Small Craft Advisory 12, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 AM AKST Fri Jan 4 2019

.ANALYSIS AND UPPER LEVELS...

The longwave pattern features a broad trough from Eastern
Siberia to Western Alaska. There is a ridge axis which spans from
Hawaii to the Central Aleutians which will serve as a break
between storms. There is a strong jet core south of 50N but a
sliver of the jet is breaking off and curving towards the Bering.
The remnants of a decaying low has brought some moisture into
portions of southwest AK and the radar at Bethel (PABC) has pinged
into a band of snow. Looking at the temperatures aloft and at the
surface, there is a cold airmass in place which has originated
from the Siberian vortex but the arctic intrusion is being
displaced away from the Western Aleutians as the next occluded
system moves in from the North Pacific. Looking at Southcentral,
specialized satellite imagery has detected several areas of fog
over portions of the Anchorage Metro Area and the Kenai Peninsula.
Last but not least, the Copper River Basin continues to stockpile
brutally cold and dense air.

&&

.MODEL DISCUSSION...
The models have converged on a solution for the Western Bering and
North Pacific for today and Saturday. The consensus is mirrored
over the Alaskan Interior through to the Gulf Of Alaska. Model
divergence begins to creep into the domain on Sunday morning but
its predominately with the decaying low near Kamchatka. The other
low near Attu and the frontal boundary pushing across the
Aleutians looks well-synced through Sunday. Other than the pesky
fog issues in Southcentral, above normal confidence with the
sensible weather highlights through the weekend.

&&

.AVIATION...
PANC...On the Nighttime Microphysics and AVHRR (which are both
satellite products) there is a bank of fog over portions of Knik
Arm, Turnagain Arm, in the Anchorage Bowl and down to PAEN. The
metars around the area all have light to calm winds which supports
the notion that its pretty stagnate in the boundary layer. The
fog situation is being actively monitored. Degraded flight
categories due to reduced visibilities and low cigs likely at PANC
this morning. The other very plausible possibility is that the
bank of fog may just remain in the vicinity. Amendments will be
made as needed.

&&

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

With high pressure over Southcentral Alaska the only issues to
look into for today are the outflow winds and fog potential.
The low level northerly flow over Cook Inlet and the Susitna
Valley caused fog to pack up along the Chugach and Kenai Mountains
overnight. Nighttime microphysics imagery from the GOES 17
satellite is showing this fog through Knik Arm and most of
Anchorage and engulfing Turnagain Arm and part of the Kenai
Peninsula. This fog may be a little stubborn to completely clear
out as the light northerly flow will persist. However, some high
clouds from a weak short wave may help to disrupt the fog as it
moves over the area later today. Fog has also developed in the
Copper River Basin and it increasing in coverage early this
morning. This fog may also be difficult to get rid of today into
tonight.

There are some outflow winds along the North Gulf coast bringing
some gusty conditions to Seward and Whittier as well as through
passes such as Thompson Pass. With the overall pattern remaining
the same though tomorrow, these winds should persist. The winds
that often develop which are not developing yet are the Matanuska
River winds. The pressure gradient shifts very slightly tonight
which could allow some to develop, but even if they do the
gradient will not be strong enough for them to be very strong.
Even so, it will be worth keeping an eye on as they can sometimes
develop even without a very tight pressure gradient. This is
especially true when the Copper River Basin temperatures are well
below zero which they already are, and will be getting colder.

&&

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

Diminishing of the snowfall rates across the Kuskokwim Delta is
already underway as a front moving into the area from the Bering
shears apart. Nevertheless due to a bit of upsloping, the snow may
persist east of Bethel along the mountains through the rest of the
morning. Further south, as the front moves through Bristol Bay
this morning, a similar effect of some upslope may allow some snow
showers to develop near King Salmon this morning and slowly drift
eastward towards the Iliamna area this afternoon. Elsewhere, some
patchy fog may develop in some of the same areas that will get
snow earlier in the day due to the extra moisture added to the
atmosphere from the snow and light and variable winds. All
precipitation will end tonight with continued seasonably cold
temperatures persisting.

