Small Craft Advisory 1025, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 PM AKST Sat Dec 24 2016

.ANALYSIS AND UPPER LEVELS...
The main upper and surface low is over the central Bering Sea
with the associated front from near Saint Matthew Island to the
Bering Sea coast to a frontal wave/surface low south of the Gulf
of Alaska. A strong zonal 160 KT jet is south of the Bering Sea
low supporting the progressive flow pattern. Another surface low
is near Unalaska Island and is rapidly moving east. This will be
the main impact driving low as it moves into the Bristol Bay
region tonight for southwest Alaska. Aloft there is ample warmer
and moist advection into south central Alaska that is taking the
area out of the cold dry pattern and shifting us to a warmer moist
pattern. Strong cross barrier flow is evident this morning across
the Kenai Peninsula resulting in the typical lee side down slope
precipitation shadow on the Cook Inlet side of the mountains.

&&

.MODEL DISCUSSION...
The models are in good synoptic level agreement through the
weekend than begin to diverge on the solution for the Bering Sea
low center Monday, though remain in good overall agreement. The
models have continued along with the theme of the low moving into
the northern Gulf of Alaska through Sunday night to Monday
afternoon and thus forecast confidence is high.

&&

.AVIATION...
PANC...The main issue will be the LLWS as the strong cross barrier
winds have set in and look to persist through the night and tomorrow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The low pressure system continues to moves eastward through the
Bristol Bay region this evening. The associated weather front has
already tracked through Kodiak Island region, and will move into
the Kenai Peninsula area this evening. This synoptic feature will
strengthen the pressure gradients resulting in strong gap winds
through the Barren Island northward into the Kenai Peninsula. Look
for strong gusty easterly winds through Portage Valley, Turnagain
Arm, and Turnagain Pass. As this front moves northward it will
bring a large swath of moisture to the Southcentral region. Look
for the temperatures being modified as the warmer air moves into
the Eastern Kenai Peninsula resulting in a transition to a wintry
mix or even all rain along the immediate coast while inland areas
remain all snow.

This looks like a long duration event for the Eastern Kenai
Peninsula as a second front enters the Gulf and keeps precip
going. Thus, have extended the Winter Weather Advisory for
Turnagain Pass area all the way through Sunday afternoon. Lower
elevations, including Portage Valley, will mix with or change to
rain for a while beginning overnight tonight. So expect improving
visibilities near sea level. With strong southeast flow the areas
in the lee of the Kenai and Chugach Mountains as well as much of
interior Southcentral will completely miss out on this event, with
dry conditions in strong downslope flow. Meanwhile, look for storm
force winds over the Barren Islands tonight, and in the northern
Gulf by Sunday morning before the winds begin to diminish as the
pressure gradients weaken heading into late Sunday midday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A potent front continues to bring gusty winds across the southwest
mainland. Due to the intensity of the low level jet, persistent
snow has fallen across the AhKlun mountains all day, while
downslope locations such as Bethel have remained mostly dry.
Temperatures have not warmed as greatly as expected, and as a
result, snow continues to persist across much of Bristol Bay and
the Kuskokwim Delta this afternoon. The snow and blowing snow
associated with this front will slowly diminish during the
overnight hours and morning hours as it pushes inland. Showers
will continues across Bristol Bay into Sunday afternoon as moist
westerly flow continues behind the front.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Broad low pressure remains in place across the Bering Sea, which
is helping to support widespread showers. By Sunday evening, cold
air advection will begin across the western Bering, and slowly
push eastward across the Aleutian Islands.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Monday evening, a weakening compact Gulf of Alaska low will
provide chances for snow showers inland, with likely rain/snow
showers along the coast. Any frontal boundary will have dissipated
by this point so any precipitation left over will be showers,
enhanced along western Prince William Sound in upslope flow. As
this system slides eastward, mostly cloudy conditions will remain
with generally light winds except for the Gulf coast where weak
gap flow will linger.

On Wednesday, a pattern change begins out west as an energetic
east Asian jet amplifies a trough over Kamchatka. The result is a
frontal boundary and large push of warm air into the arctic. As
this ridge builds into the Bering middle-late next week, the
pattern will switch from a weak outflow pattern to southwest flow.
The uncertainty lies in how far northward/eastward the warm air
progresses, and whether or not a triple point low forms to our
south, the two will go hand in hand. What is higher confidence is
it does look like we will get a trough passage from the west along
with a frontal boundary.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 125 155.
MARINE...Storm Warning 119 130 131.
 		 Gale Warning 120 125 129 132 136 137 138 139 140 141
					  150 155 170 172 173 174 175 176 177 178
					  180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...EN
LONG TERM...MTL