Heavy Freezing Spray Warning 101, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 AM AKST Sun Mar 5 2017

.ANALYSIS AND UPPER LEVELS...

The synoptic pattern continues to hold firm with a highly
amplified upper level ridge remaining over the Bering Sea and
an arctic trough over the mainland. This arctic trough continue to
dig southward which has continued to feed Southcentral and
Southwest Alaska with an abundance of cold air. A reinforcing
shortwave on the western periphery of the trough coupled with a
pretty decent jet streak will help keep temperatures well below
normal in the coming days. Abundant sunshine has created nice
diurnal thermal trends and should continue to do so as clouds will
be far and few across Southcentral Alaska. Southwest Alaska is
currently battling with a cirrus shield associated with a warm
front over the Bering Sea which has kept some locations slightly
warmer tonight. In addition, a persistent low stratus deck has
lingered over the Kuskokwim Delta and the parts of Bristol Bay.
The 12Z RAOB from PABE shows an impressive radiation inversion
which hints that it could be until later this afternoon until the
stratus deck can finally mix out.

Another shortwave has begun to show itself on water vapor this
morning across Southcentral Alaska. The extra upper-level support
will help increase outflow winds through the typical gaps. The
high wind watch for Valdez and Thompson Pass has been upgraded to
a high wind warning this morning with gusts upwards to 80 mph
expected this afternoon. In addition, both Whittier and the Copper
River Delta will see gusty winds but will remain well below
warning level.

&&

.MODEL DISCUSSION...

Models remain in excellent agreement once again thanks to the
persistent synoptic pattern. The aforementioned shortwave over
Southcentral Alaska is well handled by all numerical guidance
which gives us high confidence for the start of the strongest
winds through Valdez and Thompson Pass. In addition, the models
are handling the amplifying trough over the Alaska mainland well
which gives us high confidence that this cold, dry, and windy
period will likely continue through the coming week. The ridge
over the Bering Sea will begin to shift eastward late this week
which could cause a slight warm-up over the west coast of Alaska.
Otherwise, persistence is the key to forecasting right now. For
model choices tonight, the NAM nest was utilized due to the
increased resolution with models in excellent agreement.

&&

.AVIATION...
PANC...VFR conditions will persist. Light winds will become gusty
from the north this afternoon and become light again this evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level short wave drops south across Southcentral this
afternoon and into the Gulf. The flow aloft then becomes more
northerly in the afternoon and should enhance gap flows across
Southcentral. The Valdez/Thompson Pass areas look to once again be
the strongest through this event, though somewhat weaker than the
previous and with a shorter duration. Winds likely will increase
to warning levels in the pass by mid afternoon and should
eventually get down into town by late afternoon/early evening.
The model trend has been to delay the the onset of the winds, but
for now will continue with a slightly earlier start. Gap flows
will diminish on Monday as pressure gradients relax some. Night
time temperature forecasts continue to be difficult due to the
large differences between areas that keep the winds all night and
those that de-couple.

&&

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

Northwest flow aloft will remain dominant through Tuesday night.
Today, a shortwave trough will quickly move through with colder
air moving in behind it. This will bring a resurgence of more dry
and cold air to the region with increasing offshore winds.

&&

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

Most of the action across the Bering will be west of the Pribilof
Islands as a stalled baroclinic zone remains over the Western
Bering Sea. The front will weaken Monday evening with a series of
gale force warm fronts moving into the far Western Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The high amplitude ridge/trough pattern will continue to bring
cold, dry and locally windy conditions to Mainland Alaska through
the week. By Monday night and Tuesday, the upper level ridge axis
will extend from the eastern Aleutians, across the Pribilof Islands
and through the Gulf of Anadyr to well north of 70N. To the east
of the ridge, a very broad upper level trough will extend from the
Arctic Ocean and far Northern Canada, across Mainland and Southeast
Alaska and into Western Canada. The cold air mass currently in
place will continue to be reinforced by the northerly flow aloft
and arctic shortwaves digging down the east side of the upper
level ridge.

The entire pattern will shift gradually east through the week with
the upper level ridge axis tilting east into Southwest Alaska
Wednesday and Thursday and the strongest northerly flow progressing
east across Southcentral Alaska. The upper level ridge will bring
significant warming aloft with it as it pushes onshore bringing
an increasing potential for fog and low stratus to Southwest
Alaska from Wednesday night onward. Throughout the week, frontal
systems will continue to track across the western Aleutians and
into the Bering through the southerly flow on the west side of the
ridge.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning...131
MARINE...Heavy Freezing Spray...121 126 127 129 130 131 138 139
                                141 185
                        Storm...127
                        Gale...126 128 178 185

&&

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