Small Craft Advisory 165, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 AM AKST Thu Feb 23 2017

.ANALYSIS AND UPPER LEVELS...
The pattern is divided into two parts this morning. Over the west,
a fairly deep trough with a closed upper level low resides over
the Bering. Meanwhile, in the east, a ridge of high pressure is
building over the central mainland. The most active weather is in
between these two systems as a 145 kt jet ferries potent warm air
northward. The first warm front has already made its way up to the
Seward Peninsula. The trailing quasi-cold front is blasting
through the Alaska Peninsula stretching up towards Nunivak Island.
Most of the precipitation associated with this feature has been
rain so far, but there is colder on the backside of it which could
allow things to change back to snow. Over the Western Bering, the
parent low is quickly filling as it slings some snow showers and
storm-force gusts through the Aleutian chain.

In the east, drier and warmer is the rule under the building
ridge. Winds are relatively calm at all locations. The biggest
forecast challenge will be abundant low stratus and perhaps some
patchy fog.

&&

.MODEL DISCUSSION...
All models remain in good synoptic agreement. This is lending to
moderate forecast confidence in the big picture items. The GFS was
preferred out west for continuity. The NAM was preferred in the
east for better resolution. The remaining areas of uncertainty are
the longevity of the low stratus around the Cook Inlet today and
then the precipitation type on Friday. Models seem to be trending
towards a slightly colder scenario for Friday's front. However,
Turnagain Arm winds and the depth of the warm air could lead to a
messy mix of precipitation. This portion of the forecast was left
mostly unchanged for this package.

&&

.AVIATION...
PANC...MVFR conditions due to low stratus will persist at least
through the morning hours. With very little change to the pattern,
this stratus could last until after midnight Thu night. However,
thinking is still that as some drier air works in today, it should
be able to mix out some. The next round of precipitation will move
in late tonight bringing MVFR ceilings. Precipitation is likely
to start as a mix of rain and snow. If it is more snow than rain,
expect visibilities to drop as well.

&&

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

Temperatures across Southcentral Alaska will be warming up a bit
today as the warm core ridge moves over the area. There are some
clouds riding over the ridge but there does not appear to be any
precipitation with any of these impulses. That will change
overnight into Friday as a front moves through the area from the
west.

Tomorrow poses a challenge as to the type of precipitation that
will occur in the Cook Inlet area. While temperatures will be warm
overnight, they main batch of precipitation will come as the
colder air moves in which could mean a mixed bag of precip types
throughout the day. There is a chance for some brief freezing rain
Friday morning near Anchorage southward to Kenai as the system
moves in, but this is looking less likely. What looks most likely
is for a rain snow mix in the late morning to mid afternoon and
then have all precipitation turn to snow by late afternoon.
However precipitation will probably not last for long after it
turns to all snow.

There is a chance that the vast majority of the precipitation
will be snow with just a little rain mixed in, but the south to
southeast winds before the precipitation arrives will probably be
enough to push temperatures high enough above freezing for some
rain. However it is interesting to note that both the GFS and NAM
MOS data show all snow for Anchorage, Kenai, and points northward
for the entire event in spite of temperatures around 40.
Regardless of what happens, it will likely make for some periods
of difficult travel conditions around the region. An SPS has been
issued to address this.

&&

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

A system extending from near Hawaii will continue to spread warm
moist air across southwest AK today and tonight. Temperatures
will rise above freezing today except for portions of the lower
Kuskokwim Valley. Although some snow will persist this morning
near Hooper Bay, precipitation will change to rain along the coast
and well inland. As the front progresses inland tonight,
temperatures will drop to freezing or below and rain will change
to snow as the main band of precipitation moves east.

Partial clearing from the west Fri morning will allow some fog
formation, as snow reaches the Alaska and Aleutian ranges during
the afternoon and overnight. Beginning on Sat the next warm front
spreads a mix of snow and rain across the region through Sun.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The initial stream of tropical moisture will continue to spread
rain over the eastern Aleutians today before moving inland. Cooler
air over the western Aleutians with snow showers will spread east
tonight before moving away from land. The next gale and storm
force warm front will approach the western Aleutians tonight with
snow changing to rain. This rain will then proceed across the
chain through the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The active weather pattern over the Bering Sea will begin to move
into the Mainland this weekend as high pressure slides southward
into the North Pacific. This will allow for a gale force front
over the Bering Sea to spread gusty winds and rain/snow into the
Southwest Mainland with deep onshore flow for much of the weekend.
Then by early next week the models continue to be in good
agreement that an arctic trough will deepen over the Mainland Mon-
Tue, ushering in colder air and gusty offshore flow to much of the
Gulf Coast. What is still unclear however is the exact track that
this trough will take through the Mainland, which is having a
significant impact in the possibility of snow to the South
Mainland into the middle of next week. The EC and GEM are more
aggressive in bringing Bering Sea moisture into the Mainland ahead
of the arctic trough, and a result are more aggressive in the
potential for a snow event Mon-Tue. The GFS on the other hand has
been very consistently more aggressive in bringing cold/dry
interior air, leaving most of the South Mainland dry into the
middle of next week. For now the forecast favors the drier GFS
solution as it has been much more consistent. Forecast confidence
is below average.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm warning 177 178.
		 Gale warning 155 165 170 172 173 175 176 180 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...DEK

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
109 PM AKST Thu Feb 23 2017

AKZ101-111-241430-
Anchorage-Matanuska Valley-
Including the cities of...Anchorage...Eagle River...Indian...
Eklutna...Palmer...Wasilla...Sutton...Chickaloon
109 PM AKST Thu Feb 23 2017

...Difficult Driving Conditions possible Friday...

A weather front moving into upper Cook Inlet Friday will bring a
warmer airmass into the area on Friday. While snow is expected to
be the dominate precipitation type across the Anchorage Bowl and
Matanuska Valley, there is a chance that it could mix with rain
at times, especially during the morning hours. If any rain were to
fall on frozen surfaces, it could make for very slick conditions.
Conditions should improve by afternoon as temperatures rise to
above freezing. Snow is expected to to continue through the late
afternoon hours before diminishing in the evening. Only a couple
of inches of snow is expected across the Anchorage Bowl and
Matanuska Valley with higher amounts along the mountains.

Stay tuned for updates at weather.gov/anchorage.