Small Craft Advisory 381, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKDT Thu May 10 2018

.ANALYSIS AND UPPER LEVELS...
Spring is here, but we're stuck in a long-wave pattern that more
resembles September or October. A trough centered over the central
Bering Sea is the anchoring feature as the jet stream perturbs
around it, sending periodic disturbances from the north Pacific
into the Gulf of Alaska. A potent short-wave trough is approaching
the Alaska Peninsula/Kodiak Island, with a surface reflection
quickly wrapping up, including a solid gale-force front. The low
pressure system will be the driver of active weather in the
short-term.

Currently, one weaker front is exiting southern Alaska. Southeast
winds have briefly come into Anchorage along with a little light
rain as the upper trough passes through. There's not much of a
break, cloud-wise, between this system and the next, although it
does look like some low stratus has developed in response to the
upper moisture leaving, along with saturated low-levels in the
Cook Inlet area. The southwest mainland is in the same boat,
however, winds are rapidly increasing ahead of the incoming
system, mitigating any fog/stratus concerns.

&&

.MODEL DISCUSSION...
Guidance has come together on a solution for the developing low
approaching the AKpen, bottoming out this afternoon as it crosses
into the Bering Sea, the weakening toward the Kuskokwim Delta. The
next feature of interest is a developing triple-point/frontal
wave, which guidance is pretty locked in on as well, impacting
southern Alaska late tonight. The uncertainty in the forecast
comes on late Saturday into Sunday when the remnants of the
current Bering Sea upper trough interacts with the leftover front
from the developing system. Crazy stuff. The general consensus is
a central/eastern Gulf frontal wave/low.

&&

.AVIATION...
PANC...VFR conditions are expected to persist. Winds will be the
more variable element. Behind the front, early this morning, light
southerlies should continue until the Turnagain wind re-
establishes late morning/early afternoon. Uncertainty increases in
the evening as the approaching low in the Gulf shifts the pressure
gradient toward down-Inlet. The response should be to bend the
Turnagain wind away from Anchorage, which could bring some wind
shear into play, however it could bend the winds aloft to the
south as well. Overnight into early Friday, the Turnagain wind
will come back into the airport along with rain and lower
ceilings. Usually the southeast winds and lower ceilings don't
mix, but a moist system and favorable track make it possible.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A very active weather pattern will bring unseasonably wet and
windy conditions to much of Southcentral over the next couple
days. With an upper level short-wave trough lifting into
Southwest Alaska rain in the Anchorage to Mat-Su area early this
morning will taper off. However, the remnants of what was a
surface cold front will remain stationary, with rain continuing
out ahead of it in the Prince William Sound area.

Meanwhile, an intense low south of the Alaska Peninsula this
morning will track northwestward into Bristol bay later today,
with an occluded front pushing northward across the western Gulf.
Increasing pressure gradients way out ahead of it combine with
surface-based instability and a thermal trough inland to both
increase gap winds and bend them northward. Expect gusty winds
from Turnagain Arm into Anchorage, the Knik River Valley into
Palmer/Wasilla and along the Copper River. Rain will spread
northward with the front from Kodiak to the Kenai Peninsula.
Strong low level southeast flow will initially keep the bulk of
rain confined to the eastern Kenai Peninsula.

Down over the North Pacific, a digging short-wave will exit the
trough and head back northward with a strong upper level jet
streak, leading to development of a frontal wave along the surface
cold front. Both upper and lower level features will track
northward across the Gulf this evening then quickly inland across
Southcentral tonight. The dynamics associated with this are quite
impressive for this time of year. Expect heavy rain to spread to
Prince William Sound along with widespread very strong winds.

