Gale Warning 673, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
528 AM AKST Thu Dec 12 2019

.ANALYSIS AND UPPER LEVELS...

The upper level pattern has changed little since yesterday morning
as a large North Pacific occluded cyclone in the Gulf remains the
main driver of sensible weather across most of the region. In
conjunction with a small closed upper high over the northeast
slope of Alaska and a weak cold upper trough over the central
Bering Sea, most of the southern mainland remains under the
influence of broad easterly upper level flow. This pattern is
supporting coastal rain along the Gulf Coast with strong downslope
shadowing across most of the interior locations. The one exception
has been across the Kuskokwim Delta where some light freezing rain
and mixed precipitation is spilling over and downwind of the
Kilbuck Mountains. Otherwise, the brunt of the deformation zone
precipitation has shifted offshore from the Southwest coastline.
The strongest pressure gradient remains over the Eastern
Aleutians/western AKPEN where strong northwest winds gusting as
high as 50-60 MPH continues. Southcentral weather is rather
quiescent with some stratus and fog impacting the Copper Basin as
well above average temps continue to dominate elsewhere.

&&

.MODEL DISCUSSION...
The numerical weather models are in excellent agreement with the
general synoptic pattern and sensible weather impacts through
Saturday. They are also very well clustered with a strong 960s mb
low moving across the Aleutian Chain beginning Friday. Forecast
confidence is high.

&&

.AVIATION...
PANC...VFR conditions and northerly winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Thu and
Fri)...
The pattern continues to be dominated by an upper 960s mb low near
54N due south of Seward. This low is vertically stacked and
continues its slow drift north towards the Gulf Coast. The net
effect is a dry offshore gradient across nearly all inland areas
with continued onshore flow for the Gulf Coast. The result is dry
conditions inland with just some mid and high clouds. The lone
exception to this is over the Copper River Basin where low stratus
and fog set up yesterday morning and continue to plague areas
around Gulkana. It is quite a different story across the Gulf with
widespread steady rain and continued gale force (approximately 40
mph) winds around the low center. Rain will continue from Kodiak
Island up through Seward and into Western Prince William Sound. By
tonight, a weak wave of energy will rotate around the parent low
and through the Copper River Basin. We have increased PoPs to
likely with this feature with decent model agreement. However, we
do not expect more than roughly one inch of snow in any populated
areas.

As the parent low draws closer and closer to the southern mainland
on Fri, there will be increasing chances for some of the
precipitation to drift a bit further inland from the coast. At
this point, it still looks like much of the Cook Inlet will stay
mostly dry due to cross barrier flow. However, parts of the
interior Kenai Peninsula and over towards Thompson Pass will see
light rain by late Fri night. Higher elevations could accumulate a
couple inches of snow as the system drifts inland and weakens into
the weekend.

&&

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

A slow moving area of low pressure over the western Gulf continues
to bring gusty northwesterly winds to the Alaska Peninsula this
morning, with the strongest winds occurring through the usual
gaps and passes. Warm air overrunning colder air in place at the
surface has produced showers across Southwest with some locations
near the Kuskokwim Delta Coast seeing a brief period of freezing
rain. Winds are expected to taper off by early Friday morning as
the Gulf low tracks eastward. A weak ridge of high pressure
briefly builds over the area Friday evening before a gale force
front arrives by Saturday morning.

&&

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

While the eastern Bering remains under the influence of a low
centered in the Gulf, the western Bering continues to see
showery, northerly flow ahead of a strengthening low centered southwest
of Shemya. Gusty northwesterly flow over the eastern Bering will
eventually taper off by the evening as the Gulf low moves east.
Meanwhile, a gale force front associated with a strengthening low
reaches the western Aleutians this afternoon. The low strengthens
and tracks eastward, reaching the central Aleutians by Friday
night. A brief period of blowing snow is possible for Shemya early
on Friday, however mixed precipitation is expected for the
central and eastern Aleutians as warm southerly flow works its way
in Friday night.

&&

.MARINE (Days 3 through 5)...

For the Bering: Weather starts out quiet on Friday with a high
pressure ridge in place over the Bering Sea. On Saturday, a strong
low pressure system will skirt the southern side of the Central Aleutian
Islands as it moves west to east. It will bring gale force winds
to the area as it goes, exiting Sunday. Model agreement is good
with this system and we are confident in the forecast.

For the Gulf of Alaska: A low will be present in the Western Gulf
of Alaska on Friday, bring a large area of small craft advisory
winds with isolated gales to much of the gulf. It will also keep
sea states elevated. The low moves ashore over southwest AK Friday
night, calming winds and waves for Saturday morning. Later Saturday
into Sunday another front enters from the southwest, bringing
gales and small craft advisories back to the gulf. We are confident
in this forecast as model guidance looks to be on track.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Models have come into better, though not perfect, agreement for
the long term. Expect a continued active pattern over the
Aleutians, while quieter weather will persist over Southcentral.
Saturday, a low in the western Gulf of AK will continue to swing a
series of weak fronts toward the North Gulf Coast, bringing
periods of precipitation into Sunday. Precipitation will be
concentrated on southeast facing slopes, mainly on the Kenai
Peninsula and the Alaska Range. Meanwhile, surface high pressure
over the Yukon and interior AK will strengthen, and conditions
will trend colder over inland areas of the Southern Mainland.

Out west, a strong low south of the central Aleutians will swing
an occluded front across the chain and generate a healthy round of
precipitation over the Aleutians, the AKPEN and likely into the
Southwest Mainland Saturday night into Sunday. Precipitation
chances look highest in the Bristol Bay area, with the Kuskokwim
Valley/Delta likely staying drier in offshore flow. As the
occluded front pushes toward the Southwest Mainland, relatively
cold air in place over the mainland will enhance the pressure
gradient, and strong south/southeast winds are possible from
Kamishak Bay to as far north as the Kilbuck Mountains. There's
indication that a triple point low will form south of the AKPEN
late Saturday, enhancing precipitation over Kodiak Island and
along the south/southeast facing slopes of the Alaska/Aleutian
Range. However, it's hard to have much certainty this far out and
with run-to-run model inconsistencies.

As that secondary low pushes northeastward Sunday into Monday,
expect a swath of precipitation and strong winds in the northern
Gulf of Alaska. In addition, expect another round of precipitation
along the eastern Kenai Peninsula Sunday night into Monday...then
precipitation will likely spread towards Prince William Sound and
eastward along the coast Monday.

Another low will approach the western Aleutians Monday, and push
an occluded front across the chain through Tuesday. Since there's
still some uncertainty with the progression of the low downstream,
it's tough to discern how the upstream low will affect the
Southwest Mainland. Models differ on whether the front will
dissipate before reaching the mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warnings 155 170 172-174 176 351 352 119 120 131
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...BB/MM