Small Craft Advisory 32, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 PM AKST Wed Jan 8 2020

.ANALYSIS AND UPPER LEVELS...
A deep 500 mb closed low over the southeastern Alaskan mainland
is allowing for weak flow in the upper-levels over the Copper
River Basin. Upper-level flow remains stronger across Bristol Bay
and the Gulf of Alaska as 500 mb height gradients remain steepest
in this region. Upper- level ridging is moving across the Bering
towards the Kuskokwim Delta today. An occluded front is moving
across the ALeutians into the southern Bering Sea. Water vapor
imagery shows a well defined area of low pressure near 49N 177E
moving northeastward toward the western Aleutians/Bering.
Middleton radar shows snow showers associated with a weak polar
low moving southeastward in the central Gulf. Observations across
Thompson Pass, the Lowe River Valley, Port of Valdez and Prince
William Sound show gusty conditions from the localized funneling
of northeasterly winds associated with the aforementioned off-
shore polar low. Gusty conditions are also observed throughout the
greater Seward area including Resurrection Bay due to the
localized funneling of northerly winds. A large thermal gradient
exists across the AKPEN as southerly flow brings above freezing
temperatures across the Aleutians and western AKPEN. Observations
across the AKPEN from late this morning ranged between -18F at
King Salmon to 37F in Cold Bay. The warmest temperatures are
located in the western Aleutians with Adak and Atka.

&&

.MODEL DISCUSSION...
Models remain in good agreement at this time with respect to the
overall synoptic pattern. All models have similar solutions over
the next 84hrs with respect to the low pressure system southwest
of the Aleutians. Guidance also remains consistent with the
predominant storm track remaining far to the south as high
pressure remains in place across the Alaska mainland. The NAMNest
was favored for the wind forecast across the Copper Delta, Mat-Su
Valley and Prince William Sound as this model resolved the current
mesoscale conditions the better than the rest of guidance.

&&

.AVIATION...
PANC...Fog will be in and out overnight again. Increasing
northerly winds tomorrow give the hope that there will be less fog
around by Thursday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Cold air continues to pile up in the Copper River Basin. While
there are minimal disturbances in the upper levels, this cold air
will continue to "drain" through the usual gaps and passes along
the northern Gulf coast. This will allow for continued gusts
through the Valdez coastal waters, Thompson Pass, and the Copper
River Delta. Gusty northerly winds will also persist through
Passage Canal and Resurrection Bay. By Thursday night, the
Matanuska Valley will likely see gusty northeasterly winds through
the usual gaps as the pressure gradient strengthens. With
increased winds and cold temperatures still in place, a Wind Chill
Advisory remains in effect for Thompson Pass where wind chills
may drop to 50 below zero at times through noon on Friday.

The next low moving across the Gulf Thursday still looks to be
tracking farther south, and minimal changes to the forecast were
made regarding this feature. However as the pressure gradient
tightens along the northern Gulf coast on Friday, gap flow is
expected to increase again, especially through the Valdez coastal
waters and Copper River Delta through Friday afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A pair of lows moving through the western Bering, will help bring
warmer air to southwest Alaska over the next few days. This will
occur as each system sends an attendant front in the occlusion
process (essentially when the cold front catches up to the warm
front, and the two fronts merge, before weakening). The first such
front is approaching the coast now, with both the Bethel and King
Salmon radars showing the leading edge of snowfall approaching.
This snow will precede the front as some warmer and more moist air
aloft is thrown over this feature (over-running setup), with some
of this warming mixing down to the surface over the next 24
hours. This front will stall by Thursday afternoon from a Hooper
Bay to King Salmon line, as the frontolysis (the weakening of a
front) takes hold due to it out-running its better upper level
support.

This will be short-lived though, as yet another front pushes into
the region on Friday. Like its predecessor, it too will be
accompanied by a band of snow, with this front also maintaining
its integrity longer, reaching a Sleetmute to Homer line by
Saturday before the front once again dissolves. The net affect
will be a couple periods of snowfall followed by a gradual warming
trend over the next few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A pair of low pressure systems, and their attendant fronts, will
affect all of the Bering Sea and Aleutians over the next few days.
The first storm is an elongated area of low pressure, with two
lows both around ~972 mb, moving through the western Aleutians.
The attendant front stretched from this low through the western
Bering, before bisecting the Pribilof Islands and Alaska
Peninsula. Snow and sustained gales will precede this front as it
continues to lift north, with waves in the 15 to 20 feet range
expected.

Conditions will briefly subside following the frontal passage,
before another low moves into the area. This next low will be
located just east of Shemya Friday morning, as it deepens to ~961
mb, before reaching the western Bering by evening while having
deepened further to less than 950 mb. As this storm wraps us,
sustained storm force winds will precede the surface front, while
also developing just south of the low center itself, as dry
advection helps mix down the colder air aloft. Given a long fetch
length, waves will push 40 feet along the front across the western
Bering, with 20 to 35 feet likely for the western Aleutians.
Further east, look for waves to peak at about 15 feet east of the
Pribilof Islands and Dutch Harbor, as these locations will be
further removed from the stronger winds/best dynamics.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...
A storm in the far Western Bering will bring some storm force
winds through the central and western Aleutians on Saturday. By
Sunday, these winds will have mostly weakened overall and any
storm level winds will remain in the far western Bering. Overall,
winds across the Bering sea will be from the south or southeast
through the period, reducing the risk for ice moving southward and
freezing spray. but these winds will progressively weaken through
the weekend.

Strong north to northwesterly winds will continue across the Gulf
of Alaska through Monday. These winds will be strongest in the
gaps and may reach gale force at times.

&&

.LONG TERM FORECAST (Days 3 through 7: Sat through Wed)...
Yesterday's long term forecast mentioned a low moving across the
Bering Sea, bringing warmer temperatures to the mainland through
the weekend. Today, the forecast models are indicating that
although the low is still forecast to move across the Bering Sea,
the high currently in place over the mainland remains strong, so
the warm up won't be as dramatic as we thought yesterday. However,
by Sunday, we do have some warmer 850 hPa temperatures moving in,
which will help moderate the cold air mass. The front that moves
across the Bering will bring some upper level clouds with it, so
that will also help moderate the temperatures at the surface.
These factors lead me to believe that the record cold temperatures
for the Copper River Basin mentioned in yesterday's discussion
are pretty unlikely. Realistically we can expect daytime highs
above 0F for most of Southcentral, with the Copper River Basin
reaching near -10F and a low of the lower -30s. West of the
Alaska Range, the warmer temperatures will have much more of an
effect, with temperatures in Southwest Alaska warming into the
mid 20s in some places. The front will also bring a chance of
snow for Southwest and the lower Kuskokwim Valley on Saturday.

After Saturday, the next low to move through skirts the Aleutians
and then progresses across the Gulf of Alaska. This is not a big
weathermaker for most of Alaska, with the main concern being
outflow winds along the coast. Folks living near northerly
oriented valleys should be prepared for a windy start to the
weekend, but these will decrease as we head into early next week.
On Tuesday and Wednesday a front moves into the western Aleutians,
but again, this has been delayed slightly from yesterday's
forecast due to the strength of the high over the eastern half of
the state. For the most part, it looks like a quiet start to next
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121.
 Wind Chill Advisory 131.
MARINE...Storm Warning 176 177.
 Gale Warning 127 130 132 160 170 172>175 178>180 185.
 Heavy Freezing Spray 119>121 126 127 129 139 140 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED/EZ
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...LF