Small Craft Advisory 229, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
530 AM AKDT Sun Mar 18 2018

.ANALYSIS AND UPPER LEVELS...

A front is currently moving into Southwest Alaska this morning,
causing areas of precipitation well out ahead of it. This front is
the leading edge of cold air that will result in a pattern change
for Southern Alaska for much of the upcoming workweek. Meanwhile,
prefrontal energy is causing a few rain and snow showers,
particularly over Prince William Sound, with that shower activity
largely ended around the Cook Inlet region. The exception being
some isolated showers moving northeastward up Cook Inlet.
Precipitation associated with the main front is diminishing across
the Lower Kuskokwim Valley and Bristol Bay this morning. Plentiful
residual moisture in place across much of Southern Alaska is
causing fog and low stratus issues once again in places. A much
stronger upper level shortwave is over the western Bering this
morning. This will help to reinvigorate precipitation along the
front as it races along a 120 kt jet into Southern Alaska tonight.
More details on that below.

&&

.MODEL DISCUSSION...

The models remain in good agreement on the large-scale. Thus,
forecast challenges remain timing the small-scale features
embedded in the larger scale flow. Among these include determining
how much and the duration of precipitation today through Monday in
the Susitna Valley, with some models shutting off the
precipitation for a time this morning, but now most keeping it
going straight through into Monday. Where precipitation remains
all snow north of Talkeetna, a long-duration light snow event is
likely. Meanwhile, other forecast challenges include the strength
of the precipitation through Southwest and Southcentral tonight as
a strong upper level shortwave moves through. Precipitation type
issues are likely as temperatures hover at or above the freezing
mark. Then on Monday morning, the cold air moves in first through
Southwest Alaska, then into Southcentral, which will turn any
lingering precipitation over to snow. However, there are still
some significant differences as to how long this lasts. Behind the
front Monday through Wednesday, winds will be the primary weather
hazard, as the cold air drains through the gaps, resulting in
high wind concerns at times. With so much going on, forecast
confidence in any one aspect of the forecast is below normal.

&&

.AVIATION...

PANC...Low stratus early this morning may briefly cause MVFR
ceilings over the airport. Isolated showers along with patchy fog
cannot be ruled out. Precipitation returns tonight, with the type
still not certain. However, with southeast winds expected, rain is
most likely. MVFR to possibly briefly IFR conditions are expected.
The rain will end Monday morning as stronger winds usher in colder
temperatures.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak shortwave passing over the Southern Mainland will allow for
rain and snow showers to continue from the Susitna Valley
southward into Prince William Sound this morning. After this
passes over, most areas outside of the Susitna Valley should see
a brief break in the precipitation ahead of a much stronger upper
level trough that will move into the area by this evening. There
remains a small amount of uncertainty in how late in the evening
this precipitation will begin, but there is now good consensus of
a general deteriorating trend over all of Southcentral this
evening into early Monday morning. Precipitation type will also be
a bit of a challenge from this event, especially over the
Anchorage Bowl and Matanuska Valley where southeasterly winds from
the developing lee side low over the North Gulf should keep most
precipitation as rain or a rain/snow mix until early Monday
morning when winds should cease enough cold air surges in to
allow for light accumulations (less than an inch or so) during the
Monday morning commute. Further north over the Susitna Valley
conditions should stay cold enough to support just snow for the
event, however with temperatures mostly hovering around freezing
accumulations are expected to remain just below advisory level for
the event.

Starting late Monday morning most areas should see an abrupt
change in the weather as strong cold air advection begins to
spread in behind the departing Gulf low. This not only will allow
for any snow that does develop in Southcentral to end quickly as
drier air moves into the area, but more importantly for the
development of gusty gap winds throughout the Gulf Coast. There
remains good confidence that this low will bring widespread gale
force winds to much of the Gulf and North Gulf Coast beginning
late Monday. This will also bring strong outflow winds to the
Seward, Whittier, and Valdez areas that will persist into Tuesday.

&&

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

A front associated with a low near the Kamchatka Peninsula will
continue to move onshore on today. The precipitation shield has
moved east of the Bethel radar (PABC) with the latest metars at
Aniak and Sparrevohn recording snow this morning. Post-frontal
precipitation is anticipated today for portions of Southwest. With
colder air advecting into the region, generally expecting light snow
or snow showers but it is possible that some of the coastal areas
along Bristol Bay will see a mix. Monday will be drier across the
Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

At the surface, there is a low near Kamchatka with an occlusion
over the Northern Bering. This low is producing gale force winds
near the Western Aleutians. During the forecast period, expect
this system to shift north and eastward with the gales remaining
west of the Pribilof Islands and moving directly over St. Matthew
Island. A new wave has developed along the baroclinic zone south
of the chain but it will rapidly get absorbed into the longwave
wave pattern. A dome of high pressure will build over the North
Pacific impacting the Aleutians. Multiple locations across the
archipelago will see highs in the 40s. Warm aloft is inherently
stable and is conducive for fog formation.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The mid/long term forecast begins Tuesday with a strong upper
level ridge that is building across the Bering. East of that is an
upper level trough that is digging south of the panhandle. These
two features will be dominating the weather pattern, and become
the source of model contention as we head into the middle of the
week.

As the ridge builds to the west, strong northerly winds will
build across most of Alaska, keeping temperatures down and any
precipitation at bay. As we move closer to the weekend, guidance
really begins to diverge from any real consensus at the lower
levels. Both models wrap shortwave energy around a parent low in
the northern Bering Sea. The GFS is stronger than the EC and with
the energy and attempts to break down the ridge by Friday. The EC
on the other hand, is strong with the energy but does not cut
through the ridge, rather it dives south. In reality, this is
status quo for the GFS and the long term forecast will be shying
away from that output. We will trend closer to the ensemble output
from the EC. This trend weakens the ridge by this weekend as a
low undercuts the ridge and moves into the Gulf.

The primary takeaway of the extended forecast is that we will
undergo two pattern changes through next weekend. The first change
is as we move into the early week, and the shift to colder
temperatures with northerly winds. The second is as we move
towards the end of next week and into the weekend. The pattern
looks to shift to more of an active storm track, bringing
temperatures and chances for precipitation back up.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 120 130 131 139 174 178 185 351 411 412.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...SS