Small Craft Advisory 180, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
545 AM AKST Tue Feb 27 2018

.ANALYSIS AND UPPER LEVELS...
Snow is winding down across the southern mainland this morning as
yesterday's system rapidly tracks off to the east, yielding drier
west-northwest flow over the region. The exception to this trend
has been the Anchorage Bowl, where a persistent mesoscale
circulation over Upper Cook Inlet has kept very localized
moderate snow going over parts of south and west Anchorage for
much of the night and into the early morning hours. Otherwise, any
lingering snow showers are largely orographically driven and
limited to primarily to west facing mountain slopes. A weak
disturbance is tracking eastward across Bristol Bay, bringing
another round of snow to the Bristol Bay coast and the Alaska
Peninsula. Relatively quiet conditions are in place across the
Bering Sea as an upper level ridge begins to build overhead.
Looking out towards Kamchatka, a rapidly deepening North Pacific
low is moving almost due north into the far western Bering, with a
strong front approaching the Western Aleutians.

&&

.MODEL DISCUSSION...
Guidance has struggled mightily to resolve the mesoscale feature
bringing localized snowfall to Anchorage overnight, but otherwise
remains in strong synoptic agreement in the short term. The
pattern becomes much more amplified and transitions to dry
offshore flow dominated by cold advection and outflow winds
through Thursday across Southcentral. This will coincide with a
stout ridge building over the central Bering and shifting to the
western mainland through the middle of the week, promoting quieter
conditions over Southwest AK. Meanwhile, the front extending from
the strong Kamchatka low will gradually push eastward across the
Bering through Thursday. Given the good overall agreement, mainly
utilized the NAM to better capture localized effects with the
outflow winds in the coming days. Also incorporated the GFS
solution for the deep low in the far western part of the domain.

&&

.AVIATION...
PANC...A mesolow over Upper Cook Inlet will keep the potential for
localized moderate snow and IFR/MVFR conditions at the terminal
for the next few hours. This feature should dissipate by around
15z, yielding VFR conditions and light winds for the remainder of
the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)... Now
that the snowfall over the Anchorage Bowl from a mesolow that
developed from over Upper Cook Inlet late last night has
dissipated, most of Southcentral will still remain generally
cloudy but dry for today as a broad upper level trough moves
through the area. There is an outside chance of a few snow showers
developing mainly along the Talkeetna Mountains/Matanuska Valley
or possibly as far west as the Anchorage Bowl this evening, as the
last of the upper level trough moves overhead. This is a very low
probability. With very limited moisture, anything that does
develop, should bring only minor accumulations. On the southern
edge of this trough, a lee side low will quickly develop near
Kodiak Island this morning. This may bring a brief shot of snow
showers to the Kachemak Bay area this morning as it develops, but
the more noticeable impact will be for increasing gap winds over
the North Gulf as it races off to the east. Confidence is good
that another round of westerly gale-storm force winds will develop
from the Barren Islands southward into Kodiak Island by this
evening. These strong winds will then quickly spread into the
North Gulf Coast, bringing strong winds to the usual outflow areas
such as Seward, Whittier, and Valdez/Thompson Pass into
Wednesday. These winds will be stronger than the outflow winds
that occurred earlier this week as strong winds aloft and jet
support begin to move into the area, as well as from strong cold
air advection that will develop by Wednesday. For the rest of the
area away from the gusty winds along the North Gulf, the general
trend will be for clearing skies and offshore flow into mid week,
which should prevent any further snow or fog chances from
developing.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The upper wave moving through southwest Alaska will bring showers
to the area that will taper off throughout the day. Ridging will
then push in from the Bering helping to clear out clouds and
allow for temps to drop overnight. On Thursday a front will move
in along the coast bringing snow to the area. With the winds that
will accompany the front, blowing snow conditions may develop
along the Kuskokwim Delta coast. The front will then track east as
it dissipates Thursday evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Ridging extending from the North Pacific, across the central
Aleutians and into the western Bering will track east through the
day and make it to the eastern Bering this evening. Behind the
ridge a system will move toward the western Aleutians with its
front making it to the the islands this morning. As the main low
pressure center passes near the western Aleutians today into
tonight strong winds are expected for the islands. The front will
then slowly track across the Bering and Aleutians making it to the
eastern Bering Thursday morning. As this system tracks further
east and dissipates, more high pressure will build in over the
Bering Thursday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A trio of mature frontal boundaries will push across the Western/
Central Bering and then move into Southwestern Alaska. In each
case, the occlusion and the cold front follow a similar track,
remain intact across the Bering bringing gales and storm force
winds. The GFS has maintained decent run to run continuity with
the major synoptic features. The 12z run of the ECMWF has come
into line with the GFS with regard to the major surface features
and at 500 mb as well as with the precipitation shields. Very
confident that this progressive pattern will yield unsettled
weather across the Bering and the Aleutians most of this forecast
period. However, for the East Domain (which includes Kodiak,
Anchorage and Valdez) the long term forecast is much more
complicated and the confidence level is lower. As each frontal
boundary moves into Western AK and begins the process of decay or
frontolysis, the models handle the remnant energy different as it
propagates eastward. This is echoed by the NCEP RMOP (Relative
Measure of Predictability) for the ensemble mean forecast over the
East Domain which remains in low confidence territory the entire
forecast period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale: 119 120 121 125 127 128 130 131 132 150 155 170 173
               174 175 176 177 179 181 185 351 352 412 413 414.
         Storm: 178 411.
         Heavy Freezing Spray: 130 160 180 181 185.

