Gale Warning 456, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 AM AKDT Fri Aug 10 2018

.ANALYSIS AND UPPER LEVELS...
The trough that had been bringing rain to Southcentral Alaska has
now pushed off to the east and is replaced by ridging that has
moved in over the southern mainland. This ridging has helped to
clear out sky cover over much of the southern mainland and
brought gap winds to areas along the Aleutian Range. Near the
Pribilof Islands, a system is tracking to the northeast with its
front draped across the eastern Aleutians.

&&

.MODEL DISCUSSION...
The models are in good agreement with the system currently in the
Bering but are still having some struggles with a low moving into
the area Saturday. The second low is the remnants of Typhoon
Shanshan and while the models are in better agreement than past
days, there remains some minor timing differences. However, with
the 00Z EC run falling in line with the 00Z GFS there is higher
confidence in this track.

&&

.AVIATION...
PANC...VFR conditions will persist. Light winds will last into the
afternoon before increasing out of a south to southeast direction
as the pressure gradient increases due to the Bering low moving
toward southwest Alaska and ridging along the Gulf coast.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A flat progressive upper level ridge will cross overhead of
Southcentral today, providing some sunshine and warmer temperatures
for most areas. However, a storm system is just upstream, with a
surface low and upper level short-wave crossing the eastern Bering
Sea. A leading surface warm front will cross the western Gulf
today, then lift to the north Gulf coast tonight, with rain
spreading from Kodiak Island to Prince William Sound. At the same
time, the upper short-wave will take a further north track across
Bristol Bay today followed by interior Southcentral tonight into
Saturday morning. The combination of the two features will bring
widespread rain to Southcentral tonight through Saturday morning.
There will also be a period of gusty southeasterly gap winds ahead
of the warm front this afternoon and evening.

Following this system, attention will shift to a line of upper
waves and warm/moist tropical air moving up from the North
Pacific. With no specific strong wave to lock on to, it creates
quite the challenge for timing of rain moving back into
Southcentral on Saturday. There should be a brief break behind the
departing system, with rain likely overspreading the region again
sometime during the afternoon or evening hours. Meanwhile, a low
will intensify over the North Pacific Saturday and track northward
into the eastern Bering Sea Saturday night, setting the stage for
more rain and wind as we continue through the remainder of the
weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Periods of wet weather are expected across Southwest Alaska as two
areas of low pressure impact the region through the weekend.
Temperatures overnight have cooled into the mid to upper 40s
across the interior where a weak ridge has helped to reduce the
cloud cover. Along the coast, clouds and precipitation continue to
build, with a few radar returns showing up from Cape Newenham to
Pilot Point. The first low has slowed a bit in its forward push
toward the coast. Nonetheless, rain will overspread the coast this
morning and push inland throughout the day. As the low treks
east, southerly winds will increase across the Southwest,
especially through gap locations. Rains and winds weaken Friday
night into Saturday as the low fills in over the Kuskokwim Delta.

The warm front associated with the next low will bring rainfall to
the coast by Saturday afternoon and across the interior of
Southwest Alaska by late Saturday evening. Any brief relaxation
of the winds Saturday will end during the evening as the low
approaches. Rainfall will increase in intensity through the
nighttime hours, with the heaviest rains along the upslope sides
of the Kuskokwim Mountains and Aleutian Range. Downsloping on the
lee side of these ranges will limit some of the precipitation on
Sunday, but locations along the mountain gaps will experience the
strongest winds. Some of these gusts could approach the 50-60 mph
range out of Kamishak Gap on Sunday. The low moves north to near
Cape Newenham Sunday evening, but strong winds and locally heavy
rains will continue over nearly all of Southwest Alaska into
Monday. Minor coastal erosion may be a concern Sunday night and
Monday night from Cape Newenham to Kuskokwim Creek and from Port
Moller to Pilot Point as strong northerly winds could push water
levels one to two feet above astronomical high tide.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

Low pressure situated between Adak and St. George will trek
across the Pribilofs today and into the eastern Bering by late
Friday afternoon. This system will weaken and fill as it moves
onshore over the lower Y-K Delta Friday night.

A second, much stronger low will develop southwest of Dutch
Harbor on Saturday morning. This storm will rapidly intensify
under a strengthening upper level jet, reaching peak intensity
late Saturday evening as it crosses the Aleutians near Dutch
Harbor and enters the Bering. This system is expected to produce
strong gale force winds of 40-50 mph with gusts as high as 65 mph
Saturday afternoon through Sunday morning with the strongest
gusts on the Pacific side of the eastern Aleutians, southern
AKPEN, and the far eastern Bering into Bristol Bay. Heavy rains
are also expected for most areas around the center of the low. The
low will maintain its strength through Sunday afternoon as it
approaches the Kuskokwim Delta coastline. As the low moves toward
the coast, strong northerly winds with gusts near 50 to 60 mph
will develop and wrap around the low. The low will begin to weaken
as it moves inland Sunday evening, but the strong north to
northwest winds aided by cold air advection aloft will continue
throughout the eastern Bering into early Monday. Farther west,
high pressure builds across the western Bering and Aleutians for
Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday night through
Thursday)...

The long term forecast begins Sunday night with a 980 mb low,
enhanced with the remnants of Typhoon Shanshan, centered over the
Kuskokwim Delta coast moving further inland to the north. Models
are generally in decent agreement with the low spreading a large
swath of heavy rain across most of the southern mainland Sunday
into Monday with associated strong winds impacting most of the
region, especially along coastal areas. The low track is taking a
path that is typically conducive for strong winds through
Turnagain Arm and higher elevations along the Chugach front, as a
front sweeps north Sunday and induces pressure rises through
Prince William Sound, locally enhancing the pressure gradient
through Turnagain Arm and southcentral. There are still some model
discrepancies regarding timing and strength, but strongest winds
look to be sometime between Sunday afternoon and Monday morning.
Stay tuned for additional details.

On Monday, the low looks to stall out over the western mainland,
though models begin to diverge as to where the low ends up, with
the EC looping the low back south into Bristol Bay and eventually
weakening and ending up in the Gulf of Alaska by mid week. The GFS
and Canadian models keep the low stalled out over the western
mainland until slowly moving it southeast over southcentral and
into the gulf. Either way, cool and rainy weather is expected
through at least the first half of next week as the area will
continue to be under the influence of the low.

Looking into the end of next week, deterministic models become
unhinged with little model to model consistency or even run to run
consistency, with the EC showing a large high amplitude ridge
building across the Eastern Bering while the GFS shows another
storm system moving into the region. Until models show better
agreement, little changes will be made to the long term forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...130.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...KVP