Small Craft Advisory 59, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKST Fri Jan 18 2019

.ANALYSIS AND UPPER LEVELS...

Upper level ridging extends over most of mainland Alaska and the
Gulf of Alaska this morning. This is leading to mostly clear and
cold conditions over Southcentral with strong surface inversions
forming across the area. The stable conditions are of course
leading to fog and low stratus formation with satellite imagery
showing widespread fog over the northern Gulf Coast and Prince
William Sound, including Cordova. Fog/low clouds can also be seen
over the southern end of the Copper River Basin slowly spreading
north towards Glennallen. Dense fog has also developed this
morning over the Anchorage Bowl area with all ASOS stations in the
Anchorage area reporting a 1/4 mile or less visibility. Cold air
over the interior is leading to offshore gap winds along the
coast, including through Thompson Pass and out of the Copper River
Delta.

Further west, a front associated with a low south of Cold Bay is
draped across the eastern AKPEN bringing gusty northerly winds and
snow to the area. A ridge is located over the central
Aleutians/Bering with cold northerly flow seen at the surface. A
front is pushing across the western Aleutians bringing rain and
snow showers accompanied with southerly gale force winds.

&&

.MODEL DISCUSSION...
Models are handling the overall synoptic features well with good
agreement through most of the weekend, though they are struggling
with some of the mesoscale features like low clouds and fog over
Southcentral. Models begin to show some disagreement by the end of
Sunday as a North Pacific low south of the Aleutians moves north
towards the mainland. Timing and placement differences of this
system will have impacts on the incoming pattern change to the
southern mainland early next week. This is discussed further in
the Long Term section below.

&&

.AVIATION...
PANC...Dense fog will persist due to a strong inversion and light
winds over the Anchorage area. This stagnant pattern will cause
LIFR conditions to remain through Saturday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Sunday night)...

An upper level ridge sitting over Southcentral has led to a
strengthening and deepening of surface-based temperature inversions
over inland portions of Southcentral. This is evident in the 12Z
Anchorage sounding, with an inversion from the surface up to about
3400 feet. As is typical, areas of fog and stratus have formed
beneath the inversion, most notably in Anchorage and vicinity.
Conditions will not change much until an upper level trough digs
down from the northwest on Saturday. The cold advection and
increase in low level flow accompanying this trough should be
enough to help erode at least some of the fog. Until then, have
issued a Dense Fog Advisory for Anchorage through midday Saturday.

The presence of the upper ridge followed by strengthening offshore
flow with the upper trough will maintain mostly dry conditions
across the region through the weekend. The short-wave trough will
become neutrally tilted as it bottoms out over Southcentral
Saturday night (instead of continuing to dig down into the Gulf).
This will generate some clouds and possibly some light
precipitation out ahead of it. Have included a chance of snow
from eastern Prince William Sound into the southern Copper River
Basin Saturday night through Sunday. The dry airmass in place
ahead of this trough along with offshore winds near the surface
will limit precipitation, so anything that does fall would be
quite light.

Lastly, the combination of cold air advection with the upper
trough and tightening surface pressure gradients will lead to
strengthening gap winds along the Gulf coast and Prince William
Sound Saturday through Sunday.

&&

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

The weak frontal band stretching from the Kuskokwim Mountains
south across the Bristol Bay area will shear apart and dissipate
today as the shortwave to the north continues east and the low to
the south of the Alaska Peninsula rotates away to the southwest.
Strong northerly winds accelerating through the gaps and passes in
the eastern Aleutians and western Alaska Peninsula this morning
will turn more northeasterly and gradually diminish this afternoon
through tonight. Over mainland Southwest Alaska, breezy north
winds will also turn more northeasterly tonight through Saturday
but strengthen a bit as they do so. The cold dry northeast
offshore flow will persist through the weekend and then turn
easterly Sunday night and strengthen further as a frontal system
lifts north towards the Alaska Peninsula.

&&

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

A frontal system currently stretching across the Western
Aleutians with an associated surface low to the west of Attu will
remain over the western Aleutians today and weaken slightly as the
surface low tracks away to the northwest. Another stronger surface
low will track up from the south tonight with its frontal system
bringing storm force winds to the western Aleutians late tonight
through early Saturday morning. To the east, high amplitude upper
level ridging over the central Aleutians will build north tonight to
form a large block high covering much of the Bering. The high will
shift to the northwest over eastern Russia and the northern Bering
Saturday night and Sunday. The strong pressure gradient between
this high and a deep low south of the Aleutians will bring gale
force easterlies to much of the southern two thirds of the Bering
and the Aleutians Sunday and Sunday night.

&&

.MARINE (Days 3 through 5...Sun through Tue)...

The likelihood of gale force easterly winds across much of the
Bering remains high on Sun. Coupled with some cold air, this could
also bring increased chances of freezing spray over the northern
zones, especially near the ice edge. Some models try bring a
stronger system from the North Pacific toward the Aleutian Chain
late Sun into early next week. This could bring some storm force
winds, but the chances of that look quite low.

Model uncertainty is even higher with developing systems towards
the Gulf of AK in this time. There is high confidence that Sun
will remain mostly below small craft level winds. But by early
Mon, the model disparity is so broad that it is hard to apply
certainty into any marine area. All that can really be said for
now is that winds will be on the increase with the best chance for
gales associated with system tracking near the southern side of
Kodiak Island by Tue.

&&

.LONG TERM FORECAST (Days 4 through 7...Mon through Thu)...

A couple of key features will become important for the weather
pattern next week. The first consistent feature in model solutions
is a strong upper level Arctic low in place over the northern
half of the state by the end of the weekend. This will allow
colder temperatures to persist into early next week for most of
southern Alaska. Offshore winds will also make for even colder
wind chills in the usual places (YK Delta, Thompson Pass, and
Seward, etc.)

Accompanying this low is an upper level trough that
becomes elongated and extends over southwest Alaska through
Tuesday. A key interaction with this trough is a North Pacific
low moving over the AKPEN for Tuesday. Only the EC has held run-
to- run consistency with this feature.

While all models show the elongated trough to the north lifting
to the northwest by midweek, the GFS keeps the trough in a
positively tilted orientation over northern Alaska, while the EC
positions the trough in a N-S orientation over the Bering. This
results in the EC favoring an earlier arrival of a southern
stream of energy for the remainder of the week. The net effect
across southern Alaska will be a change from cold and dry to
warmer with better precipitation chances.

By the end of the period, models mostly fall back into agreement
with the influence of the North American ridge building along an
axis extending from Haida-Gwaii north through Interior Alaska.
Currently, the EC has shown signs of stronger, more organized
ridge development. This could lead to a deep fetch of moisture
arriving into southern Alaska by late next week with growing
confidence in much warmer temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 101
MARINE...Gales...155, 170, 172, 177, 411
         Storms...178
         Heavy Freezing Spray...160, 180, 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...KO/MO