Small Craft Advisory 107, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKST Wed Feb 1 2017

.ANALYSIS AND UPPER LEVELS...

A deep upper trough over the Western Bering Sea is amplifying a
strong ridge over the Eastern Bering Sea. A weakening upper trough
stretching from Kodiak into Southcentral will continue to rapidly
shear out and weaken as high pressure ridging over the Eastern
Bering expands and builds into Southcentral into the weekend.
Leftover moisture and weak winds across Southern Alaska have
allowed for the development of local fog, especially across
favored areas over Southwest Alaska. Showers over the North Gulf
Coast are rapidly diminishing as the aforementioned upper trough
falls apart. Out west, near hurricane force wind gusts at Adak are
diminishing as the strong warm front slowly drifts east and
weakens.

&&

.MODEL DISCUSSION...

The numerical models are in excellent agreement with both the
large scale pattern and the mesoscale pattern. Confidence is very
high except with potential fog formation across parts of inland
Southern Alaska. The NAM hi resolution 6km was favored on both
sides owing to its depiction of local wind patterns.

&&

.AVIATION...
PANC...VFR conditions will persist through the daylight hours.
There is uncertainty in the development of some fog and/or low
stratus tonight, but given the weak gradient and clearing skies,
it seems probable that patchy fog or low stratus threats will be
an increasing concern late by the overnight hours. However, the
thermal gradient between knik arm and the ambient air mass is
small, so for now the thinking is any fog will be very patchy and
not widespread.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A period of colder, dry, and cloudless conditions are setting up
over the area. These conditions will also make for gusty gap winds
across the north gulf coast. A pattern change is underway as a
large closed upper-level ridge builds over much of the state,
dominating the troposphere with unrivaled subsidence. From a
sensible weather perspective this means a prolonged period of
clear sky, and colder temperatures as radiative processes control
the surface temperatures. However, high pressure setting up over
the Interior will promote a strong surface pressure gradient over
the Chugach. The outflow pattern will affect mainly the north Gulf
coast, especially Valdez/Thompson Pass. As far as outflow events
go there are a few key ingredients missing to make it very strong,
as this will primarily be driven by a shallow cold air pool in the
Copper River Basin and the aforementioned surface pressure
gradient. The core of the upper-level jet is to the east and no
strong downward vertical motion associated with shortwave trough
passage. The strong winds will keep temperatures along the coast
elevated near or above freezing during the day, not much lower at
night.

Over in the Cook Inlet region, things are much less exciting,
meteorologically. The building ridge will promote some fog
development around Knik Arm, but a Matanuska Valley wind will
bring some dry air into the mix, potentially limiting a widespread
event. Strong radiational cooling will make for a steep
temperature inversions with near freezing temperatures around
3000 ft, and lows in the single digits near the surface. Expect
patchy fog around Cook Inlet with generally light winds and below
normal temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
High pressure over the eastern Bering is building in over mainland
Alaska where it will stay through the end of the week. As the high
moves over mainland Alaska surface flow will become dry and
offshore helping remove the lingering fog and stratus. Clear
skies will then dominate southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front over the Central Aleutians will dissipate through
Thursday as another front moves in over the Western Aleutians.
Both have a strong push of warm air that will lead to most
precipitation falling as rain. Further to the east, a shortwave
will track across the Eastern Aleutians and Alaska Peninsula
Thursday bringing snow and rain showers to the Pacific side of the
islands and Peninsula into Friday morning.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning late this week there remains very good agreement that a
blocking ridge setting up over the Mainland will bring sunny
skies throughout the area (with the exception of the Bering
Sea/Aleutians) into this weekend. The main concern from this
pattern will be for the potential for fog and stratus development
and outflow winds along the Gulf Coast. At this point it appears
that the strongest winds should occur from Eastern Prince William
Sound into the Copper River Delta where the strongest pressure
gradient will be able to tap into the available cold air in the
Copper River Basin. It is also very clear that Southcentral
temperatures under the blocking ridge will be typically for strong
inversions this time of year, with temperatures fall below normal
in the valley's while rising to near-above freezing along higher
elevations.

By early next week the models are beginning to come to a consensus
that the shortwave rotating around the blocking ridge will descend
into the Mainland as it strengthens, causing cold air to move back
into the region through the middle of next week. This trough will
remain very dry however, meaning chances of precipitation appear
to be very limited for much of the mainland into the middle of
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 172 173 174 175 176 177 178 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...DEK