Small Craft Advisory 136, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
617 AM AKST Mon Feb 13 2017

.ANALYSIS AND UPPER LEVELS...

A complicated and rapidly-evolving weather situation is unfolding
across southern Alaska this morning. The overall weather pattern
has amplified, as expected, and now consists of a strong ridge of
high pressure and associated warm air over the Panhandle, and a
digging trough with arctic air entrenched over southwest Alaska
and the Bering Sea. A large low pressure system is parked directly
south of the Alaska Peninsula, with numerous embedded shortwaves
riding a strong southerly jet stream through the Gulf and into
Southcentral Alaska. Steady and heavy precip has been falling all
along the Gulf Coast overnight. Snow changed to rain first in
Kodiak, then in Seward and Cordova around midnight. However, as of
this writing, snow continues near Whittier and Portage as well as
in Valdez with enough cold air remaining in place. Snowfall totals
overnight likely exceeded a foot in Valdez and Portage.

Over Cook Inlet, very strong northerly winds at the low levels
have been reinforcing the cold Arctic air that was in place
yesterday and resisting the approaching warm air. This has led to
dramatic temperature inversions around upper Cook Inlet, and
brings the possibility of freezing rain to many areas by later
this morning. See forecast section below for more information.

Meanwhile, gusty and cold northerly flow continues at virtually
all points west of the Alaska Range. Instability-driven snow
showers continue to barrage the north side of the Aleutians as
well as the Pribilof Islands. Over the Southwest Mainland, more
widespread lift from the storm south of the Alaska Peninsula is
bringing some fresh snow to many areas. Blowing snow is also a
concern over the Bristol Bay area with temperatures remaining very
cold.

&&

.MODEL DISCUSSION...

The models have finally come into good agreement on the track of
the low that is currently bringing all the active weather to the
area. The GFS had previously been trying to push more energy into
the Susitna Valley and weaken the winds blowing south through Cook
Inlet. The GFS has now joined the other models in keeping most of
the energy around the main low center near the AKPEN, which should
keep northerly winds in Cook Inlet throughout the day today.
The precip forecast in the typical downslope areas remains
problematic, with every model depicting a slightly different
solution, and all depicting the possibility that the typically dry
areas might see a period of precip at some point today. The
morning forecast used the NAM for larger-scale updates, with more
fine-tuning of the forecast relying mostly on conceptual models
and experience.

&&

.AVIATION...
PANC...Conditions should remain VFR for the majority of the day as
generally southeast flow and downslope conditions prevail. Wind
shear will be a concern through the afternoon, before a
strengthening pressure gradient causes the Turnagain Arm wind to
bend more strongly down the Inlet by early this evening.
There should be a period at some point during the mid to late
morning hours where a band of snow moves through the area. This
should cause ceilings to drop to low VFR before improving.

&&

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

Numerous forecast challenges are present for the next couple days
with a very complex storm system moving into Southcentral.

The setup is as follows: A slug of warm, moist air streaming
northward from the tropics is colliding headlong into a cold air
mass already in place across Southcentral. In a typical scenario,
the center of the low (currently south of the Alaska Peninsula)
would continue northward, drawing the warm air along with it and
effectively scour out the cold air.

However, in this scenario, the low is both nearly stationary, and
associated with very cold air, represented by low 500 mb heights.
As a result, the cold air is remaining very persistent, as
northerly winds stream down the Cook Inlet. Thus for the Cook
Inlet region, freezing rain becomes a concern. Also, the cold air
associated with the low is forecast to gradually work eastward,
shoving the warm air aloft out of the area by Tuesday night. Yet
another complicating factor is the level of downsloping. Currently
downsloping has kept all precipitation in the Chugach and Kenai
Mountains with relatively little getting over the mountains. The
usual setup with southeast flow also shows strong upsloping in the
western Susitna Valley as well. Freezing rain will be possible
where warm air moves above the cold air near the ground, and also
where precipitation continues outside of strong downsloping areas.

As discrete shortwaves of upper-level energy move across the area
in the broad southerly upper-level flow, it appears likely that at
times the winds may shift with the passage of these waves to
temporarily end the downsloping and allow some light precipitation
to get into Anchorage and the northern Kenai Peninsula regions.
In particular, one of those shortwaves looks to come in this
morning, allowing for some potential for freezing rain to develop.
Fortunately it looks to be light, but it may still be enough to
cause treacherous conditions on area roadways and sidewalks, which
regardless of the air temperature, remain well below freezing.
Additionally, with above freezing temperatures and some modest
snow melt, a thin layer of liquid water on top of packed down ice
on many area roadways will also make those surfaces very slippery.

