Small Craft Advisory 738, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 AM AKDT Thu Sep 27 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low in the western Bering Sea is extending a
front across the central Bering. This low is eating away at the
high that once dominated the area and pushing it eastward. A
short-wave sits over the western mainland on the western edge of
a developing upper-level high over the Gulf. This short-wave is
supporting a warm front at the surface, and is advecting a moist
airmass from the North Pacific over the eastern Aleutians and
western Gulf of Alaska. Radar is showing a swath of rain along
the frontal boundary sitting over Kodiak and the southern Kenai
Peninsula and it will move northward today. Satellite shows
widespread cloud cover over the northern Gulf of Alaska and ahead
of the low in the western Bering Sea. Winds are picking up again
slightly in the western Gulf of Alaska as the front progresses
northward. While southwesterly flow dominates the upper-levels
over Southcentral, near the surface, winds are generally coming
from the northeast.

&&

.MODEL DISCUSSION...
Short term model agreement remains good with respect to bringing
the warm front up from the Gulf of Alaska into the southern
mainland through today. This will be followed by high pressure
building over the Gulf of Alaska both at the surface and aloft
that will serve to hold the low moving up to the Alaska Peninsula
tonight out over the eastern Bering Sea. The models are consistent
and in agreement with the low over the eastern Bering Sea
elongating and weakening to the northwest through Friday morning.
There is some model disagreement with the precipitation spreading
over southwest Alaska through this afternoon as the AK- NamNest is
more aggressive with higher QPF over the Bristol Bay region this
afternoon than the High Res NMM, the Canadian and the ARW.
Considering the cross barrier flow, we will lean to the lower QPF
as there should be some low level evaporation in the downslope
flow off the Aleutian Mountains. A change in the models from last
night is a second band of moisture will track up into south
central Alaska and the Copper River Basin following this first
shot moving through today. This will keep south central Alaska,
Prince William Sound and the Copper River Basin in cloudy
conditions with a continued chance for rain.

Continuing toward the weekend the models remain on track with
ridge of high pressure extending from the Gulf of Alaska over the
central mainland. This will keep south central Alaska and the
Copper River Basin on the less cloudy and warmer side.

&&

.AVIATION...
PANC...VFR conditions with light winds are expected to continue
this morning, with no fog and stratus expected. The clouds and
rain are slowly working up the Inlet, slower than previously
expected and should spread into the airfield late this morning
around 18-20Z possibly bringing ceilings down below 4000 feet
around 21Z. There is a slight chance of ceilings around 2500 feet
around 03-06Z. This all depends on the magnitude of the down
slope flow off the Chugach as the warm front moves up.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
While the general forecast of rain for Southcentral today is
still on track, the evolution of key weather features continues
to change and affect the details of timing of rain and location of
heaviest rain. 24 hours ago it looked like the short-wave trough
in the eastern Bering would ride over top of the east Pacific
upper ridge and right across Southcentral Alaska today. However,
stronger ridging is forcing that short-wave due northward across
Southwest Alaska, with no impact to Southcentral. The atmospheric
river (with deep tropical moisture) coming up the west side of the
ridge is pointed northward right across Kodiak Island and the
western Gulf. Instead of being forced eastward by the short-wave,
the AR will shift northwestward to the eastern Bering Sea later
today as the ridge continues to build up from the south and east.
A weaker piece of the the atmospheric will still round the ridge
across the northern Gulf, but overall the rain does not look as
heavy as it would have been otherwise. The progression of the
rain into Southcentral is also slower. Thus, expect rain to start
later than originally forecast and end later.

There is quite a bit of uncertainty in the northern extent of
rain today, as heights quickly rise over the western Gulf/eastern
Bering Sea causing flow over Southcentral to shift from southwest
to westerly. Because of this, the northern Susitna Valley and
northern Copper River Basin may stay completely dry, but it's
difficult to pinpoint exactly where the northern edge of the rain
will set up. The potential for snow is looking less and less
likely for highway passes of the Copper River Basin with the later
start to precipitation and warm air advection moving up from the
Gulf during the day. The most likely place to perhaps see a wet
mix of rain and snow is along the Glenn Highway near Eureka.

While the bulk of rain will exit eastward overnight tonight, the
warm front will continue to lift northward across Southcentral,
possibly bringing another round of lighter rain right along the
front during the day Friday. With lots of variation in the model
guidance, have used a broad-brush approach to the precip forecast
on Friday. In any case, there is higher confidence in dry conditions
and clearing skies as the upper ridge continues to build northward
over mainland Alaska on Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The latest image from the King Salmon radar (PAKC) has detected
rain from Port Alsworth to Pilot Point. This precipitation is a
sliver of the atmospheric river in the North Pacific. The ridge
over the Gulf Of Alaska will amplify during the forecast period
which will keep southerly flow in the boundary layer over
Southwest Alaska through Saturday. The wettest day of the forecast
period will be today and tonight. Then there will be a break
before the next storm moves in off the Bering. With the ridge in
place expect above normal temperatures for Saturday across
Southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A wet pattern is anticipated this forecast period. There is an
occluded front over the Central Bering just west of the Pribilof
Islands that bisects the Central Aleutians. There is a second
frontal boundary over the AKPEN and Eastern Aleutians. Gale force
winds have been detected with both storms on ASCAT/advanced
scatterometer data from GOES West. The models show additional
shortwave energy diving down the backside of the upper low to the
west, helping to reinforce it. All the while, the frontal boundary
near the Eastern Aleutians will continue to push north. The upper
low remains quasi-stationary over the Bering most of the forecast
period.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning the extended period forecast Saturday night, there will
be a very high amplitude (3.5 sigma+) upper level ridge over the
state of Alaska with a deep upper trough over the Bering Sea.
Mainland high pressure will remain dominant which means dry and
beautiful with warmer than average days through the remainder of
the weekend. Moreover, with the eastern part of the ridge over the
Yukon, this will promote a northerly flow aloft which may enhance
some local offshore winds, thus ensuring a dry and pleasant
weekend.

Heading into Monday, the global numerical models have come into
much better agreement with two important features, the recurving
former Super Typhoon Trami and the breakdown of the upper
blocking high over Alaska. It does seem more likely now that a
low moving into Southwest Alaska will begin to erode the block
sometime late Monday into Tuesday with the front eventually moving
into the Gulf Coast by midweek. This should bring more clouds and
precipitation chances back to parts of Southern Alaska,
especially Southwest, so this forecast will begin trending those
chances upwards. Regarding Trami, there remains quite a bit of
model spread, but increasing likelihood that remnants of Trami
will at least bring some potential gale force winds into the
western Bering Sea Monday night into Tuesday, so this forecast
will also trend that upward to depict this change.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 165 178 180 185 412 413 120 130 131 138.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...JA