Small Craft Advisory 86, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKST Sun Jan 27 2019

.ANALYSIS AND UPPER LEVELS...

The upper levels over the state this morning are dominated by a
highly amplified pattern with a longwave trough digging south
across the Bering Sea with a high amplitude ridge seen building
across the eastern half of the mainland. In between, a 110 kt
southerly upper jet is blasting across the AKPEN and the western
mainland, helping to support a vertically stacked low over the
eastern Bering. The upper jet can be clearly seen on water vapor
and IR imagery as a thin ribbon of moisture-laden air extending
from nearly 30N to as far north as the Chucki Sea. This
atmospheric river is tapping into sub-tropical moisture and
thrusting it north over southwest AK leading to strong winds and
heavy rain over the windward (southern) mountains with distinct
downslope holes seen on the leeward sides. Further east, ridging
is leading to rather quiet weather over southcentral with just a
thick cirrus shield from a storm system well to the south keeping
temperatures somewhat mild tonight.

&&

.MODEL DISCUSSION...
Models are in good agreement through the short term forecast
period. By Monday night models begin to show some differences
regarding a surface low approaching southcentral from the Gulf of
Alaska, specifically regarding the track of the low center. This
results is differences in the wind field and QPF amounts over the
inland areas. These details should be worked out over the next
couple of runs.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the TAF
period.

&&

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

The upper level ridge over Southcentral Alaska this morning will
move to the Alcan border by the afternoon as flow aloft becomes
south-to-southwest over the region. A frontal boundary associated
with a low in the North Pacific will move to the North Gulf
coast by Sunday afternoon and bring some snow and rain to coastal
areas.
The low approaching the Gulf Sunday night will be the first of two
that move from the North Pacific into the Gulf of Alaska from the
south. This first low will move just northeast of Kodiak Island
Monday and then the second low will absorb it in the western Gulf
Monday evening and then track inland Monday night into Tuesday.
This is important because the track of the low will determine
precipitation type for the Cook Inlet region for Monday night and
Tuesday as the low heads inland. At this time the most likely low
track will be over the Western Kenai peninsula and then into the
Susitna Valley. This is a very similar track to the low that went
through the area Thursday night into Friday last week. The low
this week is not nearly as strong and therefore there will not be
the high winds associated with it as was with the previous one.
However, this track will probably allow a bit of downsloping off
the Kenai and Chugach Mountains until the low moves over the area.
Therefore there could be a mix of precipitation types from the
Kenai Peninsula northward into the Susitna Valley (including
Anchorage and the Matanuska Valley) Monday light into Tuesday.
Rain and snow are in the forecast, but there is a chance of
freezing rain as well. This is a situation worth watching as it
develops over the next two days.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Wednesday morning)

A storm located across the northern Bering will continue to move
away from the area today, with colder air in the cyclonic flow
making inroads to the Kuskokwim Delta. With decreasing dynamics as
the system pulls away, precipitation will be on a downward trend
through Monday morning as the cooler air moves in. Wouldn't be
surprised if a few locations received some very light amounts, but
this is a marginal chance.

Further south and east, another storm system will approach Kodiak
Island over the next couple of days, with an inverted trough
retrograding across the region. This will bring a reinforcing shot
of warm advection (the horizontal movement of an airmass) as
winds become easterly. This offshore flow will allow for increased
warming in the lee of the mountains, as down-sloping effects help
increase temperatures due to frictional heating. Increased lift
and moisture with the trough will aid in precipitation
development, mainly for locations east of the Kilbuck Mountains
and on the eastern/southern slopes. Look for cooler and drier
conditions to develop area-wide by early Wednesday morning, as
the system departs the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Wednesday morning)

As a Bering low lifts north of the region by Monday morning, look
for colder air to move into the area with precipitation on the
decrease. Winds will also subside due to a weakening pressure
gradient.

Our attention then turns to a sub-960 mb low approaching the
western Aleutians tonight from the west. This system will send a
surface front eastward, with sustained storm force winds
developing west of Adak by Monday morning, with some hurricane
force gusts possible. Waves will increase as they build towards 30
feet or more given a long fetch length. Widespread precipitation
with the front will reach moderate intensity at times, decreasing
visibilities. This features will reach the central
Bering/Aleutians by Tuesday evening before stalling as frontolysis
(the decaying of a front) takes place, as the parent low fills
and weakens near Shemya. Further east, relatively benign weather
is anticipated.

&&

.MARINE (Days 3 through 5)...
The models remain in good agreement with the low near the western
Aleutians Sunday night into Monday resulting in high confidence
in the area seeing storm force winds, rough seas and another
round of precipitation. A second low quickly develops in the same
source region and will take a similar track towards the Western
Aleutians. With a dome of high pressure north of Hawaii, these
storms are following the path of least resistance. For Days 3-5
expected a batch of unsettled weather for the Western and Central
Aleutians. As for the Gulf of Alaska and PWS, the confidence level
is much lower because the features that are appearing in the
models over that region simply are displaying differences with
intensity and timing.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The models continue to show a mature low moving from the eastern
waters of Japan to Western Aleutians late Sunday and Monday.
Expect this storm to bring to potentially bring storm force winds
and more precipitation to the region Monday and Tuesday.
Additionally, there is a new low spinning up in its wake that is
progged to take a similar path. Very confident that the Western
Aleutians will see a batch of unsettled weather early this
forecast period. However, the confidence level wanes for Southwest
and Southcentral Alaska where the global models are out of sync
past the 96 hr mark which is most noticeable with the myriad of
shortwaves at 500 mb. A reoccurring theme for this winter has
been fine tuning of the forecast one or two days before the event
once the models have hopefully locked onto a solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 130 131 139 173-176 180 181.
		 Storm 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...PJS