&&

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

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
1224 PM AKST TUE FEB 27 2018

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

0554 AM     SNOW             4 ENE SUTTON            61.74N 148.77W
02/27/2018  M17.0 INCH                          AK   TRAINED SPOTTER

            STORM TOTAL 17 INCHES. PERIODS OF FZRA AND PL.

0600 AM     SNOW             2 SE EAGLE RIVER        61.30N 149.51W
02/27/2018  M5.5 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL 5.5 INCHES. PERIODS OF FZRA AND PL

0625 AM     SNOW             ANCHORAGE               61.22N 149.87W
02/27/2018  M11.0 INCH                          AK   TRAINED SPOTTER

            11 IN. STORM TOTAL. CURRENTLY NO PRECIPITATION.

0640 AM     SNOW             2 SE ANCHORAGE          61.19N 149.83W
02/27/2018  M9.0 INCH                           AK   PUBLIC

            9 IN. STORM TOTAL. 4 IN. BEFORE 10PM, SAND-LIKE, HIGH
            DENSITY SNOW. 5 IN. BETWEEN 10PM MON AND 6AM TUE, VERY
            LOW DENSITY SNOW.

0700 AM     SNOW             3 NE GIRDWOOD           60.97N 149.10W
02/27/2018  M12.0 INCH                          AK   CO-OP OBSERVER

            STORM TOTAL 12.0

0700 AM     SNOW             6 SE SPENARD            61.13N 149.78W
02/27/2018  M9.0 INCH                           AK   PUBLIC

            STORM TOTAL 9.0

0756 AM     SNOW             5 SSE SPENARD           61.13N 149.83W
02/27/2018  M7.0 INCH                           AK   PUBLIC

            STORM TOTAL 7.0 INCHES. 3 INCHES BETWEEN 9 PM AND 7 AM.

0900 AM     SNOW             1 W GLEN ALPS           61.10N 149.72W
02/27/2018  M11.0 INCH                          AK   CO-OP OBSERVER

            STORM TOTAL 11 INCHES. 6 INCHES AFTER 10 PM.

0915 AM     SNOW             4 SSW ANCHORAGE         61.16N 149.94W
02/27/2018  M8.0 INCH                           AK   NWS EMPLOYEE

            STORM TOTAL 8.0 INCHES. FZRA. 5 INCHES SNOW AFTER 10 PM.

&&

KUTZ