Small Craft Advisory 753, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKDT Tue Oct 2 2018

.ANALYSIS AND UPPER LEVELS...
A stalled occluded front is strung out along the southwest side of
an upper ridge retreating northward across mainland Alaska. The
front extends roughly from northern Kodiak Island northwest to
Bristol Bay and the Kuskokwim Delta. This is bringing gusty
southeasterly winds and areas of light rain or rain showers.
Stretching of the front is leading to weakening of winds and
the breaking up of steady precipitation.

Out over the western Bering Sea a deep vertically stacked low
with estimated central pressure of around 950 mb was reinforced
this morning by a separate upper level low, short-wave and
associated jet streak. This new upper low has since phase with
the existing low. The advection of cooler more unstable air has
led to development of a few thunderstorms in the vicinity of the
western-most Aleutian Islands. It has also served to reinforce a
bent back occlusion along the south side of the low (north of Attu
Island). Based on a conceptual model of this type of feature,
would expect strengthening winds and heavy precipitation right
along the bent-back front. With the low expected to track a little
further southward as it is kicked eastward, this bent-back will
approach the Aleutians from the north and is worth watching to
see how far south it makes it.

Meanwhile, the frontal system associated with this surface low is
spreading north and east across the Bering Sea, with gale force
winds and moderate to heavy rain right ahead of the front.

&&

.MODEL DISCUSSION...
Models have a good handle on the evolution of large scale features
through Friday and overall forecast confidence is high. One of
the main forecast challenges is handling of the occluded front as
pushes onshore of Southcentral Alaska during the day Thursday.
Models differ in some of the particulars of frontal orientation
and strength/orientation of upper level trough. These will be key
to both the wind and precipitation forecast from Prince William
Sound to the Kenai Peninsula and Anchorage.

&&

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

&&

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

A historically large area of high pressure that had been
dominating the weather the past 5 days over Southcentral is
beginning to shift north, signaling a pattern change back to
typical fall weather, albeit still warmer than normal for this
time of year. A series of fronts will sweep across the region
through the rest of the week bringing cloudier and wetter weather,
especially along coastal areas. A weak front will continue to march
north across the Gulf of Alaska and Kenai Peninsula overnight
bringing rain to the Eastern Kenai coast and the Prince William
Sound. Moderate cross barrier flow will keep the usual lee side
areas dry over night into tomorrow. The front slowly dies out as
it becomes detached from its parent low and runs into the large
ridge over Eastern Mainland Alaska.

Another stronger front approaches the area from the southwest by
Wednesday night, swinging out ahead of a deep surface low over the
Bering. The front will advance further inland (north) as the
upper ridge continues to retreat into the Arctic Ocean. The
stronger front will mean increased winds through the Gulf and
stronger cross barrier flow. This will result in heavy
precipitation along the coast with a pronounced downslope hole
over the Western Kenai and Anchorage areas through most of the day
Thursday. Gap winds will kick up Thursday through the typically
favored areas, especially Turnagain Arm and the Upper Hillside,
though gusts are not expected to be above 60 mph. As the front
moves through Thursday afternoon/evening, a brief period of rain
will be seen on the lee side of the Kenai and Chugach Mountains
before drier air filters in behind it. Another weak wave will
move through on Friday keeping unsettled weather over the area
with overall cloudy conditions and showers over the Kenai and Gulf
Coast.

&&

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

A negatively tilted shortwave is moving into Southwest Alaska and
continues to bring showers to the region. Expect this trough to
move north, weaken and get absorbed into the longwave pattern.
Another storm will follow in its wake. This storm will advance
quickly bringing another round of gusty winds and a moisture rich
frontal boundary to the region. This is a fierce low and once the
surface low reaches the Central Bering, it will recurve to the
north and then back west. Expect the heaviest accumulations
Wednesday. Moist of moisture Thursday and Friday will be
post-frontal and showery in nature.

&&

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

There is an occluded front sweeping the Aleutians. The latest
ASCAT/advanced scatterometer pass detected gale force winds north
and south of the Chain. This is a moisture laden system. An
impulse of energy from Russia will phase up with this
extratropical boundary. And with time, the occlusion will become
more back bent. Colder air wrapping underneath the low will help
produce storm force winds across the western and central Aleutians
through Wednesday night as the low tracks toward the Pribilofs.
Colder air aloft combined with the lift from a series of vorticity
maxima will prove to be sufficient for thunderstorm development.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Saturday through Tuesday)

The picture is beginning to become a little more clear for the
long range portion of the forecast. Todays models runs of the GFS
and ECMWF (Euro) have a much improved agreement amongst each
other compared to days past, with the Canadian still vastly
different, and thus will be discounted. The only fly in the
ointment is the 12z Euro does try retrograde a closed mid-level
low from the northern Yukon into northeast Alaska before lifting
it north into the Arctic. Still, the overall height fields are in
reasonable agreement for the remainder of the state and the
region in general.

There are still a few question marks though regarding 3 tropical
interactions. The first is the much-talked about former Super
Typhoon trami, with the left over parent low anchoring a deep
trough across the western Bering and extending into the western
Pacific. Next is Super Typhoon Kong-Rey, which will track from the
Sea of Japan into the western Pacific before getting absorbed
into the Bering trough. This system will lead to surface
cyclogenesis south of the western tip of the AKPEN, before
deepening as it lifts across the eastern Bering. Gale to Storm
force winds and heavy rain will accompany both the trough and this
system as it rides the eastern flank of the trough. Multiple
fronts from each vort lobe cycling around the trough does look to
induce surface low pressure development with its associated
fronts lifting north through southwest and southcentral Alaska
next week with rounds of rainfall developing.

Finally, where does the remnants of category 4 Hurricane Walaka
(southwest of Hawaii currently) go, as it gets absorbed into a
central Pacific mid-level low. The past few days the models
tracked the remnants towards south central, with today it getting
shunted to the east as the east Pacific ridge tries to build
across south central. This would decrease our rainfall potential
and warm temperatures some. However, this is a new wrinkle in the
forecast and confidence in this is very low compared to the
previously mentioned two systems.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 155 160 165 170 172-174 179 180 185 412.
Storms 175-178 411 413.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...PD