Small Craft Advisory 858, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 AM AKST Thu Nov 9 2017

.ANALYSIS AND UPPER LEVELS...

A front moving eastward across the area this morning is the focus
for all active weather. Warm air has intruded well inland from the
coast on southwest winds ahead of the front, causing most areas to
be getting rain as it passes through. The front is the leading
edge of colder air streaming into Southwest Alaska from out of
Siberia. It has been modified somewhat from crossing the Bering,
so scattered snow showers are occurring in some areas behind the
front, such as Saint Paul. Broad southwest flow ahead of the front
is producing a stream of moisture that upon reaching the
mountains is causing precipitation as the moisture gets upsloped
out. This process will continue as the front moves into
Southcentral today.

In the upper levels, a digging trough is helping invigorate the
surface front as the wave supports the front from above. The
pattern is progressive this morning, so the trough will continue
moving eastward, allowing ridging to once again build across the
Bering. The cold air associated with the front will reinforce the
cold air already in place across mainland Southern Alaska,
allowing for a continued slow cooling trend in the temperatures,
which is only invigorated by the short days and broad snow cover
in Southcentral.

&&

.MODEL DISCUSSION...

Considering the progressive nature of the upper level pattern and
the numerous synoptic systems affecting Southern Alaska, the
models are in excellent agreement this morning, leading to higher
confidence in the general forecast. As usual, differences arise in
the smaller details of the forecast, such as how much snow will
impact Southcentral later today, which areas are favored, and how
quickly the snow will both develop and persist. While the models
all agree on a very quick shot of snow for most places, how
quickly it begins and ends, and how intense the snow will be
during the process is still somewhat questionable. We are using a
blend of the higher-resolution models to help answer these
questions. The uncertainty with the smaller details reduces the
confidence in those aspects of the forecast. As high pressure
builds in behind the front and the cold air is reinforced, the
model discrepancies diminish regarding the typical hazardous
weather in this pattern, namely that there will be strong winds
gusting through the gaps. Thus, confidence in the winds is higher.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through the
morning. A brief round of light snow will begin around midday,
causing ceilings and visibilities to fall to MVFR to occasionally
IFR with any heavier snow showers. The snow will end late
afternoon/early evening, with conditions returning to VFR.

&&

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

The trough swinging through the area is moving fairly quickly from
west to east and will move over the Cook Inlet and Susitna regions
this afternoon and into the Copper River Basin overnight.
While it does look likely that there will be snow from the Kenai
Peninsula northward into the Mat-Su areas (including Anchorage)
snow amounts look to be light. Most places will see in the
neighborhood of an inch with some higher elevations at 2 inches.
In short: a high likelihood of snow, but just not very much
accumulation.

Behind this trough, clouds will clear out again and winds will
increase from the north. Gusty winds will again develop across the
north Gulf coast by Friday and continue into the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A well defined cold front will continue moving through the area
this morning bringing a quick shot of precipitation to the
Kuskokwim River Valley and AKPEN. Southerly flow out ahead of the
front will bring a narrow wedge of warmer air leading to
precipitation falling as rain for locations near Bristol Bay while
snow will fall for locations in the northern Kuskokwim River
Valley, like Sleetmute and Sparrevohn, though only 1-2" of snow is
expected in the valleys with up to a foot of snow falling in the
Alaska Range.

Strong northwesterly flow will develop after the frontal passage
with a ridge of high pressure building in quickly from the west.
Colder air will filter down from the north leading to brisk
conditions across Southwest today and most of Friday. Warmer air
will ride over the top of the ridge on Friday afternoon which will
coincide with a weak disturbance moving down from the north
bringing some light rain and snow showers to the Kuskokwim River
Delta in the evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

An upper level ridge will build in over the Western and Central
Bering/Aleutians tomorrow leading to rather benign conditions for
the area. The ridge axis will shift east over the Central/Eastern
Aleutians by Friday allowing a frontal system anchored over
Russia to move over the Western Aleutians bringing another round
of gale force winds and rain to the area.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Models remain in very good agreement through much of the long term
forecast, with some minor discrepancies out west during the early
part of next week. Models continue to agree and provide higher
confidence in a long-wave ridge over much of the Bering Sea and
Aleutian chain, and a long-wave trough over Southcentral Alaska
and the Gulf Friday night through Monday morning. Several short
wave troughs traverse the top of the ridge, and drop northwest to
southeast across the state during the forecast period. The
strongest of these waves drop through the southern Mainland on
Sunday, providing some strong gusty outflow winds along the
northern Gulf and Alaska Peninsula through Monday. Some light snow
showers are expected to develop as this system tracks through
Southcentral, however the limiting factor on snowfall amounts will
be dependent on how much moisture is available. As of now, there
is limited amounts of moisture available during this time frame,
so we are expecting a quick shot of light snow showers Sunday into
Monday.

The models continue to struggle with regards to the placement and
track of a north Pacific low and its associated front impacting
the western half of the Bering and Aleutians Sunday into the
middle of next week. The slightly faster GFS track was preferred
overall, which the 12Z ECMWF trended toward and WPC guidance
leaned toward today. There remains lower confidence on the timing
and strength of the system overall, but there is higher confidence
in wet and gusty conditions making their way through the western
and central Bering/Aleutians Sunday through Tuesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales: 121 127 128 130 131 132 136 137 138 150 178 351
352

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...TP