Small Craft Advisory 401, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
850 AM AKDT Fri May 18 2018

.ANALYSIS AND UPPER LEVELS...
The jet stream remains well to the south of the Aleutians,
ultimately feeding into a complex upper low pressure in the
northeast Pacific, just south of the Alaska Peninsula/Kodiak
Island area. The other major player in the flow pattern is an
eastern Pacific ridge, extending into the Canadian Yukon. Southern
Alaska lies between these two features, not enough under the ridge
to dry us out, but with enough jet influence to continue the
moisture stream from the mid-latitudes. A blocking high pressure
sits over the northwest Bering Sea, which keeps the jet stream
from buckling and encourages it to stay to the south. While the
"complex" low currently shows two circulations on water vapor
satellite, there are FOUR other candidates to join the party in
coming days.

From a sensible weather perspective, what does the mean? A
continuation of clouds and the same pattern we've been in for the
last month plus. A weak, decaying front remains draped from
Bristol Bay through Southcentral Alaska keeping very light rain
over much of that corridor. Generally light winds exist across the
area, save for some diurnally-driven gap winds.

&&

.MODEL DISCUSSION...
Given the complexity of the synoptic pattern described above, it
is no surprise the guidance is struggling with the details out
three to four days. The general pattern IS well agreed upon, with
a surface low developing south of Kodiak Island, however, the
details are very unclear. The position, strength of the low center
is all over the southern Gulf, as well as various frontal timing.

&&

.AVIATION...
PANC...Steady light rain and calm winds have encouraged low
stratus to develop around the northern Cook Inlet/Knik Arm area.
Now that it is established, there is no real mechanism for
dissipation or advecting it out until at least late this morning
or early this afternoon. Upper level clouds will limit surface
heating, taking away the mixing aspect, and the Turnagain wind
isn't expected to come back until late this afternoon/early
evening. While the current TAF has conditions improving from IFR
by late morning, it is possible that this one could take longer to
lift out.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Little overall break in the synoptic pattern is expected over the
next couple of days. The mean position of the broad negatively
tilted upper long wave trough over the Northeast Pacific remains
fairly steady. This will allow for a progression of upper waves
in conjunction with surface low/fronts to roll northwest across
the gulf through Saturday. Which means that coastal areas will
remain relatively wet with periods of wetting rains while inland
areas, especially the Copper River Basin, should remain somewhat
drier. That said, models are hinting at the potential for
increased chances moisture making it across the Chugach into the
Anchorage Bowl/Matanuska Valley this afternoon and early evening
just ahead of an upper wave. Strong cross barrier flow should put
an end to that late this evening.

A surface low over the Northeast Pacific moves to just south of
the eastern Alaska Peninsula tonight. This feature will strengthen
pressure gradients and bring windy conditions across many areas
tonight through Saturday afternoon. Expect brisk to strong gap
flows in favored locals (Turnagain Arm, Knik River Valley, and
along the Copper River). Winds in most areas will diminish by
Saturday evening.

Conditions on Saturday will be windy and warm with highs in the
low to mid 60s across the Copper River Basin. Rh values however,
should remain just above critical fire weather thresholds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

There are some showers on the Bethel radar this morning while the
King Salmon radar has remained relatively quiet. Today there will
be a lull in precipitation activity. However, additional moisture
will work its way into the region as an upper level shortwave
pushes across the AKPEN this evening. This next wave of
precipitation will have move lift and a different dynamic setup
from yesterday, so expect more convection Saturday and Sunday.
With the longwave trough dominating the region, expect additional
shortwaves to get ejected keeping wet and unsettled weather in
place through the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
A very progressive pattern is on deck this forecast period. There
is a pair of surface lows near Shemya and Cold Bay. Upstream there
is a third low east of Japan. The Northern Pacific will continue
to be active bringing moisture and at times gusty winds to the
Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The overall pattern will transition to a more progressive one over
the weekend, as the upper level ridge over southeast Alaska pushes
further eastward with a strengthening upper low tracking into the
southwest Gulf by Sunday evening. This will allow several systems
to rotate into the southern mainland over the weekend and into
next week. Strong southeasterly onshore flow associated with these
systems will favor coastal areas for precipitation, while the
typical lee-side locations will downslope out and remain mostly
dry, though seeing continued cloudy and cooler conditions.

Further west, an upper level ridge will amplify and track over the
western Aleutians to start the week, and start weakening as it
progresses eastward through mid-week. During this time, another
large upper level low will begin to track over the western
Aleutians Tuesday night. Models are in fair agreement now that
this low will closely follow the decaying ridge eastward, and
into the north Pacific by Thursday. it is looking more likely that
the persistent pattern of cooler, cloudy, and unsettled weather
that has plagued the southern mainland for nearly the past month
will remain in place through next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...ML
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...TP