Small Craft Advisory 351, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 PM AKDT Sun Apr 29 2018

.ANALYSIS AND UPPER LEVELS...
A deep vertically stacked low over the northern Bering Sea is
lifting slowly northward. A strong upper level jet streak,
extending from the Central Aleutians northeast into Southwest
Alaska, is helping drive the associated surface frontal system
eastward across Southwest Alaska and into the western Gulf and
Southcentral Alaska. Widespread rain over Southwest Alaska is
spreading to Southcentral, with colder air changing precip over
to snow across the Kuskokwim Delta. Southeast low level flow and
gap winds are increasing over Southcentral as the surface frontal
system approaches, leading to downslope drying in the lee of the
Kenai and Chugach Mtns. However, deep southwest flow above the
low level southeast flow combined with dynamic lift at the tip of
the upper jet will largely overcome the downslope component and
bring a brief period of light rain to these areas this afternoon
or evening. Lastly, strong and gusty southwest winds behind the
cold front are pushing across the eastern Bering and into the
Kuskokwim Delta. This is expected to lead to high surf, especially
further north in vicinity of Hooper Bay and particularly at high
tide Monday morning.

&&

.MODEL DISCUSSION...
The key model difference in the short-term is the handling of an
upper short-wave as it crosses Southwest Alaska Monday night and
approaches Southcentral Alaska early Tuesday. There are
differences in orientation and speed of this trough which affects
the track of a developing surface low over the Gulf and
precipitation distribution over Southcentral. On the 12Z model
cycle the NAM is an outlier with a more positively tilted upper
short-wave and much faster progression which would keep the
surface low and bulk of the precipitation on Tuesday from Prince
William Sound eastward. The other slower solutions would bring
rain to the population corridor, from the western Kenai northward
to Anchorage and the Matanuska Valley. For now, will side with the
wetter solution.

Otherwise, models are in good agreement with large scale features
and the primary forecast challenge is timing as the very
progressive upper level flow continues.

&&

.AVIATION...
PANC...There may be a brief period of MVFR conditions in rain
near the time of frontal passage (between 02Z and 04Z).
Otherwise, unstable air moving in behind the front will cause
ceilings to rise heading through the evening and overnight hours.
Southeasterly winds will become more southerly behind the front
and will gradually diminish overnight. As temps warm during the
day Monday, winds will likely become gusty again with the
increased instability.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The front currently tracking through the Cook Inlet to MAT-SU
corridor will track through the Copper River Basin tonight. This
decaying system will continue to produce rainy and gusty conditions
through late this evening before tapering off to scattered showers
though Monday morning. Winds through favored gaps (Turnagain Arm,
Knik Arm, along the Copper River, etc.) will begin to diminish
this evening as the pressure gradient slackens. However, the gap
winds will briefly pick back up Monday afternoon as a weaker lee-
side trough sets up over the western Kenai during the day.

Broad southwesterly flow in the mid to upper levels will continue
to favor higher precipitation values along the typical upsloped
areas of the northern Susitna Valley and along the Talkeetna
Mountains. Rain is expected mostly for the lower elevations across
southcentral, with a few exceptions over the northern Susitna
Valley and parts of the Copper River Basin. Chulitna north could
see a couple of inches of snow with several inches over the
Talkeetna Mountains. Coastal areas will once again be the big
winners with regards to precip amounts this evening, prior to
tapering off after frontal passage tonight. The threat of showers
continues into Monday evening, and briefly tapers off before the
next incoming wave of precipitation tracks in by Tuesday morning.
This system will bring another round of precipitation to
southcentral Alaska through Tuesday evening.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon and Tue)...
Unsettled weather will persist across the area to start the week.
The strong low near St Lawrence Island is pushing a front through
the area this evening. While steady precipitation will come to an
end as the front races quickly east, expect plenty of showers to
move in behind it. 850 mb (5000') temperatures started the morning
around -3C. By this afternoon, they will have dropped below -6C
and they will continue to fall tonight as more unseasonably cold
air moves in. This cold air is also helping to make for rather
unstable conditions which is resulting in gusty winds across all
of SW AK. These winds will continue to work on the water and make
for some higher surf along the west coast. The SPS remains in
effect to address this. Showers will favor west-facing slopes
tonight as they transition from rain to snow. The pattern will not
change much on Mon with cool west-to-southwest winds and showers.
A weak short wave pivots around the low center late Mon into Tue.
This should allow the snow showers to become slightly more
widespread through the day Tue, which will start out rather cold.
However, we don't expect any records minimum temperatures as
stout westerly (onshore) flow and clouds will mitigate
temperatures from fully realizing the synoptic airmass. Apparent
temperatures/wind chills will reach near 20F though, so a rather
cold start to May.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Mon and
Tue)...
The 974 mb low has just about reached peak intensity near St
Lawrence Island and should continue to slowly weaken to start the
week. However, it has produced a rather cold trough across the
entire Bering Sea. On the south side of the low, there is still a
small area of storm-force winds this evening. However winds should
quickly abate as the storm weakens and starts to push out of the
area on Mon. Cold air moving over the open waters will leave just
about everywhere vulnerable to snow showers and gusty northwest to
southwest winds. An upper-level shortwave dropping in from the
Kamchatka area should help to consolidate some showers near the
Pribilof Islands Mon afternoon/evening. This shortwave will then
pull some rain up from the North Pacific and just glance along the
southern side of the Alaska Peninsula late Mon-Tue.

The next low in the series takes shape near the Western Aleutians
Mon afternoon. We backed off significantly on the intensity of
this feature today as it does not have nearly the thermal
structure and upper-level jet support it showed yesterday. It
will bring another round of rain and snow to the West and Central
Aleutians Tue as it pulls up some warmer air. It will keep the
door open for yet another west-to-east moving system on Wed.

&&

.LONG TERM FORECAST (Days 3 through 7: Wednesday through Sunday)...

The models start out the long term portion of the forecast already
in significant disagreement on the minor features impacting the
forecast area. However, there are still important areas where
there is good agreement despite this. A low over the central
Bering Wednesday morning will begin impacting the Southwest
mainland late in the day with rain and snow. It is replacing a
rather cold air mass for this time of year, so many areas should
at least start out as snow. Meanwhile nearly all of Southcentral
should get a rare dry day on Wednesday.

On Thursday, as the low becomes vertically stacked off the coast
of Southwest Alaska, the influx of energy in the upper levels
ahead of the low will get much of Southcentral into widespread
rain and mountain snow starting as early as Wednesday night.
Since the details are still very much in doubt, the strength of
the downsloping winds for Anchorage still leave how much rain (if
any) that Anchorage, the Matanuska Valley, and the western Kenai
Peninsula receive very much in doubt. A secondary round of upper
level energy may make Friday a repeat day for most areas east of
the Kuskokwim Mountains, but again the strength of the southeast
winds through the mountains will be a very important deciding
factor as to how much precipitation is received. Right now the
development of a low over the northern Gulf Friday suggests
downsloping will be more prevalent Friday.

The weekend again appears to be a wet one for much of southern
Mainland Alaska with south to southwesterly upper level flow
resulting in continuous onshore flow. The models agree on that,
but not on when and where the precipitation will fall. Right now,
it looks that Sunday will be the drier of the two weekend days,
though scattered to numerous showers are still expected Sunday
over much of the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 179 180 181. Storm 185.

&&

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