Small Craft Advisory 103, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKST Wed Jan 31 2018

.ANALYSIS AND UPPER LEVELS...
The upper level blocking high over the Bering Sea has drifted
slightly northward today, putting the center near the Gulf of
Anadyr in eastern Russia. The upper level low extends out of
Canada and into the Gulf of Alaska where a 500 mb upper level
cut-off low is now situated just south of Kodiak Island.
At the surface a small but potent low brought snowfall to the
southern Kenai Peninsula and Kodiak Island earlier today. This
low is moving over Kodiak Island this afternoon. A large portion
of the eastern Bering Sea is clear of clouds today with just some
stratus along the southern part Bristol Bay and right along the
Alaska Peninsula.

&&

.MODEL DISCUSSION...
Models are still excelling in their handling of this very slow to
change pattern. The low in the western Gulf of Alaska is better
defined and able to be tracked today. The next issue doe not
arrive until early this weekend when a low moves to the central
Aleutian Islands. The strength of the blocking ridge is causing
some different solutions for this low from the different models.
However, all of them tend to impede its movement northward until
at least late in the weekend.

&&

.AVIATION...
PANC...VFR conditions will persist. Northerly winds should be at
their peak through early evening and then diminish some overnight.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
For much of Southcentral a persistent weather pattern is in
place. The longwave pattern features a dome of high pressure over
the Bering with an upper level trough extending from Nunavut
Canada and into the Gulf Of Alaska. At 500 mb, there is an
elongated vortex just south of Kodiak Island with potent
shortwaves associated with it. Although the surface low is weak,
the affiliate mid- level energy has been slinging moisture into
Kodiak and this wrap around moisture will continue this forecast
period bringing additional new snow to the island. There are some
pockets of warm air which will make a rain/snow or short periods
of rain possible. In the wake of this low's passage, a second
impulse will move out of the North Pacific towards Kodiak late
Friday. Elsewhere, the Anchorage Metro Area and the Mat-Su Valleys
will remain dry the next two days with afternoon high temps
struggling to reach the 20F benchmark. Other sensible weather high
lights include the gusty winds in the Matanuska Valley, through
Thompson Pass, the Copper River Delta, Valdez and across multiple
marine zones near the Western Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Similar to the past few days, the forecast remains on track. An
area of high pressure will shift from eastern Siberia into the
Chukchi Sea by Thursday afternoon. Further south, a weak low
southeast of Kodiak Island will get absorbed into a larger
northward-moving Pacific low while it strengthens. This low will
attempt to send a warm front towards the AKpen Thursday, never
making it that far north. However, isentropic lift along and ahead
of said front, in an over-running setup, will provide sufficient
lift for precipitation to develop over the Alaska Peninsula and
Bristol Bay region by the end of the week. Otherwise, look for
minimal changes in temperatures with a dry offshore flow pattern
continuing.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As a pair of lows start to affect the Aleutians and AKpen, the
pressure gradient will gradually tighten between these features
and high pressure over eastern Siberia and the Chukchi Sea. Winds
will steadily increase over the next few days, with precipitation
potential increasing for much of the chain and southern Bering by
weeks end. Otherwise, look for temperatures to moderate slightly.

&&

.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...Storm 138 Gales 127 130 131 139 175 414. Heavy freezing
spray 126-128 130 139 180 181 185 414.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA