Small Craft Advisory 905, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKST Sat Nov 17 2018

.ANALYSIS AND UPPER LEVELS...
The pattern remains dominated by southerly flow bringing up plenty
of warm air and moisture from the North Pacific. This is all
courtesy of a potent upper level jet at 30,000' (300mb). The jet
is mostly zonal at upwards of 150 kts south of the Aleutians. This
is allowing a broad trough to sit over the Bering. However, the
jet then takes a sharp turn northward towards Southcentral at
about 90 kts. Several wave of energy are visible on satellite
riding up on this jet stream. The initial front produced several
inches of snow over the southern Copper River Basin. Then a
trailing shortwave brought some light freezing rain this morning
to parts of the Anchorage Bowl and Matanuska Valley. The next wave
is already visible on satellite just approaching the southern tip
of the Kenai Peninsula. The warm, moist air is also impacting
Southwest AK with rain falling all the way from Bethel down to
King Salmon.

Its only out in the western portion of the aforementioned trough
over the Western Aleutians that is seeing any real cold air to
speak of. The pattern will remain progressive and dynamic through
the week. This could ultimately allow some of this cold air to
return to the mainland by week's end.

&&

.MODEL DISCUSSION...Models are not handling things well today.
Several pieces of energy are moving into the North Pacific and
then turning sharply towards Kodiak Island/Kenai Peninsula. As has
often been the case this Fall, this track has proven challenging
for the models to lock onto a solution. This morning's wave that
moved through Southcentral was not picked up at all by the models.
They look to be in better agreement with the weak one tracking
towards the Kenai this evening. However, by late Sun-Mon, there
are marked differences in how they handled the stronger system in
the Gulf of AK. This forecast package leans towards the more
consistent EC and GFS, trying to paint the closed surface low
tracking due south of Seward. The QPF on the GFS seemed a bit
heavy while the NAM was excessively drying things due to some
downsloping. So we took a middle-ground approach with
precipitation amounts as well. All models do agree that this
system appears warm enough that all locations near sea-level will
see rain (versus snow).

&&

.AVIATION...
PANC...One weak passing wave will trigger some showers this
evening, but ceilings should remain VFR. When the stronger front
arrives early Sun morning, rain intensity should increase. This
will allow ceilings to slowly lower, possibly reaching MVFR by
daybreak. There could be some light wind shear by Sun morning as
well with surface winds staying out of the northeast as mountain-
top switches to southeast.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Southerly flow between upper level troughing over Southwest Alaska
and the eastern Bering and ridging over the Southeast Alaska
Panhandle and western Canada will continue to direct warm, moist
air up into Southcentral Alaska. The next frontal wave currently
over Kodiak Island will lift north through the Gulf tonight and
then continue further inland into Southcentral Alaska Sunday. The
next frontal wave will track north into the Gulf Sunday night,
continuing north to stall in the vicinity of the Kenai Peninsula
Monday night and Tuesday.

Temperature across the Anchorage Bowl and through much of the
Matanuska valley have warmed into the mid to upper 30s this
afternoon with low 40s along the Hillside with only a couple areas
such as the Eagle River Valley and Willow area lingering near 30.
While precipitation in these colder valleys may start out with
some light freezing rain mixed in, precipitation with the next
frontal system will be predominately rain with temperatures
remaining steady or warming slightly tonight through Sunday.

A bit more low level cold air remains trapped in the Copper River
Basin with overrunning mixed precipitation types continuing to be
a concern, however precipitation amount and coverage in that area
will initially be more limited through Sunday morning. Precipitation
spreading across the Copper River Basin with a weak frontal wave
Sunday afternoon and a stronger one late Sunday night could be
more problematic depending on how persistent the low level cold
air is.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The upper level short-wave trough that stretches from the
Kuskokwim Delta through Bristol Bay will push northeast through
the area tonight. The parent upper low over the central Bering Sea
will drift slowly towards the southeast. The associated low level
front will weaken as it moves through. This will bring rain,
snow, and freezing rain to southwest Alaska. The freezing rain
will mostly fall in the lower Kuskokwim valley, with some also
possible north of Bethel as these areas still have fairly cool
temperatures at the surface. South of these areas the
precipitation will be mainly rain with quite warm temperatures,
except some snow in the higher elevations. On Sunday and Sunday
night, the upper level flow will remain southerly with the upper
low only slowly moving eastward. Precipitation will diminish to
scattered showers, and the snow level will slowly fall as heights
diminish. The upper low will open up into a trough by Monday
morning, with a low moving eastward across the northern Bering
becoming the main low. This low will then move into southwest
Alaska Monday night and Tuesday, with fairly strong northwesterly
flow behind it.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The upper low over the central Bering will move southeast tonight
and into southwest Alaska by late in the day Sunday. A ridge will
build briefly behind this low, and ahead of the next low. This
next low will send a fairly weak front through the Bering from
west to east Sunday morning through Monday morning, dissipating in
the far eastern Bering by Sunday afternoon. Another front will
push across the southern Bering and Aleutians Sunday evening
through Monday, reaching the Alaska Peninsula Monday evening. This
front will be stronger than the previous one with both winds and
precipitation. By Tuesday a flat ridge will be across most of the
Bering and Aleutians, with west to northwest winds at the surface.
The ridge is weak enough that some scattered showers will persist
in the eastern Bering and eastern Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7/Tuesday night through
Saturday)...

Some change for the positive today in the long term, in that
models are finally showing a decent picture of what might happen
next week. Going with a GFS/ECMWF solution for the long term as
those have been consistent enough the past few days to put some
trust in them. Both show troughing over southcentral AK through
Wednesday and ridging over the western and central Aleutians
through the middle of the week.

As with yesterday, ridging slowly works its way across the
Aleutians, to be over SW AK on Friday. This was similar to
yesterday's solution, so am able to put some stock in it. This
means a general cooling trend and finally perhaps some clearing of
weather later next week for southcentral and SW AK, as the ridge
will bring in some cool, dry air from the north. Confidence still
needs to grow with the above. With the way models have been
handling the weather lately, need to see this same solution for
another day or two to really buy it. The western Aleutians are
also a wash after the middle of next week, with models varying
widely with the track of the next low after ridging moves east out
of the area midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory...152.
MARINE...Gales...119 130 155 165 170 172 179.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
LONG TERM...BB