On the way behind it is another front that looks to be an
interesting example of meteorological de ja vu all over again. The
front will approach the Kuskokwim Delta from the west Saturday
night, potentially causing briefly moderate snowfall rates around
Bethel overnight. Then the front will rapidly weaken as it tracks
eastward, but lingering snow showers along the west sides of the
mountains may persist into the day on Sunday. Thus, it appears to
be nearly a repeat of the event going on right now for most of the
area. Elsewhere, winds look to remain light and variable for
nearly all of Southwest Alaska through Sunday with the exception
of some gustier conditions along the coast Saturday evening as the
front moves in from the Bering.

&&

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

Not much is going on across the vast majority of the Bering this
morning as high pressure over the eastern two-thirds of the Basin
keeps weather conditions quiet. However, the first of two storm
systems is quickly approaching the western and central Aleutians
that will keep the area active for the next several days. The
strengthening storm system will produce widespread gale force
winds with storm force gusts over most of that area as it tracks
to the west of the Date Line this afternoon. Behind the low, a
"sting jet" is already appearing on satellite, which itself is
expected to produce storm force winds and gusts approaching
hurricane force over the Western Aleutians starting tonight and
persisting into the day Saturday. The storm weakens and moves to
the northwest, nearing the Kamchatka Peninsula Saturday night. The
leading front associated with this low will be the one approaching
mainland Alaska Saturday night as well with that front shearing
apart as it moves into Southwest Alaska.

Even as this first low weakens, the second one will be right on
its heels on Saturday night. Currently the center is expected to
track over or near Shemya early Sunday morning. Most of the
strongest winds with this system will be east of the low in the
warmer southerly flow out ahead of the center. This means the
strongest winds which currently also look to get to storm force
will take aim mainly on the central Aleutians, including Adak and
Atka, late Saturday night into the day on Sunday. The low center
will slowly continue moving into the northwestern Bering Sunday
night as its leading front sweeps across the Bering, bringing
gusty southeasterly winds to the Pribilofs and Eastern Aleutians
Sunday night, then rapidly weakens and shears apart beyond that as
it approaches the Kuskokwim Delta coast and Alaska Peninsula
Monday morning.

&&

.LONG TERM FORECAST (Days 4 through 7)...
An Arctic high over northern AK and Canada will continue to bring
cold dry conditions and offshore flow to Southcentral and
Southwest AK through early next week. Expect temperatures to be
below climo normal for the first time this winter for most of
Southcentral. The last few runs of the GFS and EC have positioned
the axis of the high over the Yukon, indicating very cold
conditions in the Copper River Basin. Meanwhile, models continue
to indicate a storm force, or high end gale force low impacting
the Aleutians and western Bering Sea Sunday then winds should
diminish and precipitation will transition to a showery pattern
Monday as the low shifts northward into the central Bering Sea.
Expect a series of weak fronts associated with the low to bring
onshore flow and potential for precipitation to the AKPEN, Bristol
Bay, and Y-K Delta areas early next week. There's uncertainty
with the timing of these waves given the dependence on the exact
progression of that main low, which has differed between various
models and runs. High pressure over Southcentral will erode by
mid next week, bringing increasing chances for precipitation by
the second half of the week.

&&

.MARINE (Days 3 through 5)...

A storm force low will move into the western Bering Sea Sunday
then track north toward Eastern Siberia by Monday. The front will
bring a swath of southerly gale/storm force winds to the Central
Bering Sea before the front exits the north Bering Sea Monday
night. Much quieter weather moves into the Bering Tuesday as the
storm track shifts south.

Over the Gulf waters, the weather regime will remain quiet as
north/northwest offshore winds remain below gale force. A weak
front moves into the Gulf waters on Tuesday, but the front will
be weakening and also currently looks to remain below gale force.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 165 170 171 172 173 174 175 176 177 179 181 411 412
413 414. Storm 178 185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
MARINE/LONG TERM...JA