Small Craft Advisory 177, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
724 AM AKST Mon Feb 26 2018

.ANALYSIS AND UPPER LEVELS...
Snow has overspread much of the southern mainland this morning as
an upper level trough races eastward from the Bering Sea embedded
in quasi-zonal flow aloft. A surface reflection of this feature
is moving inland over the Y-K Delta region with an occluded front
extending eastward into Cook Inlet and the central Gulf.
Widespread gale to storm force winds are present from the eastern
Bering Sea into the Gulf of Alaska beneath a 130+ knot upper level
jet riding along the base of the trough. A showery regime
encompasses much of the Bering in westerly flow in the wake of the
trough, with another much weaker disturbance moving into the
western Aleutians.

&&

.MODEL DISCUSSION...
Models remain in good agreement in the short term as zonal flow
gives way to a more amplified pattern by the middle of the week.
The upper trough will track quickly across the mainland today,
followed by several weak shortwaves which will help to keep snow
showers going into Monday night. The pattern will begin to amplify
on Tuesday as the longwave trough is reinforced by shortwave
energy digging down from the interior, allowing flow to become
more north-northwesterly with time. Meanwhile, another deep
Kamchatka low will help to pump up a stout ridge over the Bering.
All of this will yield a drier offshore flow pattern dominated by
cold air advection and outflow winds by Wednesday. Utilized a
combination of the NAM and GFS for the morning forecast package.

&&

.AVIATION...
PANC...Snow will redevelop this morning after a brief break prior
to 15z. Snow will be focused mainly over eastern portions of the
Anchorage Bowl, with the terminal remaining on the western edge of
the precipitation. This is expected to yield prevailing MVFR
conditions at the terminal, with occasional fluctuations to IFR or
VFR over the course of the day. Conditions will improve by late
this evening as the system moves away from the area, yielding VFR
conditions for the remainder for the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
After a brief break early this morning, snow is expected to
redevelop from the Kenai Peninsula northward into the Anchorage
Bowl and MatSu Valleys later this morning. This second round of
snow is expected to bring the lower snow ratios that were
initially expected from this snow event, but have yet to occur in
some places due to gusty southerly winds that developed over West
Anchorage late last night, bringing increased supercooled liquid
water into the lower levels. This second band of snow will also
bring a shift to southwesterly flow up Cook Inlet, a much more
favorable flow for orographically enhanced snow for East Anchorage
and the Matanuska Valley. As a result of this enhancement areas
west of the Seward Highway will likely stay on the very western
edge of most of snowfall from this system and should see notably
lower accumulations by the time it is all said and done this
evening. Further north over the Susitna Valley, the expected
accumulations were increased significantly to just below advisory
criteria this morning due to stronger southwest flow than
initially expected. By this evening the models are beginning to
indicate that this system will linger longer into the evening,
especially in the Matanuska Valley which will continue
southwesterly-westerly flow with cyclonic flow aloft until late
into the evening. and as a result the end time of the Anchorage
and Matanuska Valley advisories were pushed back into the evening.

The other concern in the forecast will be for the development
outflow winds behind the Gulf low beginning today from Kachemak
Bay southward into Kodiak Island. There is still good agreement in
widespread storm to hurricane force gusts developing through the
Barren Islands by this afternoon, with storm force westerly gusts
for Kodiak Island. There should then be a brief lull in the
westerly winds on Tuesday morning as a lee side low develops off
of Kodiak Island, but outflow winds will quickly ramp up again
and spread into the North Gulf Coast Tuesday evening. These winds
will then be the primary forecast challenge looking into the
middle of the week as a surge of cold air begins to descend into
the South Mainland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The low over southwest Alaska will continue its eastward track
through the area bringing snow as it passes. There are also strong
winds associated with the low which is bringing blowing snow to
southwest Alaska. This afternoon the low will have moved off to
the east and snow and winds will diminish. This setup will also
bring cold northerly flow to the region. Tuesday night ridging
from the Bering will move over the area and help to clear out the
cloud cover. Then late on Wednesday, winds will switch to the
south and bring warmer air to southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Zonal flow and snow showers look to dominate the Bering and
Aleutians until Tuesday morning when ridging builds in over the
central Bering and a new system approaches the western Aleutians
from the North Pacific. The ridging will clear out precip as it
tracks east. Meanwhile, the new system will pull up enough warm
air from the south to bring a mix of rain and snow to the islands.
As the main low center tracks north, to the west of Shemya, it
will bring strong winds with gusts in the 60 kt range Tuesday
night. The front with this system will move over the central
Aleutians Wednesday and the eastern Aleutians by Wednesday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The GFS has maintained decent run to run continuity with the major
synoptic features. At the onset of this forecast period, the
latest iteration of the GFS ensembles has a tight clustering with
the low over the Western Bering, the high over the Yukon/NW
Territories, and the low over SE AK. There is a third low that all
of the global models are hinting at coming into the North Pacific
but there has been too much variance even with the more stable
model guidance. Any way you slice it, a progressive and amplified
pattern is on tap most of this period. The primary sensible
weather highlight to look for this package will be the storm force
and potentially hurricane force winds over the Western and
Central Bering as the low passes by Shemya and the surface
pressure possibly drops below 950 mb Wednesday. The GFS and the
ECMWF really bring that deepening trough with multiple shortwaves
across the Bering and into Western Alaska through Day 6.
Confidence is building that this unsettled pattern will
materialize.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111.
MARINE...Storm Warning 120 130 131 132 138 160 165 178 180 351 352 414.
Gale Warning 119 136 137 139 141 150 155 170 171 172 175 177 179 411.
Heavy Freezing Spray 130 180 181 185.

&&

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

PRELIMINARY LOCAL STORM REPORT...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
1234 PM AKST MON FEB 26 2018

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

0400 AM     SNOW             1 W GLEN ALPS           61.10N 149.73W
02/26/2018  E2.0 INCH                           AK   DEPT OF HIGHWAYS

            DEARMOUN AT HILLSIDE

0400 AM     SNOW             4 S ANCHORAGE           61.16N 149.89W
02/26/2018  M2.0 INCH                           AK   DEPT OF HIGHWAYS

            C STREET AT RASPBERRY

0500 AM     SNOW             4 W PALMER              61.59N 149.24W
02/26/2018  E3.0 INCH                           AK   NWS EMPLOYEE

            MIXED WITH FREEZING RAIN

0600 AM     SNOW             6 NNE EAGLE RIVER       61.39N 149.48W
02/26/2018  M1.6 INCH                           AK   COCORAHS

0630 AM     SNOW             1 SSW BIG LAKE          61.51N 149.96W
02/26/2018  M4.0 INCH                           AK   NWS EMPLOYEE

0655 AM     SNOW             BIG LAKE                61.52N 149.95W
02/26/2018  M4.0 INCH                           AK   NWS EMPLOYEE

            4 IN OF SNOW. CURRENTLY FREEZING DRIZZLE.

0700 AM     SNOW             9 S TALKEETNA           62.21N 150.09W
02/26/2018  M6.3 INCH                           AK   COCORAHS

0800 AM     SNOW             3 ESE MEADOW LAKES      61.60N 149.51W
02/26/2018  M2.5 INCH                           AK   COCORAHS

0830 AM     SNOW             5 NNW BUTTE             61.61N 149.10W
02/26/2018  M4.0 INCH                           AK   COCORAHS

1115 AM     SNOW             27 NNE CHASE            62.82N 149.91W
02/26/2018  M4.0 INCH                           AK   TRAINED SPOTTER

&&

KUTZ