Small Craft Advisory 915, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKST Tue Nov 20 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked, occluding low spins at the center of a long
wave trough that encompasses a vast majority of the Gulf of
Alaska. Cooler, drier air is wrapping around the low which is
enhancing the occlusion process. Scattered showers are seen on
radar pushing across the Gulf Coast rotating around the top of the
surface low. Clearer skies are present further inland leading
rapid surface cooling and fog development over portions of the
Copper River Basin, the Anchorage Bowl, and the Western Kenai
Peninsula. A second long wave trough is over western mainland
Alaska and the eastern Aleutians. This trough stems from a
vertically stacked low north of Mekoryuk Island. Behind the
trough, northerly flow is advecting cold air over the Bering and
AK-PEN. At the surface, a shortwave trough is embedded within the
flow of the larger, main western trough. A north-south oriented
front sits over the AK-PEN and eastern Aleutians. High pressure is
building quickly over the western Bering.

&&

.MODEL DISCUSSION...

Models are again in decent agreement this morning with the
development and progression of the synoptic level features. There
are some differences by Wednesday/Thursday regarding a broad
trough over the Gulf and how several weak surface lows interact
with each other, though the only differences in the sensible
weather revolve around the magnitude of the northwesterly outflow
winds coming out of Kamishak Bay.

&&

.AVIATION...
PANC...Clearing skies overnight has lead to the enhancement of a
surface inversion and consequently fog development over the Knik
Arm, which has advected east over the terminal. The fog/stratus
will do the typical ebb and flow through the morning before slowly
burning off around noon, though low stratus may linger in the
area through the afternoon.

&&

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

A fairly well organized low over the Gulf drifts to the eastern
Gulf coast tonight then weakens and retrogrades west Wednesday. A
weak deformation zone with rain and snow over the western Susitna
Valley will slowly weaken through today. Locations along the North
Gulf Coast will see the lions share of precipitation (mainly
showers) with the low skirting the northern Gulf through
Wednesday. Inland areas, except for the aforementioned western
Susitna Valley, will generally see a mixed showers through today,
with a nudge toward snow snow will slightly cooler air. Showers
will diminish over most inland areas tonight and Wednesday. Patchy
fog is expected in may areas this morning across the Cook Inlet
Region as well as the MAT-SU and Anchorage Bowl. May see a
recurrence tonight, but not on such a large scale.

Westerly gales over the offshore Gulf waters will diminish through
today. Winds over the northern marine coastal waters will see
some increase today and tonight and may approach gales, but should
be on a weakening trend Wednesday morning.

&&

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

Two weak fronts keep higher chances for snow in the forecast
through Wednesday morning, then a cold dry pattern sets up
through Thursday night. The first front moves inland across
Bristol Bay this morning bringing light snow as it moves east
across the Aleutian Range this afternoon. At the same time, a
second low dives south along the coast crossing over the Alaska
Peninsula tonight. Only trace accumulations are expected as
dynamics remain weak for these two systems. Patchy fog across the
region is expected through Wednesday morning then winds become
northerly on the backside of the low pulling a cold air mass
across the region.

&&

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

Low pressure along the eastern Bering will advect a colder air
mass toward the Alaska Peninsula bringing snow and rain showers
through Wednesday afternoon; then the upper level low moves
southeast to the Gulf. To the west, the next strong front moves
to the western Aleutians late tonight bringing max gale force
easterly winds and rain along the frontal boundary. This front
begins to weaken on takes a slow northeast track on Wednesday.
However, a re-enforcing North Pacific low will kick this system
into motion Wednesday night with gale force winds and rain
spreading across to the Alaska Peninsula and Pribilof Islands by
Thursday night.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

The long-term forecast remains an uncertain one during the Friday
through Monday time frame. While all the models have a good handle
on day 4 with seasonably cold and quiet weather across mainland
Alaska, the uncertainty develops behind where a strong low
tracking eastward along the Aleutians goes, how strong it gets,
and how quickly it moves. Each of the models has a different
solution to this. As a new low over the north Pacific approaches
it, the low is expected to deepen as the two lows merge. The GFS
and EC both keep the low drifting eastward, while the Canadian
tracks it faster into the Gulf. In addition, there is significant
uncertainty by Sunday and Monday as to exactly where the cold air
sets up, with the GFS by far the warmest, the Canadian the
coldest, and the EC in the middle. Most of the models agree the
seasonably cold and quiet weather for Thanksgiving and Friday
turns warmer and wetter by the weekend over Southern mainland
Alaska, but how much so remains a question with significant
uncertainty.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 119 177 178 351 411.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP/RA
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...JW