Small Craft Advisory 853, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
1114 AM AKDT Thu Nov 1 2018

.ANALYSIS AND UPPER LEVELS...

A ridge of high pressure located across southwest Alaska this
morning was building into the Mat-Su Valleys south through the
Cook Inlet region, as an area of low pressure is quickly lifting
into the northeastern Gulf ahead of the building ridge. This
mornings soundings show pretty stout low-level inversions with
abundant dry air across these locations.

Further west, an area of low pressure was lifting north through
the central Aleutians, with Gale Force winds already observed
south of the chain in the observations. The latest scatterometer
pass indicates these winds extend north into the southern Bering
as well. Finally, a strengthening area of low pressure was located
south of Shemya, with satellite imagery showing cloud tops
continuing to cool.

&&

.MODEL DISCUSSION...

Overall the models remain in good agreement at the surface and
aloft. The main forecast issue with the models revolves around
when and where a triple point low develops southwest or near
Kodiak Island, and how fast its attendant front swings northward
towards the southcentral coast. The GFS continues to be slightly
faster with this feature, but, the latest NAM is now showing a
quicker low development which suggests maybe it's trying to trend
towards the GFS. In any cast, forecast confidence is above average
given the good continuity.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
High pressure at the surface and aloft will continue over the
Southcentral mainland into Friday. The offshore surface gradient
is stronger today than it has been, resulting in the gap winds
increasing. Weak flow over the Copper Basin will keep threats for
fog and low stratus in place into Friday at least. A front will
move into the northern Gulf on Friday, and the upper level high
will begin to retreat northward. The front will stretch out and
be oriented east-to-west just off the north Gulf coast Friday
night. Precipitation will reach Kodiak Island during the day
Friday, and to the north Gulf coast Friday night. Dry conditions
are expected for most inland areas as the front stalls.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today through
Friday Night)...

High pressure remains established across Southwest Alaska this
morning. The first forecast challenge remains the ongoing fog
across Bethel and the Kuskokwim Delta. Offshore winds, albeit
fairly light, are not having much effect on the fog so far, so
it's likely it will persist into the morning, with perhaps some
patchy fog redeveloping again tonight. A weakening front and
associated low will keep things breezy along the coast through
Friday. On Friday morning, a front associated with a strong low
over the eastern Aleutians will bring warmer air and rain chances
to Bristol Bay, with higher elevation snows for the Alaska Range
and the Kuskokwim Mountains. As the front presses inland, it will
weaken, but in the unlikely event some of that moisture gets over
the Kuskokwim Mountains into the Kuskokwim Delta Friday evening
and Friday night, there is a slight chance for some patchy
freezing rain as the warm air associated with the front is
unlikely to succeed at scouring out all of the cold air near the
surface. Gusty gap winds through Lake Iliamna will increase Friday
afternoon and night as the front moves in.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Today
through Friday Night)...

An area of low pressure currently spinning south of Adak this
morning will be the focus for all of the Bering's weather over the
next two days. A dissipating front, currently over the eastern
Bering, will be absorbed into the strengthening low as tracks
south of Dutch Harbor this evening. Much of the strongest winds
associated with the low will be with the occluded front that moves
east of the low into the Alaska Peninsula tonight. Widespread
gale force winds (35-45 kt) are expected, with a brief period of
storm force winds (50-60 kt) as well south of the Alaska Peninsula
tonight into the day Friday. As the center of the low approaches
the Pacific side of Unalaska Island Friday morning, it will stall
out there for the rest of the day Friday. This will allow for a
prolonged period of strong gale force winds across much of the
southern Bering from Bristol Bay through almost to Atka from late
tonight through into Friday evening. The weakening low at this
point will then begin a slower eastward movement, with the center
remaining south of the Aleutians and Alaska Peninsula into
Saturday morning. Since the low will bring in slightly warmer air,
all areas impacted by the low should see only rain as the
dominant precipitation type.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The extended period begins Saturday evening with a ridge of high
pressure situated over the Alaska mainland. Near Kodiak Island, a
surface low is beginning to fill as it loses upper-level support
with the jet now racing west to east well south of the state. A
decaying occlusion remains draped across the Gulf east of the low
with a secondary surface low over the Alaska Panhandle. farther
west, conditions remain unsettled as an inverted trough lifts
north across the Bering while a mature low begins to slide east
from Kamchatka. Between the two, a weak ridge lies across the
central Bering. However, low-level moisture and a west-northwest
flow is keeping showers across this part of the chain.

Moving forward through the weekend, the low near Kodiak moves west
to east across the Gulf. Uncertainties in the forecast remain as
they have in previous days, though, as the GFS continues to
strengthen the low and track it farther into the northern Gulf
than the EC, NAM, and Canadian. Given the combination of colder
surface temperatures, the strength of the high pressure over the
mainland interior, and current outflow winds, the GFS is seen as
the outlier in bringing the low and associated occlusion into the
northern Gulf. With the EC favored, this would suggest limited
moisture for southcentral, with precipitation confined to the
coast this weekend. Farther west, the latest GFS/GEFS runs are
also the most aggressive with the placement of the Kamchatka low
and its associated front draped across the Bering. This, too, is
seen as the outlier, with the latest EC runs favored. This would
place the front over the central Bering and Aleutians by Sunday
with a weaker trough along the southwest coast.

Uncertainty within the guidance grows quite a bit moving into
early next week as the GFS continues to erode the high pressure
over the mainland and push the trough over the Bering east. The EC
and to a lesser extent the FV3 amplify the long-wave pattern,
keeping a stout trough over the Bering and ridge in place over the
Alaska mainland. While guidance from the GFS and EC would keep
conditions unsettled across the Bering to the southwest coast
(strong westerly winds and plenty of precipitation), the
differences become more pronounced downstream. To put it
differently, the GFS solutions favor shortwaves pushing across the
southwest and eventually into the northern Gulf and across
southcentral beginning early Monday lingering through midweek. The
EC would favor a drier solution, holding off on the advancement
of the trough through the interior until Tuesday. But the EC also
has a distinct south to north flow east of the AKPEN which would
help advect more moisture and warmer air into southcentral by mid
week.

Given the issues with the GFS in the short term, it is difficult
to lend it any credence through the mid term. Therefore, the
forecast into early next week is weighted toward the EC solution.
However, it is also difficult to buy into the large amplitude
solution of the EC moving beyond Monday. Therefore, after Monday
a compromise between the EC and GFS is favored. This would begin
to erode the high from the south and west late Monday and
eventually bring moisture back into the mainland interior and
southern coast by Tuesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm warning 150 155
 Gale Warning 130 131 132 136 138 160 165 170 171 172 179 180

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...TM