Small Craft Advisory 85, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 PM AKST Sat Jan 26 2019

.ANALYSIS AND UPPER LEVELS...

A strong, vertically stacked low positioned just west of St. Paul
Island is the main weather maker this evening. This upper level low
is extending a long wave over the Aleutian Chain and into the
North Pacific. Cold air is beginning to wrap around the backside
of the low over the western Bering. Jet stream support stems from
a southerly oriented segment of the jet, just to the east of the
low where winds are reaching 115 kts. IR satellite imagery shows
the area of the jet well, as an area of dense cloud cover draped
over the eastern Bering and western mainland. At 500 mb, there is
an increased gradient in the flow on the eastern side of the low
over the eastern Bering. At the surface a warm front, ahead of the
surface low, is pushing over western Alaska, and the central and
eastern Aleutians. This front is bringing gusty winds and
precipitation to much of southwest Alaska as well as the Lower
Kuskokwim Delta, Bristol Bay and AK-PEN.

While the west is in an active state, Southcentral and the Gulf
are fairly calm as a ridge remains stationary over the Gulf.
Upper air soundings from Anchorage and Kodiak show a vertical
column of dry air and an unstable atmosphere which is aiding in
the dissipation of cloud cover and precipitation throughout
Southcentral. Scattered showers still remain inland and along
the western Gulf coast and Kenai Peninsula.

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.MODEL DISCUSSION...
Models are in relatively good agreement through Monday afternoon.
General synoptic wind fields adequately capture marine winds. QPF
also differs some over the mainland, but significantly.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Monday)...

High pressure over the southeastern Gulf and a low over the Bering
are the two primary weather features controlling the broad weather
pattern over Southcentral this evening. Southerly winds and
associated cloud cover are moving across the Cook Inlet region.
Southcentral will be on the western edge of the cloud cover
though, so periods with relatively clear skies are expected,
primarily late tonight into Sunday morning. The clear skies and
additional melting from the generally above freezing temperatures
today will add moisture to the lower atmosphere that may promote
the development of some patchy fog for much of the area. The
Turnagain Arm wind briefly moved into West Anchorage this
afternoon, but should move back south out of the area tonight.
However, the proximity of wind should help to keep fog from
developing in the Anchorage Bowl itself.

On Sunday morning, the high moves into the Panhandle as a weak
North Pacific low makes its way northward towards the western
Gulf. The center of the low will move to near Kodiak on Monday
morning, but associated rain will begin along the Kenai Peninsula
Sunday night. It may have just enough energy to get the barrier
jet along the north Gulf Coast to get to gale force on Monday
morning. A stronger low moves north out of the Pacific Monday
night which will have a lot more subtropical moisture associated
with it. In fact, an atmospheric river may develop Monday night,
and somewhere along the north Gulf Coast, primarily Prince William
Sound, may get a period of heavy rain going into Tuesday.

Southeasterly winds across the Chugach are expected to continue
from now right through the day on Monday. Fortunately, they're not
expected to be particularly strong at any point. The unfortunate
side to that though is that when precipitation starts up at the
coast Sunday night, some of it may "slop-over". With increasing
northerly winds and temperatures near freezing, there is
considerable uncertainty as to what form any precipitation that
falls in Anchorage and the Mat-Su, if any, will be.

&&

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

The main challenges in the short term forecast surround precipitation
types and gusty winds as low pressure lifts northeast tonight
through Sunday. Temperatures across the Southwest are not going to
trend down much overnight as a deep subtropical jet keeps a
warmer air mass pumping across the region. Therefore, snow will be
limited as the next wave pushes inland from the Bering tonight.
However, there is still some uncertainty in the timing for a
colder air mass to advect into the Kuskokwim Delta coast toward
Sunday morning. Blowing snow could significantly reduce visibilities
for coastal areas if the cooler air makes it around faster than
expected.

The next front moves toward Bristol Bay and the Kuskokwim Valley
from the Gulf on Monday. Southerly flow tapers down and shifts to
the east with this next system. Mixed precipitation will be light
in the lee of the mountains due to downsloping.

&&

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

A stacked low over the central Bering lifts north tonight bringing
gale to storm force southerly winds across much of the Bering.
Precipitation will mainly be rain as this storm is pulling warmer
air from the North Pacific. Colder air advects across as this low
exits north early Monday morning and winds shift to the north.

The main forecast challenge through Monday afternoon surrounds a
strong storm system progged to move across the western Aleutians
Sunday evening with a slow track through Monday. Moderate rain and
storm force marine winds are expected to develop along the
western half of the Aleutian Chain when the boundary moves
through late Sunday. The associated surface low deepens to around
955 mb early Monday morning before slowly filling on a northward
track...with the center approximately 200 miles south of Shemya at
960 mb late Monday afternoon.

&&

.MARINE (Days 3 through 5)...
The models remain in good agreement with the low near the western
Aleutians Sunday night into Monday resulting in high confidence
in the area seeing storm force winds, rough seas and another
round of precipitation. A second cyclone quickly develops in the
same source region and will take a similar track towards the
Western Aleutians. With a dome of high pressure north of Hawaii,
these storms are following the path of least resistance. For Days
3-5 expected a batch of unsettled weather for the Western and
Central Aleutians. As for the Gulf of Alaska and PWS, the
confidence level is much lower because the features that are
appearing in the models over that region simply are displaying
differences with intensity and timing.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The models continue to show a mature low moving from the eastern
waters of Japan to Western Aleutians late Sunday and Monday.
Expect this storm to bring to potentially bring storm force winds
and more precipitation to the region Monday and Tuesday.
Additionally, there is a new low spinning up in its wake that is
progged to take a similar path. Very confident that the Western
Aleutians will see a batch of unsettled weather early this
forecast period. However, the confidence level wanes for Southwest
and Southcentral Alaska where the global models are out of sync
past the 96 hr mark which is most noticeable with the myriad of
shortwaves at 500mb. A reoccurring theme for this winter has been
fine tuning of the forecast one or two days before the event once
the models have really locked onto a solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 165 170 177 178.
		 Gale Warning 150 155 160 171 172 174 175 176 180 181.
	     FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/BC
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...PS