Yet another forecast challenge is evaluating the likelihood of a
precipitation type changeover at all in Valdez/Thompson Pass. The
forecast models are all over the spectrum with this. If you follow
the GFS, Valdez changes over to rain later this morning and snow
totals are held to essentially what has already fallen. The NAM
however keeps Valdez all snow for the next 3 days, which when
taking an atmospheric river's worth of moisture and dropping all
if it in the form of snow would result in totals requiring a
yardstick to measure from this system alone. Fortunately for snow
lovers, the GFS solution is showing temperatures getting up well
into the 40s Monday night into Tuesday morning, which is
completely unrealistic given the transient nature of the warm air.
However, the NAM solution of keeping the precipitation type all
snow for the next 3 days and beyond with temperatures never
reaching freezing also appears unlikely, as the warm air that does
get there should have some effect of warming the temperatures and
changing over the precipitation type, at least for a time. So for
now the best course of action is a middle ground, with the
expectation of some changeover to rain sometime Monday, but not
enough to inhibit Valdez and Thompson Pass picking up at least a
foot of new snow. Thus the largest degree of uncertainty is
determining when exactly the precipitation changes over. The
longer it takes, the more the snow piles up.

Then as if all that wasn't enough, on Tuesday night and especially Wednesday
when the cold air returns from the southwest at all levels, that
flow will put the previously downsloped areas of Anchorage and the
Matanuska Valley on the upslope side of things. As the wave
causing the precipitation passes through, snow should break out
across much of Southcentral, likely favoring the higher elevations
of Hillside, Eagle River, and the Matanuska Valley. Several inches
of snow will be possible. Stay tuned.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A cold upper level low centered near Nunivak Island early this
morning drops south, crossing the western tip of the Alaska
Peninsula this evening as a surface low south of Kodiak Island
rotates north crossing the Alaska Peninsula on its way into
Bristol Bay. A frontal band associated the surface low is
currently rotating through Southwest Alaska. The low will stall
around Dillingham where it will remain through this evening
continuing to bring snow, breezy northerly winds and some areas of
blowing snow. The surface low will follow the path of the upper
level low rotating west out of Bristol Bay, back across the
western tip of the Alaska Peninsula on Tuesday, only to swing back
to the east and continue this spiraling path through Wednesday.
The net result for Southwest Alaska will be continued northerly
flow with several more frontal bands rotating through and bringing
snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As the cold upper level low and the surface low south of Kodiak
Island spiral around each other, their mean center will become
established over the Alaska Peninsula as they rotate around each
other through midweek. A front associated with this system will
push into the Pribilof Islands this evening bringing snow, strong
north winds and blizzard conditions through Tuesday. With the
northerly flow continuing south into the eastern Aleutians and
western Alaska Peninsula expecting to see some blowing snow in
those locations as well. Cold northerly flow and snow showers will
continue over the western Bering through midweek.

&&

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

By Tuesday evening there will be a large closed upper low centered
over the Eastern Aleutians with a broad southerly flow across
Southcentral. Colder air wrapping around the low will result in a
mix of rain and snow chances from the coast inland. Across
southwest Alaska, enough cold air will be in place for all snow.
Over the Bering, continued cyclonic flow aloft with cold air
streaming over warmer waters will support widespread snow
showers. By Wednesday and Wednesday night a series of weak
disturbances along a baroclinic zone in the Gulf of Alaska will
work northward toward the coast. Much uncertainty remains with the
track of these shortwave troughs, so for now will hold with inland
snow chances heading through the weekend as another arctic trough
moves into Southwest Alaska Friday then stalls. Colder air will
then remain in place across southern Alaska through next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 195
Winter Weather Advisory 131 145 161
MARINE...Storms 119 125
Gales 130 155 173 174 179 180 181 185 351 352 412 413 414
Heavy Freezing Spray 139 165 179 185 412 414

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...JA

PRELIMINARY LOCAL STORM REPORT...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
133 PM AKST MON FEB 13 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0912 AM     SNOW             VALDEZ                  61.08N 146.30W
02/13/2017  M20.0 INCH                          AK   CO-OP OBSERVER

            20 INCHES SINCE 3 PM YESTERDAY. HEAVY SNOW REPORTED
            OVERNIGHT.

1049 AM     SNOW             6 NW BENTALIT           61.62N 150.75W
02/13/2017  M7.5 INCH                           AK   PUBLIC

            7.5 INCHES STORM TOTAL SNOWFALL SINCE SNOW BEGAN AT 4PM
            YESTERDAY. HEAVY SNOW CONTINUING TO FALL AT TIME OF
            REPORT.

1245 PM     SNOW             5 ESE RIDGEWAY          60.51N 150.93W
02/13/2017  M4.0 INCH                           AK   NWS EMPLOYEE

            4 INCHES MEASURED IN LAST 10 HOURS. FREQUENT MODERATE
            SNOW SHOWERS. WINDY AT TIMES WITH BLOWING AND DRIFTING
            SNOW. SNOWING HARD AT TIME OF REPORT.

&&

RDUELL