Small Craft Advisory 756, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 AM AKDT Wed Oct 3 2018

.ANALYSIS AND UPPER LEVELS...

A large upper level low over the Bering continues to be the main
story for our weather today. The leading frontal boundary is
currently lifting inland over southwest Alaska, spreading in rain
across the AKPEN and along the southwest coast. Surface
observations indicate persistent southeasterly winds along
coastal areas of the AKPEN.

Ahead of this front, the remnants of an occluded front have moved
into the central Gulf, generating a weak shortwave over
Southcentral. As a result, a few showers are visible on radar
imagery this morning along the southern coast of Kenai Peninsula
and over Prince William Sound. Some showers are also spreading in
across the Susitna Valley.

&&

.MODEL DISCUSSION...

Models remain in generally good agreement with large scale
synoptic features. The forecast challenge continues to be the
timing of the front associated with the Bering low moving into
Southcentral on Thursday. At this time, the GFS solution was
preferred.

&&

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

&&

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

The weather the next few days will be the story of a series of
fronts that emanate from a large low in the Bering Sea which move
from south-to-north over Southcentral Alaska. The first front
moves over the region yesterday and is weakening over the inland
part of southcentral Alaska and Prince William Sound. This weak
front is bringing some rain to the windward sides of the Kenai
and Chugach Mountains with only a few sprinkles making it to the
leeward sides.

A second, and stronger, front will be moving over Kodiak Island
later this morning and to the Barren Islands by the evening. This
front should bring Gale force winds to marine areas north of
Kodiak beginning this evening and then spread Gales to the north
Gulf Coast with the development of a barrier jet. Also of note is
that there will be an area of instability along the front in the
Gulf of Alaska which may be enough to kick off some isolated
thunderstorms over parts of the Gulf on Thursday.

Once again, the rain will start off on just over the mountains and
windward slopes, but on Thursday an upper level wave will increase
the lifting over Cook Inlet and the Susitna Valley. This wave will
likely bring some rain to the lee of the mountains as well.

&&

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

A negatively tilted shortwave continues to move closer to
Southwest Alaska this morning bringing showers to the central
AKPEN and west coast this morning. A few embedded thunderstorms
are also possible throughout the Bristol Bay region this afternoon
as the surface trough moves into the Southwest. The surface trough
and upper-level shortwave will weaken as it moves further north
and west across the interior late Wednesday and becomes absorbed
into the longwave pattern. Another shortwave trough quickly
advancing east underneath the broad low over the Bering will
follow in its wake, bringing another round of showers, gusty
southerly winds, and a few isolated thunderstorms along the
surface front. This storm will advance quickly bringing another
round of gusty winds and a moisture rich frontal boundary to the
region Wednesday night into Thursday. As this trough lifts north
on Thursday, another shortwave will move into the Southwest behind
it, reinforcing the precipitation and southerly winds through
Thursday night. By Friday, the main upper-level low over the
Bering retreats to the west, allowing a more organized north
Pacific low to move across the eastern Bering. This low will
weaken as it enters the eastern Bering, but more rain will move
over the region for Friday as its associated occluded front slides
across the AKPEN.

&&

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

A long-wave trough with a vertically stacked low sitting over the
central Bering will continue to dominate the weather pattern over
the region through the end of the week. A vigorous shortwave
undercutting the low will produce showers and isolated
thunderstorms across the central Bering, Aleutians, and Pribilofs
today. This system will also continue to produce storm-force
winds across the western and central Aleutians with hurricane-
force wind gusts likely at Adak this morning. This shortwave lifts
northeast into the eastern Bering tonight with another following
on its heels into the eastern Bering for Thursday as the main low
lifts north toward Saint Matthew Island. As it does, a second
upper-level low will drop south over the western Bering and
Aleutians, bringing more showers and gusty westerly winds. Farther
east, a north Pacific low will quickly strengthen as it moves
north toward the eastern Aleutians with showers and gusty
southerly winds developing east of Nikolski late Thursday. Both
the low over the eastern Aleutians and the shortwave father west
lift northeast for Friday as the axis of the longwave trough drifts
west. As this center moves west, it will help steer a Kamchatka
Low into the western Aleutians and a second low into the eastern
Aleutians and AKPEN as the work week draws to a close.

&&

.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...High Wind Warning 187.
MARINE...Storm 173 175 176 177 178
         Gale 160 165 170 172 174 179 181 185

&&

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