Gale Warning 645, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 AM AKST Mon Nov 25 2019

.ANALYSIS AND UPPER LEVELS...
At the surface, there is a decaying low near Cordova, a mature
low in the north Pacific and a second mature low near Kamchatka.
On the latest ASCAT (Advanced Scatterometer) has detected an area
of gale force winds over the Barren Islands and in the Gulf Of
Alaska associated a gap wind scenario. ASCAT has also detected a
broad area of gales in the Bering associated with the Kamchatka
low. There is a 150 knot jet core in the north Pacific and a 160
knot jet core nosing its way into the Western Bering. In the wake
of the arctic front, substantially colder temperatures have been
observed across the Anchorage Bowl. At Merrill Field the
temperature was 17 degrees Fahrenheit at midnight which is 15
degrees colder than the previous midnight observation for Merrill
Field.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement with regards to the front
moving across the Bering today and Tuesday. However, there is a
low in the North Pacific and the models diverge with the
evolution of this feature. The current thinking is to hedge
towards the ECMWF and the NAM over the GFS which appears to be an
outlier at this time.

&&

.AVIATION...
PANC...VFR conditions and light winds should persist this TAF
package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With the upper level low near the Alcan border, the arctic front
has moved into Southcentral Alaska bringing in colder
temperatures for the next two days. A surface trough just offshore
from the northern Gulf coast will keep some northerly outflow
winds through channeled terrain along the Gulf coast.

This pattern is the break in between systems as the next system
will begin to move into the region Tuesday night and Wednesday.
This system will be a very complex one and will have the potential
for both areas of heavy snowfall as well as mixed precipitation
type.

&&

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

The weather pattern across Southwest Alaska is about to undergo a
radical change towards the warmer and wetter. A strong front
associated with a low is tracking across the Bering this morning.
This front will cause the wind flow to turn onshore/southwesterly.
Warmer air will replace the current cold air mass rather quickly.
The warmer than normal sea surface temperatures off the coast
will further support the warmer air mass that will be encroaching
into Southwest Alaska. Snow will begin along the coast of the
Kuskokwim Delta this afternoon. Overnight, the warmer air will
advance inland, changing the snow into rain as temperatures warm
through the day and into tonight into the 30s. Depending on the
intensity of the precipitation, heavier rates may allow for snow
to mix in, though above freezing surface temperature should limit
accumulations across the Kuskokwim Delta. Precipitation will
overspread the Lower Kuskokwim Valley and northern inland areas of
Bristol Bay tonight into Tuesday morning. Since the main front is
expected to stall out along the coast, the heaviest precipitation
will also remain over the Kuskokwim Delta, with only remnants
encroaching inland. Thus, while the Lower Kuskokwim Valley and
northern Bristol Bay should remain mostly snow through this event,
the precipitation amounts will be much lesser than areas further
west, where there is greater confidence that rain will be the
dominant precipitation type. Thus, there are no winter weather
advisories in effect for Southwest Alaska.

Coastal Flood Advisories are in effect for the coast everywhere
from Cape Newenham northward through Hooper Bay. 40-55 mph winds
will be directed directly into the coast of the Kuskokwim Delta.
This combined with the new moon phase will cause already
astronomically high tides to rise well above where they would be
with no wind. Near Cape Newenham and Goodnews Bay, the winds will
be somewhat parallel to the coast, so high tides will only be 2-4
feet above a normal high tide. Further north in the Kuskokwim Bay
on up through Hooper Bay, the winds will be stronger and plowing
right into the coast, this will support tides of 4-7 feet above a
normal high tide, especially Tuesday morning. Thus, coastal
flooding will likely be the greatest impact for the area.

The front stalls over the Kuskokwim Delta on Tuesday, while
further east, any precipitation will be tied to waves moving north
along the front, and thus will be more intermittent. On Tuesday
night through Wednesday, the front will resume drifting eastward.
At this point the model differences are much greater, pertaining
to how a low tracks up Shelikof Strait on Wednesday. These large
model differences result in much more uncertainty to how long and
how intense the precipitation over Southwest Alaska will be on
Wednesday. A westward track along with encroaching cold air from a
cold front may result in significant snowfall accumulations along
the Alaska Range, while the models that keep the low further east
in the Gulf will direct that precipitation into Southcentral, and
only snow showers will remain over Southwest Alaska. This will be
refined in future forecasts.

Looking ahead to Thanksgiving Day, despite the aforementioned
model differences, most of them are suggesting another polar air
mass settles into the area, so seasonably cold and dry weather is
likely.

&&

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

The main theme in the Bering will be the front currently over the
western Bering that will slowly track over the eastern Bering
through Wednesday. Today is when the front will be strongest,
where storm force winds are expected from the western Aleutians
now across the open waters of the Bering today and impacting
Nunivak Island this evening. The winds and associated high seas
will be the primary impacts from the front, beyond the front's
slow movement resulting in a prolonged period of gales for the
eastern Bering through the day Tuesday. Behind this front, a
reinforcing Siberian air mass will stream eastward across the
Bering through Wednesday. This should return much of the Bering
back to widespread snow showers with frequent breaks and sunshine
in between. This pattern will persist through Wednesday, when
another warm front moves into the western Aleutians. Not much
precipitation is expected with this front, with a strong high in
place over the central Aleutians.

&&

.MARINE (Days 3 through 5)...
Southerly gales are expected to spread from west to east across
the Bering and Aleutians Thursday and Friday as a front works its
way across the area. Northerly Gales are expected south of the
eastern Aleutians and Alaska Peninsula on Thursday and Friday with
strong inflow into a low over the southern Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Shortwave energy initially over the eastern Aleutians/Bering Sea
into the North Pacific on Wednesday should progress eastward and
weaken while a mid-latitude system to the south remains separated
from most of this energy and eventually drops southeastward.
Toward the end of the week and weekend the pattern will become
more dominated by a deep upper low moving into or just inland from
the extreme western/northern Bering. The overall trough containing
the upper low should extend south into the mid-latitude Pacific,
with the trough axis most likely reaching the eastern Bering
around next Mon. Expect a downstream ridge aloft to cross the
mainland around Fri-Sat and continue into western Canada. For the
southern Alaska mainland this will lead to a warming trend
Wednesday and Thursday, with warmer than normal temperatures
continuing through the weekend. Precipitation type will therefore
be an issue for most of the extended forecast.

Operational model runs and individual ensemble members display a
decent amount of spread and run-to-run variability to temper
confidence in the forecast, and thus it is a little below
average.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory: 155 161.
MARINE...Storm Warning: 177-179 181 185 411-414.
Gale Warning: 127 130 131 165 170 173 175 176 180 351.

&&

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