Gale Warning 727, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKST Fri Dec 13 2019

.ANALYSIS AND UPPER LEVELS...
A weakening low continues to lift slowly northward across the
western Gulf, with showers and strong winds out ahead of it across
the northern to western Gulf. With easterly flow, showers extend
onshore across the eastern Kenai Peninsula, with a few showers
slopping over the mountains to the western Kenai and Anchorage.
As per the 12Z Anchorage sounding, a warm above freezing layer
is still in place, so precipitation is falling primarily as rain.
Marginally cooler air aloft is advecting in from the east, but
it isn't enough to completely erode the warm layer. A surface
trough extends east-northeast from the Gulf low. This is leading
to strengthening winds out ahead of it, particularly along the
north Gulf coast. Steady rain also accompanies the trough.

Weak upper level waves transiting Southwest Alaska are producing
areas of mostly very light precipitation. Colder air is moving in
from the Bering with a well defined, but weak trough axis. This is
slowly transitioning precip to snow. However, for the time being
there is a wintry mix of rain, snow, and freezing rain across the
region.

Out west, a high amplitude trough is amplifying over the north
central Pacific, as a strong short-wave digs down the back side.
As a result, a new surface low has spun up on the front side of
the trough and is tracking toward the Eastern Aleutians (Dutch
Harbor). A frontal boundary out ahead of this low is strung out
along the Aleutians and southern Bering Sea, leading to rain and
gale force winds.

&&

.MODEL DISCUSSION...
Models are in good agreement with handling of large scale features
through Saturday. Solutions then diverge with track and strength
of a new surface low moving from the Pacific toward Kodiak Island
and the Alaska Peninsula Saturday night through Sunday. All
solutions are much stronger with this low than yesterday, which
makes sense based on the high amplitude pattern. However, the GFS
solution seems too strong. For now, will take an average position
and depth. While forecast confidence is low in the vicinity of
Kodiak and the Alaska Peninsula, there is much better agreement
elsewhere and forecast confidence is quite a bit higher.

&&

.AVIATION...
PANC...There is a chance of light precipitation as the upper low
over the Gulf opens into a trough and crosses overhead of
Anchorage between about 12Z and 18Z Saturday. However, low level
southeasterly flow will persist, so VFR ceilings should persist.
Precipitation-type is also a challenge, with forecast soundings
maintaining at least a small above freezing layer. Could see
rain, freezing rain, or even snow - but precipitation will be very
light and possibly not even measurable. As the upper wave passes
by, Turnagain winds will bend into Anchorage, which will cause
temperatures to jump to well above freezing. These winds will be
relatively short-lived. As a front moves into the southwestern
Gulf Saturday afternoon, down Inlet pressure gradients will
strengthen and push the Turnagain wind back over the northern
Inlet.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The low pressure system in the central Gulf is currently south of
Seward as it continues its progression towards the northern Gulf
coast. This low will continue to produce gusty east-southeasterly
winds across the Gulf as the low looks to make landfall. Rain
continues to fall across the coastal regions from Kodiak City to
western Prince William Sound and Cordova. Waves of upper level
energy will continue to rotate around this area of low pressure to
provide enough support for periods of very light rainfall or
sprinkles to make its way to the leeward side of the Chugach and
Kenai mountains this evening. This energy combined with a shift in
wind direction from southeasterly to southerly aloft, will make
for increased chances of precipitation around the Anchorage Bowl
overnight tonight and into tomorrow. This wind direction will
continue to bring warmer temperatures up Cook Inlet which should
keep precipitation in the lower elevations as liquid across
Anchorage and the Mat Valley. Uncertainty with precipitation type
remains as the trough lifts north and the Turnagain Arm wind bends
over Anchorage, as the transition to a southeasterly wind over
Anchorage would likely push temperatures a few degrees above
freezing and keep any rainfall from freezing at the surface.
However, if the wind does not bend into the area, temperatures
could stay right around the freezing mark and allow for some
pockets of freezing rain. If we are to see pockets of freezing
rain across the lowest elevations, any accumulation looks like it
will be light. Higher elevations of the passes will remain cold
enough that snow accumulations of a few inches are likely.

A front associated with an area of low pressure centered over the
Aleutian islands will cross the Gulf and make its way inland
Saturday into Sunday. Due to this, the wind direction will shift
back to southeasterly which looks to downslope over Anchorage and
western Kenai Peninsula similar to earlier this week.
Additionally, Turnagain Arm winds will pick up with peak gusts of
50 mph expected Saturday night. Due to the downsloping, expect
little to no precipitation across the region except for coastal
and mountainous locations.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Scattered rain and snow showers will continue through late
evening as upper level short-waves continue to move overhead from
low pressure over the Gulf. Patchy fog is expected to redevelop
along the coastal areas again tonight. The next front moves across
the Alaska Peninsula late tonight with the boundary spreading
rain to Bristol Bay Saturday morning. Gusty southeasterly winds
will increase as the front quickly moves north to the Kuskokwim
Delta early afternoon. The Kuskokwim Valley will not see much
precipitation from this front as the occluded system pulls the
boundary northwestward across the Bering, the eastward extent of
the boundary shearing apart. Winds across much of the Valley will
keep a light east to northeast flow, with only the Kuskokwim
Mountains expected to see snow and gusty southeasterly flow.

The second front moves to the Southwest coast Sunday morning which
will re-enforce gusty southeasterly flow and bring another round
of light precipitation. The Kuskokwim Valley will again see a
similar wind pattern across the region as the fast moving front
follows a similar track as the first one. Winds and precipitation
taper down late Monday night as the second surface low slowly
fills as begins to drift east toward the gulf.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An active weather pattern is currently underway with the next
storm system organizing south of the Central Aleutians. The low
associated with this system rapidly deepens to 955 mb as it lifts
across the Eastern Aleutians tonight. Storm force winds are
expected around the core of the low through Saturday, then winds
taper down with the system remaining nearly stationary and filling
through Saturday night. Another low develops and moves north late
Saturday night. This low crosses over the Alaska Peninsula to the
Southern Bering early Sunday, but is kicked to the east Sunday
night as yet another North pacific low races up to the Central
Aleutians by Sunday evening. Precipitation will mainly be rain
around the core of these next several systems, with mixed
precipitation or snow across the northern and western areas of the
Bering as these low keep a tap into the colder air mass over
Kamchatka.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...
The storm track during this period will be from the Aleutian
Islands eastward to the Gulf of Alaska. High end small craft
advisories or gale warnings will be the norm as each storm system
moves through. Forecast confidence with low and frontal positions
is above average.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The global models are in decent agreement with the synoptic
pattern for north Pacific and Bering through Thursday December
19th. At the onset of this forecast period there are two lows, one
near the AKPEN and a Kamchatka low. There is a dome of high
pressure between the two. There are some minor difference with the
timing of two different cyclones but the forecast intensity is
only a few millibars off, and the forecast track is very similar.
Expect these systems to move from west to east with the first
storm tracking into the Gulf Of Alaska. All the while, the second
storm will head into the Bering with the frontal boundary pushing
across the Aleutian chain. For Southcentral Alaska, temperatures
will be near or slightly above normal. For the Anchorage metro
area the chances of snow in this forecast period look very slim
given the current run of the models.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 165 170 172>174.
 Gale Warning 119 120 125 130>132 136>138 150 155 160 171 175>180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...SB/PJS