Small Craft Advisory 104, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
417 AM AKST Thu Feb 1 2018

.ANALYSIS AND UPPER LEVELS...

A strong upper level high centered over eastern Russia extends
eastward over western Alaska. Meanwhile a weak upper level low
centered south of Chirikof Island is moving westward. This upper
level pattern is keeping broad northeasterly flow ongoing over
much of the area. The upper high is pumping upper level warm air
over the area, while general low pressure over the Gulf continues
a cold northerly offshore wind through the gaps. Thus, with cold
air underneath warm air above it, much of Southern Alaska remains
in a stable, dry weather pattern consisting of seasonable
temperatures with lots of sunshine and clear skies.

With the upper level low, broad easterly flow is keeping a
constant feed of moisture from off the Gulf heading into Kodiak.
However, the Gulf flow is also warming temperatures up, so current
precipitation is a rain/snow mix with above freezing temperatures.
The Matanuska Valley wind is also continuing through Palmer and
Wasilla as the northeast flow drains cold air out of the Copper
River Basin down the valley. Drainage winds near gale force are
also ongoing through the Valdez Narrows, Valdez Arm, and the
Copper River Delta.

&&

.MODEL DISCUSSION...

Model agreement remains very good as the pattern remains very
steady state due to the developing rex block in the upper level
pattern. So while the synoptic large-scale pattern is high
confidence, smaller scale features remain the forecast challenge
this morning. Among the challenges are the strength, duration, and
location of the Matanuska Valley wind, winds through many of the
other gaps, precipitation type and intensity through Kodiak, and
how low temperatures drop in any interior areas where the winds go
slack. As such, forecast confidence is broadly high, but lower in
some of the local-effect areas.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The strong outflow winds that have blown through many of the usual
spots in Southcentral (such as the Matanuska Valley, Thompson
Pass, and the North Gulf Coast) for most of this week will begin
to diminish starting this afternoon. This will occur as the
pressure gradient weakens behind the incoming North Pacific low
that will graze the Gulf as it tracks into the Panhandle. For
Thompson Pass and the Matanuska Valley, gusty winds will continue
into Friday, however, these should be noticeably weaker than the
past few days. Precipitation will continue to generally be limited
today to the Kodiak Island area where a shift to onshore flow has
brought temperatures above freezing with a rain/snow mix,
allowing for the expected accumulations to be significantly
lowered and the potential for blowing snow removed from the
forecast. The only other chance for precipitation over the next
few days will be further north in the Gulf towards Cordova this
morning, as a weak trough passing overhead from the Yukon may
bring a some periods of light snow to area. This will only be a
brief shot this morning though, shifting the focus for
precipitation back to Kodiak Island where it will remain for the
rest of the work week.

&&

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

Gusty offshore winds will continue through this evening, before
decreasing some as the pressure gradient decreases somewhat. This
is due to the flat ridge of high pressure to our north decreasing
some in magnitude. This should allow temperatures to cool at
night a few degrees tonight and tomorrow night, especially for
the interior valleys as mixing decreases. Otherwise, the main
item of interest will be an approaching area of low pressure for
the weekend, with models differing on how far north does it make
it and their associated warm fronts (Bristol Bay per GFS versus
AKpen via NAM). However, the overall theme is for moderating
temperatures and increasing potential for precipitation by the
weekend.

&&

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

The rather benign pattern looks to change by the weekend, as a
pair of lows affect the region. The general theme is for broad
high pressure to weaken as one low moves into western portions of
the chain/Bering, with another low making it to the peninsula or
eastern Bering. A warm front will be accompanying each of these
lows, with winds and precipitation potential increasing ahead of
them. Timing and placement issues remain large at this time.
However, the overall theme for a shift in pattern is consistent
amongst all of the numerical guidance.

&&

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

Beginning the long term forecast period Friday night, the strong
omega blocking high will still be in place over Eastern Siberia
and northwest Alaska. There will be a weakening upper trough
(deforming and stretching) over Southern Alaska with a strong
arctic surface high over the interior. A stationary front will
stretch from the central North Pacific to Kodiak Island and the
Eastern Gulf of Alaska. Energetic shortwave troughs will slowly
but surely push the front farther north toward the Gulf Coast,
slowly eroding the arctic high in place Sunday. There is quite a
bit of uncertainty, still, regarding how far north the front makes
it. Another day with the same model discrepancies as, once again,
the GFS and ECMWF models have "flipped", now the ECMWF mostly
remains dry for the Gulf Coast and inland (with precipitation
favoring Kodiak Island and the Alaska Peninsula/Western Alaska
Range) while the GFS wants to move the front farther northward.
For now, the WPC approach was favored which does bring a low
chance for precipitation to the coast by the end of the weekend,
although keeping inland locales dry. There should be a gradual
uptick in temperatures next week as the arctic high weakens, but
the general dry pattern will remain in place with no snow likely
in the near future for inland locations as the blocking high
remains dominant.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 130 138 150 172.
		 Brisk Wind Advisory 181.
		 Heavy Freezing Spray Warning 121 126 127 139 160 180 185 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JA