Gale Warning 713, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
731 AM AKST Thu Dec 7 2017

.ANALYSIS AND UPPER LEVELS...
The weather over the mainland continues to be dominated by a very
amplified pattern with a broad trough over the Bering Sea
pressing up against a ridge stretching from the west coast of the
Lower 48 the British Columbia. Between these two primary features,
a deep southerly 150 kt jet is surging moisture from as far as
30N northward into the Gulf. Developing under this jet is a wave
that is beginning to move into the Gulf, which will also be the
focus for another round of rain and snow for much of the South
mainland today, especially along the Gulf Coast which will see
continued heavy precipitation. Over the Bering Sea and North
Pacific, three distinct lows can be seen rotating under the upper
level trough. The most significant of which is beginning to
approach the AKpen where it will make landfall this afternoon,
bringing snow to southwest Alaska.

&&

.MODEL DISCUSSION...
The models are in very good synoptic agreement into Friday. The
main issues are how they handle precipitation and precipitation
type over the Gulf Coast and Southcentral today. The main
difference in the models is with how far west the rain/snow line
will move through the Kenai Peninsula this afternoon once strong
warm air advection moves in from the Gulf. There is very good
agreement that this warm air advection will be sufficient to
switch the precipitation type to all rain over the Anchorage
Bowl and Matanuska Valley. This is less clear further west over
the Cook Inlet coast of the Kenai Peninsula. At this point it
appears that the rain/snow line will remain just east of the Kenai
area, however the confidence in where and when the rain/freezing
rain will transition to snow is not good. Model agreement then
begins to break down on Friday afternoon, especially when looking
at the NAM which has been consistently slower and deeper with a
weak low moving into the Gulf.

&&

.AVIATION...
PANC...Ceilings will continue falling this morning as a front
over the North Gulf Coast makes landfall. This will begin by bring
LLWS into the airport by mid-morning that will persist through
this afternoon. Precipitation will begin around midday, likely in
the form of rain with temperatures hovering around freezing at
the surface. This will bring the potential for up to one-tenth of
an inch of ice accumulation to exposed surfaces through this
afternoon. LLWS will end later this afternoon and temperatures
will rise to above freezing.

&&

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

The active pattern continues with another bout of mixed
precipitation types, warming temperatures, and locally windy
conditions on tap today and Friday. A series of waves will track
north across Southcentral through this evening. This will likely
bring another round of snowfall, although not as heavy as
Wednesdays snow, but should reach advisory levels over the Copper
River Basin until Friday morning and Thompson Pass through Friday
afternoon. Moderate to locally heavy snowfall is also likely over
the Susitna Valley north and west of the Parks Highway through
this evening. These waves will also push warmer air further north
producing areas of freezing rain across many inland areas
including the western Kenai Peninsula. The warming should be
enough across the Anchorage Bowl and Matanuska Valley to change
to dominate precipitation type to rain by afternoon. Precipitation
from the Kenai Peninsula to Mat-Su corridor will diminish overnight
and the remaining areas Friday. Elsewhere, coastal areas will
remain as rain with locally rainfall along the North Gulf Coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frontal system rotating across the Alaska Peninsula early this
morning will spread snow across Southwest Alaska today as it
continues to lift to the northwest. The heaviest band of QPF looks
to set up running from Bristol Bay north-northeast across
Dillingham and into the lower Kuskokwim Valley with accumulations
of around 4 inches out of the initial frontal band today and
another very localized 3 to 5 inches tonight out of a deformation
forming to the north of the surface low tracking north into
Bristol Bay. Another frontal system will rotate up over the Alaska
Peninsula Friday afternoon and evening but looks to stay far
enough south to mostly miss mainland Southwest Alaska. The next
front following a similar track but extending slightly further
north will rotate through Saturday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A frontal system south of the central Aleutians will track off to
the east-southeast today through Friday as ridging builds over the
western Aleutians and Bering. Brisk northwest winds will
strengthen along the east side of the ridge on Friday spreading
from the western Bering into the central Aleutians. A rather
strong frontal system associated with a likely sub-950 mb low
will approach the western Aleutians Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The pattern will remain largely unchanged into the middle of next
week as a stout ridge persists over the west coast of the CONUS
and numerous systems track northward towards the Alaska mainland
in deep southerly flow. This means a continuation of warm
temperatures running well above seasonal normal values for much of
the southern mainland, with rainy conditions along the Gulf
coast and snow possibly mixing in at higher elevations. Many
inland locations will remain on the drier side due to fairly
persistent downslope flow, with only sporadic opportunities for
showers as various systems move through. The most noteworthy
system to monitor in the long term looks to be a deep low tracking
through the western Gulf Sunday night, bringing the potential for
very windy conditions to Anchorage and other favored locations
through Monday afternoon depending on how the system evolves.
Meanwhile, the Bering will also remain in an unsettled pattern
through next week, though no major systems are expected with the
exception of a strong North Pacific low which will provide a
glancing blow to the Western Aleutians on Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 121 131 141 and 145.
MARINE...Gale 125 130 139 175.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...CB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
204 PM AKST THU DEC 07 2017

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

0158 PM     FREEZING RAIN    2 S TALKEETNA           62.31N 150.09W
12/07/2017  E0.10 INCH                          AK   EMERGENCY MNGR

            FREEZE RAIN OCCURRING ON THE PARKS HIGHWAY. VERY SLICK
            CONDITIONS. HIGHWAY SHUT DOWN AT MILE 97 FOR MAJOR
            ACCIDENT CAUSED BY SLICK ROADS.

&&

MOTTENWE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
303 PM AKST THU DEC 07 2017

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

0258 PM     FREEZING RAIN    10 NE ANCHORAGE         61.32N 149.66W
12/07/2017  U0.00 INCH                          AK   DEPT OF HIGHWAYS

            FREEZING RAIN ON THE GLENN HIGHWAY CAUSING SLICK DRIVING
            CONDITIONS.

&&

AHILL