
amounts at elevation, especially along the windward side of the
Kenai and Chugach Mountains due to orographic enhancement. The
rain will finally taper off by Monday morning as the upper-level
trough retreats to the west and the ridge downstream extends
across the northern Gulf.

&&

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

The longwave pattern is highly amplified this forecast period.
There is a surface low south of Cold Bay this morning that has a
fetch of modified tropical moisture wrapped into it. This low will
continue to deepen as it moves eastward and an inverted trough
will set up over the AKPen and Southwest Alaska. The
precipitation shield will impact the AKPen by noon today. The
expectation is that this low will bring a wintry mix due to the
fact that a cold airmass has been in place over SW AK and the low
has a subtropical origin. Expect snow, freezing rain (possibly
sleet), and rain. A Winter Weather Advisory has been issued for
this event for zones 152 and 161. Aniak (in 152) will remain all
snow for this event with 8 to 14 inches expected. Whereas
Dillingham (in 161) will see multiple types of precipitation.

&&

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

This morning there is a surface low south of Cold Bay. The front
attached to this mature low is currently moving across the
Aleutian Chain and will continue moving eastward Friday night.
Then the surface low track pivots north. This system is expected
to bring storm force winds to the coastal waters south of the
Eastern Aleutians and southern Alaska Peninsula Friday and Friday
night. Rough seas will accompany this storm. There is a second
surface low in the north Bering that will move towards St Matthew
Island. The models all bring Siberian air over the Bering. Cold
air over relatively warmer air is inherently unstable, showers are
possible.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Aleutians/Bering...A broad area of low pressure over the Bering
through Tuesday. Gusty winds with areas of Westerly gales
associated with a front moving across the Bering through Tuesday.
Northerly gales with the rear flank of the Gulf low linger over
the Eastern Bering and Bristol Bay, diminishing. Seas to 18 feet,
subsiding. Forecast confidence is moderate.

Gulf of Alaska...A strong nearly stationary low over the Western
Gulf brings widespread gales over the Gulf, diminishing Monday.
Seas to 25 feet, subsiding. An area of gales moves over the
Northern Gulf and North Gulf Coast for Tuesday, as the low center
moves towards Kodiak Island. Seas to 22 feet, subsiding.

&&

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

Solutions struggled quite a bit with the placement of the lows,
the distribution of rain and mixed rain/snow, and wind speed for
the long term. However, GFS, GEM and ECMWF highlighted the
potential for increased precipitation and winds over Pribilof
Islands, Southwest, Kodiak Island, Kenai Peninsula, Anchorage
Bowl and Prince William Sound in response to two separate lows and
fronts over the Gulf and Bering Sea in the coming days. Over the
Gulf, a strong North Pacific low and its associated front will
usher strong winds and higher gusts across the western Gulf
through Prince William Sound beginning Mon afternoon into Tue
evening. Interestingly, a ridge over the state could weaken the
expansion of clouds and rain over Southcentral on Mon, creating
dry downsloping to the north of Anchorage Bowl and across Copper
River Basin into Mon night. Meanwhile, a developing low near the
central Aleutians Tue afternoon will build across the eastern
Bering, causing strong winds and increased precipitation across
Southwest Coast, Pribilof Islands and the Aleutian Chain into Thu
afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning...101, 125.
         Winter Weather Advisory...152, 161.
         Flood Watch...121, 125.
         Special Weather Statement...155.
MARINE...Storms...155.
         Gales... 119, 120, 125, 129, 130, 131, 132, 136-139, 141,
                  150, 160, 165, 170-174, 180, 351, 352, 414.
         Marine Weather Statement...177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...MK/CB