Small Craft Advisory 88, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKST Sat Jan 27 2018

.ANALYSIS AND UPPER LEVELS...

A dominant, highly amplified Bering Sea upper level ridge axis
and an associated upper trough over Alaska continues to drive the
sensible weather pattern. These height anomalies are keeping an
offshore pressure gradient in place over the southern part of the
state, keeping skies on the clearer side as well. There is a
marginal gale force front barely clipping the extreme Western
Aleutians, as well as a developing gale force front which will
clip the far southern Gulf of Alaska tomorrow, but otherwise,
the weather is rather quiet, for now. However, offshore flow is
expected to increase dramatically Sunday and Monday as strong
northerly shortwaves drop south and push cold, dense air through
favored gaps.

&&

.MODEL DISCUSSION...
Models are in very good agreement with the large scale pattern
through Wednesday. There will be a series of northerly shortwave
troughs moving through Southcentral Sunday through Tuesday, and
there is some discrepancy with the exact timing these shortwave
troughs move over the region which will impact the onset of very
strong offshore winds. However, the differences are minor so the
overall impact is minimal. For this forecast, the NAM nest was
favored for the high resolution wind fields.

&&

.AVIATION...
PANC...VFR conditions and light north to northeast winds will
persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The ridging over the Bering will build in over mainland Alaska
from the northwest through Sunday and will push into Southcentral
Alaska helping to enhance the outflow wind pattern. With areas
already seeing outflow winds due to pressure gradient, this
additional forcing will help amplify winds even more Sunday night
into Monday. One of the key factors behind this is that the
ridging will help bring multiple shortwaves over Southcentral
Alaska as it builds in. Some of the most impacted areas during the
outflow will be the Matanuska Valley, the Copper River delta, and
the Thompson Pass/Valdez area. The other impact of this pattern is
that the majority of precip will stay south of the southern
mainland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Tonight through Monday night)

Very little change is seen in the latest numerical runs, as they
continue to maintain dry conditions across the region with an
offshore flow continuing. Temperatures are expected to continue
moderating through Monday, with wind speeds remaining light enough
to preclude any wind chill headlines.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Tonight through Monday night)

A ridge of high pressure will continue to provide mainly dry
conditions with light winds through Monday morning for the eastern
two-thirds of the Bering Sea and Aleutians/AKpen. Thereafter, a
few shortwaves aloft will drop south southwest across the region
on the eastern periphery of the ridge, which will allow gap winds
to approach gale force by Monday afternoon south of the
chain/peninsula. Otherwise, a pair of lows riding north along a
quasi-stationary front west of the chain will allow gales to
persist near Shemya and points westward. A few spotter showers
can't be ruled out here from time to time either.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Starting Tuesday, a high amplitude upper level ridge will be
centered over the Bering while a long wave trough will be located
over the ALCAN and southeastern Alaska, leading to continued cold
and dry northerly off shore winds. By Wednesday, a shift in the
upper level pattern will begin as a ridge over the Western US
Coast builds back in which will act to tilt the trough/ridge
couplet over Alaska on its side, with the longwave trough moving over
southern Alaska and the gulf, and the longwave ridge moving over
the northern half of the state. There is some disagreement as to
what happens next, with previous model runs showing the trough
moving off to the west which would allow a return to southerly
flow into southcentral by late next week with warmer and wetter
conditions. The EC on the other hand has been consistently much less
aggressive in breaking down what looks to be an area of strong high
pressure with significant cold air associated with it over the Yukon
Territory. This is a scenario that the models are typically too
aggressive to break down ridging over the Mainland, so as a result
the EC is the favored model beyond the middle of next week as it has
consistently been the slowest and further south with frontal
positions, and closest to the current forecast. This preference
keeps ridging cold air/dry air over the South Mainland with the
potential for outflow winds along the Gulf Coast. The Aleutians and
Bering on the other hand would would see a return to a more warmer,
wetter, and more active pattern into next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 128 160 172 178 352 411.
         Heavy Freezing Spray 127 160 180.

&&

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