Storm Warning 205, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 AM AKST Tue Dec 13 2016

.ANALYSIS AND UPPER LEVELS...
The ridge of high pressure over the Southeast Mainland continues
to bring clear and cold conditions to much of Southcentral this
morning along with dense fog and very low stratus once again from
Northern Cook Inlet into the Anchorage Bowl again. This ridge is
beginning to slowly slide the east as it strengthens in advance
of a storm force front spreading into the Eastern Bering. The
associated low has moved northward through the Western Aleutians,
bringing gale-storm force winds and rain from the eastern
Aleutians into the Central and Western Bering. Cold and dry air
can be see feeding around the western edge of this low, but this
is beginning to be flow is beginning to be cut-off by a rapidly
approaching North Pacific low.

&&

.MODEL DISCUSSION...
Very little has changed in either model agreement or forecast
confidence through mid week as the models remain in very good
synoptic agreement. The NAM continues to be slightly weaker than
every other model with the current western Aleutian low, and was
once again treated as an outlier. This weak bias continues with
the next western Aleutian low for late Wed-Thu, so the stronger
and more consistent GFS was used to increase winds and waves
throughout the western half of the Bering with good confidence.
Looking into late in the work week the models remain in very good
agreement with the track of the second low through the western
Bering and eastward progression of the Mainland ridging, leading
to good forecast confidence into Friday.

&&

.AVIATION...
PANC...Areas of fog with LIFR vis/ceiling remain in place early
this morning, but a 3500 ft cloud deck is headed up Cook Inlet
and may lead to gradual improvement when it arrives between 15Z
and 17z. Even so, there is nothing to completely clear out the
fog, so expect it to linger and possibly move back in tonight
as clouds decrease once again and a ridge builds back overhead.
Forecast confidence is low.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weakening upper level trough over Southwest Alaska has started
to split into two troughs, with the southern trough crossing
Kodiak Island and the northern one headed to Interior Alaska.
While Kodiak Island can expect areas of light precip, expect a
dry trough passage for the rest of Southcentral. An amplifying
ridge ahead of the deep storm system along the Aleutians will
then build in and reinforce the dry weather across the region.
There is an area of weak isentropic lift on the leading edge of
the ridge which moves up Cook Inlet this afternoon and evening, so
have introduced flurries to the forecast. The persistent fog in
the Anchorage area should break up a bit as low to mid level
clouds move in, but conditions will remain favorable for fog as
these clouds depart tonight. Lastly, as the ridge amplifies
northerly flow and subsidence will increase over Southcentral,
leading to an uptick in gap winds tonight through Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The weather pattern continues to shift to more active, as the
next front over the Eastern Bering begins to push inland over the
Southwest mainland today. This system will bring the next round of
precipitation to the Alaska Peninsula and the Southwest coast this
afternoon, along with some gusty southerly winds along the coast
and Alaska Peninsula as the weakening front passes. Precipitation
will remain mostly snow over the inland areas. However, with the
warmer southerly flow, temperatures will be more likely to
moderate along the southwest coast and Alaska Peninsula where a
rain/snow mix if not all rain may be observed. New snow
accumulations are expected between 1 to 2 inches mainly over
interior portions of the southwest mainland on Wednesday.

As the warmer air moves over the cold surfaces over the southwest
mainland, there will be the potential for some patchy fog to
develop early this morning or parts of the southwest mainland. A
sharp ridge will then build in ahead of the next system today,
allowing for more warm southwesterly flow to help stabilize the
atmosphere and keeping fog in the forecast for much of the day.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Several stout systems will impact the area over the next several
days. The first is a rapidly occluding system centered east of
Shemya, with a associated warm front slowly weakening as it
tracks through the Eastern Aleutians this morning and into the
Alaska Peninsula this afternoon. This system will continue to
bring gusty winds and precipitation to the Western and Central
Aleutians/Bering and Alaska Peninsula, before slowly falling
apart as it drifts north over the far Western Bering by Wednesday.
A slightly stronger North Pacific low and associated front will
quickly approach the Western and Central Aleutians Wednesday
afternoon/evening. Current models are indicating widespread rain
and hurricane force gusts over the Western and Central Aleutians
as this next system lifts northward into the Western Bering
for Thursday.

&&

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

A shift in the pattern will be well underway Wednesday night and
Thursday as the high latitude blocking high currently over Alaska
weakens and shifts east. A weakening shortwave trough axis over
Southwest Alaska will be moving east into Southcentral. There is
uncertainty with the exact timing and strength of this trough, but
right now there certainly will be a chance for light "overrunning"
light snow across all of Southcentral as this system tracks
through Thursday and Thursday night. Meanwhile, a deep and near
hurricane force low will move into the Western Bering/Aleutians
Wednesday night. This will be the second in a series of strong
lows which will impact the region through early next week.

Sometime late Friday into Saturday, the occluded front associated
with the strong Bering low will merge with a subtropical warm
front then move into the Northern Gulf Coast Friday and Saturday.
There remains uncertainty with track and strength of this feature,
but there it is likely to bring widespread rain and snow to all of
Southern Alaska. As usual, amounts and the exact precipitation
type will be challenging, especially along the coasts where warm
air will have an easier time eroding the cold arctic air mass
currently in place. Thereafter, the storm track will remain in
place as an even deeper low moves into the Gulf of Alaska Sunday
night into Monday.

Bottom line...expect a dramatic uptick in weather activity as this
storm cycle unfolds late this week into the early parts of next
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 177 178.
         Gale Warning 155 165 170 172 173 174 175 176 179 180 181 185.
         Heavy Freezing Spray Warning 127.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
113 PM AKST Tue Dec 13 2016

AKZ121-150200-
Western Kenai Peninsula-
Including the cities of...Kenai...Soldotna...Homer...
Cooper Landing
113 PM AKST Tue Dec 13 2016

...Ice jam continues on the Kenai River in Soldotna...

An ice jam on the Kenai River beneath the Sterling Highway bridge
in Soldotna continues to hold as of midday Tuesday. Reports
indicate the water is high and the river near bankfull upstream of
the bridge. Water is beginning to encroach on some yards in the
area.

The recent cold snap has kicked ice production on the rivers along
the western Kenai peninsula into high gear. These flows of ice
moving down the river can become jammed at bridges or other
natural constrictions in the river and cause water levels to rise.
Some reports indicate this could be happening on the Kasilof River
in addition to the Kenai River. While there are no active river
gages on the Kasilof River, water levels at the gage on the Kenai
River at the bridge in Soldotna have risen nearly 7 ft since
Sunday afternoon. With temperatures gradually warming throughout
the rest of this week the most likely scenario is that the jam
will remain in place but water levels will stabilize near where
they are now or slowly fall.

Residents and other interested parties upstream and downstream on
the Kenai river in the vicinity of the Sterling highway bridge in
Soldotna should take extra precautions and prepare for the
possibility of additional water level rises. In the event the ice
jam releases, water levels will rise downstream of the bridge and
will also be accompanied by substantial movement of ice pans and
chunks. Ladders, boats, and other recreational equipment near the
river bank should be secured.

This statement will be updated as necessary.