Small Craft Advisory 166, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKST Sat Feb 23 2019

.ANALYSIS AND UPPER LEVELS...
The ridge that was building over mainland Alaska yesterday is
becoming more amplified and stretching much farther north. This is
keeping calm winds and dry conditions spread across Southcentral
and interior locations of southwest Alaska. The long fetch of
moisture surging from the North Pacific to the northern Bering
continues to be situated between the boundary of this ridge over
Mainland and a trough over the Bering. Precipitation associated
with this front is mostly falling as rain as temperatures remain
above freezing over the central Aleutians, along with strong,
southerly winds.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Monday
afternoon. There are some minor differences on the location and
speed of a strong surface low moving across the central/western
Aleutians Sunday night and northward across the central Bering on
Monday. Also, with a ridge and subsidence over Southcentral, some
fog is expected and the models generally do not do very well with
low level moisture. The forecast confidence for the afternoon
package is above normal, except below normal for fog in
Southcentral.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. LIFR conditions in fog are probable after midnight with
a strong inversion and light winds. Conditions are expected to
improve early Sunday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Monday night)

The main forecast challenge over the next couple of day is how
dense and widespread will the fog be during the early morning
hours. Early this morning a mid-level ridge was poking into the
AKPEN at 552 dkm, with it having increased in strength this
afternoon as the latest analysis shows 558 dkm having shifted
eastward while poking north of the Alaska Range. This trend looks
to continue with it peaking in strength around 564 dkm by
tomorrow, while shifting eastward early next week.

The subsequent mid-level warming will help strengthen the
nocturnal inversion (a layer of air where the temperature
increases with height, versus the usual decrease in height), while
the eastward gradual shift allows a more southerly flow to
eventually develop, adding to the low-level moisture for coastal
locations. That said, the short story is fog will become more
widespread and increasingly dense for some of the typically
favored areas, over the next couple of days. The only potential
limiting factor to this is high-level cirrus dropping south in the
flow aloft, but, the visible satellite imagery this afternoon
shows this is on a downward trend.

&&

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

A vigorous occluded front will approach the coast the Kusko Delta
coast (and Western Capes) this evening and tonight, bringing a
mix of rain and snow. Temperatures will be very marginal, but with
30-45 MPH S winds, there will be some reduction in visibility
with falling snow. The front will stall out then retrograde back
west Sunday evening, keeping a rain and snow threat across the
Kusko Delta, especially along the immediate coastline. The
remainder of Southwest will generally see a strong downslope
shadow with SE winds. This general pattern will persist all the
way into Tuesday.

&&

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

A gale force front is crossing the Pribilof Islands and moving
northeast toward the coastline this evening. This front stalls
along the coast, with attention quickly turning to a developing
storm force low which will move into the western and central Bering
Sea Sunday morning, and will remain in place through Monday as it
slowly tracks north and weakens. However, before that, storm force
winds will reach near hurricane force gusts Sunday afternoon
across the Central Aleutians, but look marginal for a high wind
warning type event. Strong northwest winds and cold air behind the
low will support snow and blowing snow over the Near Island group
including Shemya, with wraparound cold air advection snow showers
lasting through Monday.

&&

.MARINE (Days 3 through 5)...

There is a bit more divergence between the numerical guidance as
compared to yesterday, especially in the strength and timing of
the low tracking through the Bering Wed/Thu. Still, a general
consensus remains that the active pattern will persist with strong
southerly winds and significant wave heights over the Bering
through midweek. Confidence remains above average that winds and
waves will slowly diminish in intensity and height as the first
low tracks toward eastern Russia late Tuesday. Confidence also
remains high that a second deepening low moving across the
central Aleutians and into the central and eastern Bering late
Tuesday through Wednesday will bring widespread southerly gales
and wave heights building to 20 feet. The discrepancy lies in
whether this system deepens enough to produce an area of storm-
force winds across the central Bering and Pribilofs on Wednesday
before lifting north. Nonetheless, expect at the very least strong
southerly gales, and areas of rain spreading north and east
across the Bering midweek with this system. Westerly winds and
waves again relax in the wake of this system as another deepening
low brings southeasterly gales across the western Aleutians by the
end of the week.

High pressure anchored over the Gulf of Alaska will allow
generally westerly, sub small-craft winds along the coast from
Kodiak Island to Cordova through the end of the week with only
a slight chance of scattered precipitation south of Kodiak Island.

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

The longwave trough/ridge pattern extending from the western
Bering to the eastern Gulf will remain in place through midweek.
After that, model guidance begins to diverge in how this pattern
evolves moving into the second half of next week. The GFS is most
bullish progressing the main trough east, splitting the ridge,
and bringing a phased shortwave into the northern Gulf. The EC and
Canadian, while showing something similar, is slower with the
movement of these features and holds the ridge over the Gulf a bit
longer. For what it is worth, the deterministic GFS and its
ensemble means have been inconsistent over the past 24 hours or so
with the timing, placement, and strength of the developing
shortwaves in the Bering. This has resulted in a forecast weighted
a bit more towards the EC/GEM solutions, with the ridge holding
on for now.

For Southcentral, expect the tranquil pattern to continue with a
few periods of higher clouds passing overhead as weakened
shortwaves crest over the ridge. This pattern will also lend to
persistent large diurnal temperature swings with average to
slightly below average overnight lows and slightly above average
daytime highs. The biggest challenge to the forecast will
continue to be fog, especially as the high drifts to the east
through next week. A southerly flow on the back side of the ridge
could advect just enough moisture to combine with the overall weak
surface flow to allow overnight/early morning fog to develop
along Cook Inlet and portions of the MAT-SU and Copper River
valleys. The southerly flow may also bring moisture to produce a
few scattered showers across Kodiak Island by the end of the week.

For the AKPEN and southwest coast, expect periods of strong
southerly winds and precipitation (mainly rain) as the occluded
fronts attached to lows tracking north across the Bering slow and
weaken as they to push inland and run up against the ridge. The
first of these will impact the coast late Monday through Wednesday
before drifting east across the southwest interior. The second
arrives overnight Wednesday and lingers through early Friday
before weakening as it, too, drifts east. Behind it, a ridge then
attempts to build in for the weekend.

For the Bering and Aleutians, one departing low will give way to a
second, gale-force low tracking north across the central Aleutians
and into the Bering beginning early Wednesday. Expect strong,
southerly gales (with areas of storm force winds possible) moving
northeast from the central Aleutians to the Pribilofs and eastern
Bering. Broad, westerly small-craft winds and areas of showers
continue across the Bering late week as this second storm lifts
north and a weaker shortwave slides underneath the main low.
Southeasterly gales redevelop along the western Aleutians by the
weekend as another deepening NOrth Pacific low tracks east toward
the chain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 150-170 172 180-185.
		 Storm 173-178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...TM