Small Craft Advisory 825, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKDT Wed Oct 24 2018

.ANALYSIS AND UPPER LEVELS...

The low pressure system responsible for yesterday's gusty winds
(over hurricane-force in some locations) and widespread rainfall
along coastal Southcentral has moved well inland overnight and has
decreased in intensity. As a result, rainfall is also diminishing
in coverage. Winds along Cook Inlet and in the Anchorage Bowl
have also switched to the southeast in response to the low moving
west of the region. The change in wind direction has also allowed
for downsloping which has brought milder air into portions of
Southcentral throughout the nighttime and overnight hours.
Anchorage broke its October 23rd record high of 54 degrees as the
temperature climbed to 55 degrees around 9pm last night!

Elsewhere, a weak trough extending across the northern Gulf
combined with increased vorticity advecting south to north has
allowed showers to persist from Cordova, through Prince William
Sound, to Seward this morning.

Farther south, a developing 150kt jet streak amplifying over the
Pacific well south of the state is driving the intensification of
another low moving into the eastern Pacific. This is the low that
is expected to continue to intensify as it takes a similar track
to the system that affected Southcentral yesterday.

Farther west, an area of weak low pressure over Dillingham
continues to produce clouds and a few scattered showers across the
Bristol Bay region this morning. Another area of low pressure
southwest of Dutch Harbor is also bringing clouds and showers to
the eastern Aleutians. Colder air advecting in behind this system
has also produced an area of widespread small craft winds and
gales over the central and western Bering.

&&

.MODEL DISCUSSION...

Individual model run to run consistency has improved over the
past 24 hrs when it comes to the development and track of the next
low in the northern Gulf. Even the SREF and GEFS are in line with
their deterministic counterparts. However, this does not mean
that there is great confidence in guidance overall. For example,
the GFS and Canadian continue to support a stronger low with a
track that takes the center of circulation from the eastern Gulf
toward the Barren Islands by early Friday. The NAM continues to
produce a weaker low with a more northerly track, crossing Seward
late Thursday. The EC splits the difference but then moves the low
southwest towards Kodiak Island. What is for certain, is that
guidance does place a low in the northern Gulf by Thursday
afternoon, but the differences in position mean differences in
precipitation pattern and amounts and wind fields. For this
forecast package, a blend weighted heavily toward the GFS was used
for the aforementioned low. This was done given how the GFS
outperformed other guidance with the previous two systems.

&&

.AVIATION...
PANC...VFR conditions will persist through the day with an
isolated shower in the vicinity of the airport complex possible
through the first half of the day. Gusty southeasterly winds this
morning will diminish by midday then turn to the north and
increase into the afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Rain showers and gusty winds will diminish this morning as the
powerful low that moved through Southcentral Alaska yesterday
continues to weaken and move away from the area. The next low is
racing toward the Gulf of Alaska and will move into the Central
Gulf overnight tonight. The track of this low should not only be
considerably farther south as compared to yesterday's low, but it
will stall in the western Gulf near Kodiak Island Thursday night
into Friday as another low develops southeast of the Gulf and
becomes the main Gulf low. The low yesterday punched into western
Alaska and is near Norton Sound early Wednesday morning.

What all this means is that this next low will keep the strongest
winds farther offshore in the marine areas through tonight. The
low will be come a bit elongated as it moves through the Gulf
and, due to this structure, the strongest winds will not move
into the Prince William Sound area until Thursday afternoon and
evening, but should not be as strong as yesterday.

This pattern for the low also means that Kodiak will see heavier
rain and stronger winds than the last few lows to move through the
Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A weak trough across the Southwest brings relatively quiet weather
across the region today. A few showers will develop as several shortwaves
propagate northward, however little to no accumulation can be
expected. Patchy fog is possible tonight as winds remain light
and temperatures cool to near dew point values. Conditions should
improve quickly by sunrise on Thursday as the environment becomes
more unstable with the next approaching low in the Gulf. Northerly
flow increases for the area by Thursday afternoon with stronger
gusts developing along the coast during the evening. Precipitation
chances increase Thursday night into Friday along the southwest
coast and for the Alaska and Aleutian Ranges as two fronts
approach from the east and west.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main challenge in the short term forecast is wind speed as
the polar jet noses across the Bering. Winds were increased to
storm force for the Central Aleutians tonight as a significant mid
level wave dives south to the chain. This wave closes off a new
low center which amplifies the synoptic pattern as it travels to
near 40N before taking a turn to the east. A ridge quickly builds
across the western waters in response which will allow winds to
taper late Thursday. At the same time, the trough will be
tightening across the eastern Bering with northerly flow
increasing. New disturbances develop and slide south across the
Bering Strait to the Alaska Peninsula, bringing gusty winds along
with rain and snow showers across the northern waters and rain
showers to the south. The next front to watch for organizes
quickly over the North Pacific and races toward the western
Aleutians on Friday. The ridge in place over the western waters
blocks this system, but winds are expected to increase out of the
southeast for the western half of the Aleutian Chain on
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
(Friday through Tuesday)

The forecast remains complicated as model continuity continues to
be in poor agreement starting this weekend. The main issues are
related to how quickly the models break down the persistent Bering
trough, as a series of compact and strong Arctic vort maxes tries
to make inroads further south. Yesterday, the general consensus
was that this would occur towards the end of the forecast period
as they were essentially trekking from the North Slope towards
the Yukon border, allowing the trough to deamplify some as it
repositions eastward.

However, the runs today show these disturbances taking a more
meridional approach as they dive south, basically emerging from
eastern Siberia and diving straight into the Bering trough,
allowing it to persist longer as they re-energize it. Adding to the
mix are significant differences in the 500mb height fields. The
same is true (albeit slightly better than aloft) with the low-
level pressure fields in relation to surface low placement and
timing (as one would expect). Finally, with the majority of these
systems remaining over water where they can't be sampled by our
land-based radiosonde network, which is a large part of the model
initializations, it's no surprise their forecasts continue to go
awry.

One should also keep in mind that typically the models have a
fast bias in pattern change events, so it wouldn't be surprising
if this wet pattern of multiple systems lifting north into the
Gulf continued into at least the middle of next week. Stay tuned!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 120 131 132 172 173 174 178 179 181 185 411
412 413 414.
Storm Warning 119 175 176 177 351 352.

&&

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