Small Craft Advisory 207, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Thu Mar 14 2019

.ANALYSIS AND UPPER LEVELS...

The dominate feature is a vertically stacked storm force low that
is spinning over the western Gulf of Alaska. Several shortwaves
are present within the flow around the low at the upper levels;
the most notable is stretched over Southcentral. As the surface
low migrates westward over the Barren Islands, it is producing
some mild convection which is aiding in widespread precipitation.
Cold air wrapping around the low is keeping many areas under snow,
especially over the inland areas. In juxtaposition, the coastal
areas, being impacted by warmer sea surface temperatures, are
sticking to a rain or rain-snow mix. Generalized east-
southeasterly flow is predominate in the northern Gulf which is
creating gusty gap winds through favored passes and along Turnagain
Arm. A strong northeast to southwest pressure gradient between the
low currently near the Barren Islands and a ridge extending across
the Copper River Basin is also driving a northeasterly surface
wind out of the Mat-Su valleys and a down inlet flow along Cook
Inlet. For the Anchorage Bowl, the northerly winds are more
relaxed for the moment as the Turnagain Arm winds bend south, away
from town and the core of the strongest Mat-Su winds remain to the
north.

A second less dominate feature is a low that is stationed over
the northern Bering. This low is is extending a trough over the
eastern Bering. This trough is associated with an arm of the jet
stream that is northwesterly oriented at 30,000 ft. A weak
occluded front, aligned with the trough, is spread over the
eastern Bering and Aleutians. This front is creating gusty winds
and precipitation through the eastern Aleutians, AK-Pen and the
southwestern coastline. Elsewhere in southwest, conditions remain
fairly quiet.

&&

.MODEL DISCUSSION...

Models continue to be in good agreement through the first 48
hours as the low in the Gulf moves inland over Southcentral today.
The next system is a Pacific low and associated front that are
advancing toward the Gulf on Friday. With this low, there is model
variability which, in turn, creates discrepancies in the
solutions as the storm develops. The main difference lies with the
track of the Pacific low as it moves into the Gulf on Saturday.
The GFS is the quickest and most westerly solution while the NAM,
and Canadian are in lock-step further east and slower. The
differences in low position and pace is creating some uncertainty,
especially in regards to winds in the northern Gulf on Saturday.

&&

.AVIATION...
PANC... General VFR conditions will persist with ceilings
expected to remain at or above 5,000ft AGL through the TAF
package. Low level wind shear (LLWS) is again possible this
morning preceding frontal passage. By 21z, the front is expected
to lift north across Anchorage, bringing the southeasterly
Turnagain Arm winds back over the airport complex. Scattered
showers are also possible at this time, lingering into the
evening, but are not expected to reduce ceilings or visibility.
LLWS is possible again tomorrow morning in advance of the next
frontal systems as southeasterly winds aloft increase with surface
winds remaining out of the north.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Thu and
Fri)...
An impressive, tightly wrapped occluded low continues to spin
just south of the southern tip of the Kenai Peninsula. East of the
low center, strong winds and very heavy precipitation continue
this morning. Several feet of snow have already fallen through
Turnagain Pass and more is on the way this morning. However,
temperatures are slowly warming across the area as the warm air
right around the center of the low moves in. This will limit
additional accumulations to some extent. The other impressive
facet of this system has been the gusty east to southeast winds.
Mountain top winds will continue to blow between 60 and 70 mph
this morning before slowly tapering off this afternoon. Winds out
of the Matanuska Valley down through Anchorage will continue
early today as long as the low center remains to our south. But as
the storm lifts through, expect winds to shift out of the
southeast and bring another round of gusts to these locations.

With little reprieve, another system is already on track to impact
many of the same areas heading into the day Fri. The recent model
runs have come into better agreement that this will actually be
another consolidated low pressure center (vice just an open
shortwave). This means it will have some of the same offshore
gradient as the current system. This gradient serves to reinforce
some cooler air from inland areas. So even though the system
itself will sling a warm airmass towards us with 850 mb (5,000')
temperatures rising to -2 to -5 degrees Celsius, cold air will do
its best to hang on at the surface. So once again, it will be a
battle between how much above freezing the coastal locales will
get and thus, how much snow they can accumulate. Certainly places
like Turnagain Pass and Valdez are in for another round of snow,
but the exact accumulations still need to be ironed out. Cordova
will also be right on the edge of rain and snow. With ample QPF
forecasted for them, we will be monitoring that situation closely.

For Anchorage and other inland population centers heading towards
the end of the week, expect more warm, windy, and mostly dry
conditions. Once again, east to southeast winds coming over the
Chugach and Kenai Mountains will create downslope drying. Gusty
winds will ramp up again from Palmer through Anchorage, but the
strongest winds will once again be confined to the higher
elevations and the Anchorage Hillside.

&&

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

The longwave pattern features an elongated trough from the Gulf Of
Alaska to NE Russia. Looking at 500 mb, there is a negatively
tilted trough axis from the western Alaska Range bisecting the
Gulf Of Alaska. There is second upper level perturbation between
the Pribilofs and St Matthew Island with another negatively tilted
trough axis extending towards the AKPEN. Then there is another
upper level disturbance north of Kamchatka with the trough axis
extending into the northern Bering. At the surface, there really
isn't an organized frontal boundary, just a few troughs which is
seen in the latest ASCAT/Advanced scatterometer pass and in the
isobars at MSL pressure of the models. And of course wrap around
moisture potential from the storm in the Gulf Of Alaska which will
be a factor later in the forecast period.

&&

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

There is a low southwest of Attu that will advect warm moist air
towards the western and central Aleutians. But the models keep the
actual surface low south of the chain. Meanwhile, over the
northern Bering there is a surface low between St Matthew and St.
Lawrence Island and the models just have this low wobbling and
swaying over the Bering as it continues to weaken. Looking at the
1000MB- 850MB thickness values, cold air will remain steadfast
from NE Russia across much of the Bering and into Western Alaska.
Cold air over warm water is inherently unstable, so expect more
convection/showers over the Bering.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Main impacts will be located in the northern Gulf of Alaska with
the next round of low pressure on Saturday. Gale force winds and
20 foot seas are expected in the north Gulf of Alaska, with gale
force winds in the lower Cook Inlet as well. Confidence is high in
this forecast. However confidence drops after this system moves
through due to lack of model consistency. As of now, it looks like
we can still expect some small craft to gale force wind along the
coast through Monday.

The Bering Sea is expected to remain generally quiet with continued
weak northerly flow on Saturday. Some weak areas of low pressure
are then expected to move through, but will not be strong enough
to cause much of an impact.

&&

.LONG TERM FORECAST (Days 3 through 7/Saturday through Wednesday)...

Issues with model consistency continues to be the storyline for
the long term. We are still expecting a longwave trough to be over
the Bering Sea, with weak areas of low pressure circulating
through the trough. Southcentral AK will continue maintain the
pattern of warm and wet weather from a series of lows tracking
north through the Gulf of Alaska. However, models are not near
consistent enough to be able to forecast for individual weather
systems both in the Bering and southcentral for the extended
forecast. Thus, a blend of models continues to be used for a
consensus approach and the particulars of each system will have to
be dealt with when they enter the short term.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 125 130 139 140 351.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/TM
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...BB