Small Craft Advisory 128, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
715 PM AKST Mon Feb 11 2019

.ANALYSIS AND UPPER LEVELS...

The vertically stacked low that traveled through the Bering Sea
overnight has moved into the northern Bering this evening. Cold
air is wrapping around the low and is advecting over the western
Bering. The jet stream is northerly oriented with the highest
winds blowing over St. Paul at 150 kts at 30,000 ft. This jet,
combined with the flow around the low is bringing strong
southwesterly flow aloft to southwest Alaska. A positively tilted
ridge aloft is building over the AK-Pen and Gulf of Alaska. As
this ridge builds, the isoheight gradient between the low in the
Bering and the ridge in the Gulf is increasing, intensifying the
onshore flow. This southwesterly fetch is pushing moist and warm
air over the AK-pen and into the Gulf.

At the surface, the onshore flow continues as a front pushes
inland over southwestern Alaska. This front is creating gusty
winds and precipitation over the Kuskokwim Delta and a threat for
coastal flooding in the region and hurricane-force gusts over St.
Paul Island. In Southcentral, the ridge is keeping conditions
mild and dry. Satellite imagery shows the clear skies throughout
Southcentral and the Gulf.

&&

.MODEL DISCUSSION...

Models remain in good agreement as the current active pattern
dissipates and a new one forms just south of the Aleutians
Wednesday. This consistency weans as a front moves over the
western Aleutians Wednesday. With this storm, models struggle to
agree on wind intensity. The GFS/NAM are much stronger than the
EC/Canadian. This pattern continues through Thursday morning.

&&

.AVIATION...
PANC...

Ground fog has been toying with the Anchorage runway since just
before sunrise today. This should continue until another weak
front passes over around 0300 with a brief opportunity for snow,
ending after midnight. Following this, southeast winds should
become gusty off the Turnagain arm, but limit any fog development
through the remainder of the forecast period. A short period of
MVFR is expected around this time, but the majority of the
forecast will remain VFR (barring a sneak attack from the Cook
Inlet fog monster again).

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Monday
night through Wednesday)...

Largely quiet weather exists again today for southcentral Alaska.
This benign weather will continue for the next few days, with a
cooling trend also in the forecast. A weak front will push
through southcentral overnight tonight, but as with the last one
not expecting much out of it. Just some light snow showers with
little accumulation around Anchorage and perhaps and inch or two
of snow in the MatSu. Most of this snow will occur in areas of
upslope flow like the Talkeetna Mountains.

After this front passes, weak high pressure will keep most areas
in southcentral on the sunny side, but some low stratus is expected
to hang around the Copper River Basin. Fog concerns will also
diminish, with turnagain arm winds keeping fog out of
Anchorage...though patchy fog may still develop around the fog
prone area of Knik Arm.

The next couple of days will see a low pressure system work its
way east across the Gulf of Alaska. Model runs today are
strengthening this low and winds behind it. This low will couple
with northwesterly flow to pull down cold air from Interior AK,
which will steadily cool temperatures starting tomorrow night for
all of southcentral AK. This will also set up strong offshore
winds behind the low around Shelikof Strait and gap areas exposed
to northerly flow. The same will be true for areas like Valdez and
the Copper River Delta as cold air pools in the Copper River
Basin and pushes through mountain valleys. So colder but
precipitation free the next few days.

&&

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

The hurricane force Bering low that brought active weather over
the last few days has lifted north, however its associated front
has moved inland over Southwest. This front continues to bring
residual rain and snow showers over areas across Southwest and the
northern AKPEN this afternoon. Southerly winds over the eastern
Bering will eventually become more westerly this evening with
gusts up to 60 mph over the Kuskokwim Delta coast tonight as cold
air wraps in around the bottom of the low. Although there is some
shore fast ice in place along the coast, the southwesterly wind
direction and long fetch have prompted the issuance of a coastal
flood advisory, specifically for the northern Kuskokwim Delta from
Hooper Bay south to Hazen Bay through Tue morning.

By Tue afternoon, a North Pacific low lifts north towards the
southern AKPEN. This system will bring accumulating snow and gusty
north to northwesterly outflow winds through the usual gaps and
passes along the southern AKPEN coast. The combination of snow and
gusty winds will also lead to areas of blowing snow over the
western AKPEN Tuesday night as well. This fast moving low quickly
shifts eastward into the Gulf by Wed morning, allowing for a
ridge to build over Southwest for Wed evening.

&&

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

A strong Bering low continues to move northward, advancing a
front inland over the western coast of Alaska. A strong low level
jet on the south side of the low continues to bring a large area
of westerly gales with an embedded swath of storm force winds over
the Bering. As the low exits the Bering, storm force winds will
persist over the northern Bering before tapering off by Tue
morning.

Lingering snow showers are expected to persist over much of the
Bering through Tue night before diminishing Wednesday morning as
a ridge briefly builds in over the central Bering for Wed
afternoon. The next front associated with another storm force low
moves in over the western Aleutians by Wed morning and advances
eastward by Wed afternoon, bringing relatively warmer, southerly
flow to the western and central Aleutians later in the week.

&&

.MARINE (Days 3 through 5)...

The period begins Thursday with a large intense low over the
western Bering Sea and another strong low southeast of the Gulf
of Alaska. Model guidance is in very good agreement for this
extended time frame lending higher confidence in the forecast for
the days 3 through 5 timeframe. Gale to storm force wind is likely with
the Bering storm Thursday and in advance of the front as it
progresses across the Bering Sea through Friday. Gales are likely
across the Gulf of Alaska Thursday, then expect benign
conditions, sub gales, throughout the Gulf of Alaska and adjacent
coastal areas.

&&

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

The main message for the longer term is the continuation of a
very active weather pattern.

A large intense low in the western Bering Sea Thursday with the
associated front extending through the Bering Sea across the
eastern Aleutians will continue along the north to northeast
trajectory. This storm and front will produce gale to storm force
wind in the Bering Sea and along the Aleutians. The Front looks
to make landfall along the Bering Sea coast Thursday as it weakens
significantly. All the models are in general agreement with a
secondary trough that develops into a front Thursday with the
formation of a frontal low Thursday night. This is where there is
some disagreement in the model solutions with the GFS supporting a
stronger low than the EC. At this point we will lean toward the
EC solution maintaining continuity in the forecast until we see
more of a model push for the stronger frontal low solution.
Following this Bering Sea storm another compact strong low will
be heading toward the western Aleutians Friday. We will maintain
continuity here and stick with WPC and the EC as we progress
through the weekend. Early next week the models converge back into
better agreement bringing a moderate low into the Seward
Peninsula region.

Over the Gulf of Alaska and southern mainland the main driving
factor will be the low southeast of the Gulf of Alaska and high
pressure over the mainland Thursday. This combination will
support colder conditions over the southern mainland along with
moderate outflow wind through gaps in the coastal mountains. At
this point we expect temperatures to cool around 10 to 20 degrees
over much of the southern mainland Thursday and Friday. Heading
through the weekend warmer air will be pushed across the southern
mainland from the Bering resulting in warming temperatures around
10 to 15 degrees. The models are in generally good agreement
Sunday bringing a weak low into the northern Gulf of Alaska
followed by a weak ridge of high pressure Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory 155.
MARINE...Storm 181 185 412. Gales 119 131 150 155 172 177-180 351
411 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP/KO
MARINE/LONG TERM...JW