Small Craft Advisory 188, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
953 AM AKST Fri Mar 3 2017

.ANALYSIS AND UPPER LEVELS...

The large-scale omega blocking pattern remains well entrenched
across Alaska and the Bering Sea with high pressure situated over
the Bering Sea and an upper-level low situated over the Gulf of
Alaska, and another near Shemya. This is very reminiscent to the
pattern observed yesterday morning as little has changed due to
the nature of the blocking pattern. Colder air continues to filter
southward from the Arctic with 850 mb temperatures continuing to
gradually cool each day. Large diurnal temperature trends were
observed on Thursday with abundant sunlight under clear skies.

Looking at the water vapor and infrared satellite imagery this
morning, a shortwave on the western periphery of the upper level
low over the Gulf of Alaska has helped to amplify the winds over
Kodiak Island. Winds upwards to 50 kts have been reported thus far
but should gradually trend downwards after the shortwave pushes
through this afternoon. In addition, outflow winds have been
blowing strongly across much of Southcentral Alaska with sfc high
pressure building across the state and an area of low pressure
slowly dissipating as it pushes toward the Alaska Panhandle. The
low-level jet has been strengthening over the Prince William Sound
region this morning which hints at another round of hurricane
force wind gusts this morning near Valdez and surrounding areas.

&&

.MODEL DISCUSSION...

Models remain in very good agreement thanks in large part to the
current blocking pattern. A gradually breakdown of the blocking
pattern is expected to begin this weekend but looks to open up
much of the Alaska mainland to a reinforcing shot of cold air from
the Arctic. Some models hint that this reinforcing shot of colder
air could be colder than the current airmass. The consensus is
that this cold pattern likely continues through next weekend.
Models are struggling with how quickly the winds diminish across
Southcentral Alaska this afternoon as offshore flow begins to
weaken. Based on the diurnal trends and favorable RAOB soundings
for low-level mixing, the GFS is likely too fast as gusty winds
shouldn't begin to subside until this evening with the loss of
sunlight. The NAM was favored for forecast updates this morning
due to the aforementioned reasoning and it's higher resolution.

&&

.AVIATION...
PANC...VFR conditions will persist through the forecast period.
Gusty northerly winds will continue through much of this afternoon
and early evening but should finally begin to diminish tonight.

&&

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

Sunny days and clear nights will persist through Saturday night.
The upper ridge over western Alaska amplifies some today sending a
northerly jet across Southcentral and enhancing some gap flows.
The ridge weakens this evening and the main upper wind support
shifts toward the western Gulf. Warning level winds in Valdez and
Thompson Pass should end by evening. Gap flows in general across
the remainder of Southcentral are weakening also. That said,
rather brisk and gusty winds will continue across the North Gulf
Coast and eastern Kenai Pen, with gales over the western Gulf
through Saturday morning. Cold overnight lows with good daytime
temperature recovery can be expected, especially in areas
sheltered from the wind.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Mostly clear skies with diminishing northerly winds will be the
story into Saturday as high pressure remains just to the west.
The strongest winds will occur over Bristol Bay this morning, but
will begin to diminish by this afternoon as high pressure from
the Bering Sea begins to move in. This should also allow
temperatures to fall below zero throughout most of the area
tonight as much stronger inversions are allowed to develop. A weak
shortwave moving over the area on Saturday may bring some light
snow showers to the area, however these will be very quick moving
as a 150 kt jet sets up overhead, so only very minor accumulations
are expected. On Sunday, the models are beginning to back off on
the possibility of snowfall from a rapidly weakening front over
the Bering, and it now appears that most areas should remain dry
with the exception of the AK pen coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The omega block that has protected the eastern half of the Bering
this week will begin to dissipate tonight, allowing for a
weakening front to bring rain and snow into the Pribilof Islands
for Saturday. This will be a fairly short respite from Central
Bering blocking however as a weaker omega block develops on
Sunday-early next week. This pattern will once again keep
rain/snow and gale force winds for much of the western
Bering/Aleutians, while winds to the east will be much lighter
with mostly dry conditions.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By Saturday evening, the high amplitude upper level ridge will
have tilted back to extend from the North Pacific, across the
eastern Aleutians to run northwest across the Bering to eastern
Russia. An equally large scale upper level trough will extend
from the Arctic, across much of Mainland Alaska, through western
Canada to the Washington and Oregon Coast. An upper level trough
associated with a particularly cold arctic low over Banks Island
in far northern Canada will dig south into the Yukon Territory
and eastern interior Alaska Sunday. This upper trough will both
reinforce the very cold air mass and strengthen the surface high
pressure inland. Additionally, the passage of this trough across
the northern Gulf Coast late Sunday night/Monday morning will
bring another rather strong push of offshore Gap winds through
Monday night. The offshore gradient will diminish gradually
Tuesday through Wednesday, however will still be quite sufficient
for the cold, dry offshore flow pattern to persist over
Southcentral Alaska through the end of the week and beyond.

The negatively tilted ridge over the Bering will become more
upright and shift slightly east putting southwest Alaska under
northerly flow aloft Monday and Tuesday. The ridge will continue
to tilt east shifting the flow aloft to northeasterly Wednesday
and Thursday with the axis of the ridge rotating into southwest
Alaska by Friday. Over the Bering, stratus underneath the ridge
will be a forecasting challenge through the week with the stratus
reaching the Kuskokwim Delta Coast towards the end of the week.
Several frontal systems will move through the western Aleutians
through the week but will be prevented from making it further east
by the large upper level ridge in the Bering.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning...131
MARINE...Heavy Freezing Spray...121 126 127 129 130 138 139 150
                                160
         Storm...127 131 132
         Gale...119 120 120 125 126 128 130 136 137 138 139 150
                173 174 175 176 177 185

&&

SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...JR