While downslope flow will initially change keep the lee of the
Kenai and Chugach Mountains dry, this will rapidly change early
Friday morning as the surface wave (possibly a closed low by this
point) tracks inland across the Kenai Peninsula then by Anchorage,
with flow shifting to the south and allowing rain to spread over
the western Kenai, Anchorage, and the Mat-Su. In the wake of these
very strong features, colder low level air will move in behind,
increasing instability. This will lead to cloudy and showery
weather across the region on Friday along with gusty winds.
Pressure gradients will slowly slacken Friday night through
Saturday, allowing winds to diminish. However, continued low
level instability combined with a series of weak upper waves
moving through the trough will maintain generally cloudy skies
with areas of showers.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Thu
through Sat)... Today we will see the pattern shift from
dominance from a low over the Central Bering to a new, compact low
blasting through from the North Pacific. The day will start out
dry for most areas as a shortwave ridge between these two features
passes through. But the King Salmon radar is already showing
returns with the system approaching the Alaska Peninsula. This
system will continue to deepen through this morning to a
respectable upper 960 mb low.  By late this morning and certainly
into this afternoon, these strong east to southeast winds will
cross into Bristol Bay with especially gusty conditions along the
terrain. This system will also bring another surge of warm air. So
while there will be widespread precipitation associated with the
system, it should fall as all rain except for in the highest
elevations. The strong southeast winds will also provide
downsloping conditions for many locations in the lee of the
terrain severely limiting rainfall totals.

By tonight, the low will start to weaken as it begins its trek
along the Western Capes. As flow switches around to more of a
southwesterly direction off of Bristol Bay, it will bring in some
weak cold air advection with an unstable airmass. This will bring
widespread showers across most of the area, but especially
interior Bristol Bay. It will also allow gusty conditions to
continue as some stronger winds mix down from aloft on Friday.
Conditions won't change much into Sat as the low slowly drifts
north along the YK-Delta coast. Colder air will continue to make
its way in from the Bering making for cool and mostly cloudy start
to the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3...Thu through Sat)... The low that established residence over
the Bering will finally yield the territory to an up and coming
low from the North Pacific. Winds will continue to increase
across the area in response to this and a storm warning remains in
effect for marine zone 150.For the Western Aleutians, the
departing low will leave the area under cool, showery, west-
northwest flow today. As the developing low crosses the Alaska
Peninsula and into the extreme easter Bering tonight, it will
bring a swath of gale- force winds with its leading warm front. It
will also bring heavy rain to much of the Alaska Peninsula and
parts of the Eastern Aleutians. The low will reorient the longwave
trough over the entire Bering Sea. This will leave most of the
area from Dutch Harbor through Shemya under benign northerly flow
going into Fri. Over the Eastern Bering, the wrap-around flow
around the low will bring another area of gale-force winds Fri. It
will leave all of the Eastern Bering under cooler, showery
conditions into Sat. The Pribilof Islands will be right on the
edge of more steady precipitation. We did increase the PoPs for
them Fri as models have trended just a bit further west with this
deformation boundary.

By Saturday, a decaying warm front will approach the Western
Aleutians. This will allow winds to switch to more of a south to
southeast direction and bring another round of light rain.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The extended portion of the forecast begins Saturday with the
weakening low lingering just offshore of the Y-K Delta coast,
promoting continued showery conditions for much of the southern
mainland and localized gusty winds through favorable gap flow
areas including Turnagain Arm. By Sunday, a vigorous shortwave
trough rounding the base of the Bering trough will track rapidly
to the northeast Gulf coast and help to spin up a compact surface
low. It appears at this time that the primary impacts from this
feature will be largely confined to the panhandle, but a period of
heavier rain and gusty winds cannot be ruled out along the north
Gulf coast. Forecast details become a little more muddled by the
start of next week, but the general pattern generally looks to
maintain some semblance of longwave troughing over the eastern
Bering Sea and keep southern Alaska under the influence of moist
southerly flow. This will keep Southwest and Southcentral AK in a
generally cloudy and showery regime well into next week.
Meanwhile, a broad North Pacific low will be tracking south of the
Aleutians, keeping the threat for rainy and windy conditions over
the chain for much of the long term while the remainder of the
Bering experiences more benign showery conditions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 119 130 131 138 150.
Gale 120 125 128 132 136 137 155 160 165.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...CB