Area Forecast Discussion
Issued by NWS Anchorage, AK

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14
000
FXAK68 PAFC 240142
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKST Thu Nov 23 2017

.ANALYSIS AND UPPER LEVELS...
The broad low that has slowly moved through the Bering this week
has reached its peak intensity and is beginning to slowly weaken
as it moves southward. This system brought high winds from the
Pribilof Islands southward into the Eastern Aleutians overnight,
including gusts near 95 mph in the Dutch Harbor/Unalaska area.
These winds have now spread into the AKpen where warning level
gusts continue to occur. On the northern side of this low, the
snow and blowing snow that moved through the Southwest Mainland
overnight is diminishing from north to south, however gusty
northerly winds remain from the continued tight pressure
gradient. This Bering low spawned a secondary triple point low
early this morning over the Western Gulf, bringing some light snow
the Southern Kenai Peninsula, and will be the focus for
increasing outflow winds for Kodiak Island and the North Gulf
Coast as it moves to the east and intensifies.

&&

.MODEL DISCUSSION...
The models are in reasonable agreement with the track of the
triple point low as it develops in the Gulf this evening,
especially when compared to the runs 24 hours ago as the models
all keep this low centered in the Gulf through Friday. As a result
there is increasing confidence that, with the further north
solution, stronger outflow winds will develop than initially
expected (especially in the Valdez area). The next forecast
challenge comes from another system projected to deepen in the
Bering late this weekend, with large differences between the much
weaker/faster NAM and deeper/slower EC/GEM. Forecast changes today
mostly used the GFS to trend towards slightly slower/deeper
solution, but not as slow as the EC is indicating.

&&

.AVIATION...
PANC...Areas of low stratus should depart the Anchorage area for
good this evening as offshore flow develops. This will bring
gusty N-NE winds into the airport beginning this evening which
are expected continue into Friday with persistent down-inlet flow.

&&

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

A strong low that is moving south away from the Alaska Peninsula
is in the process of transferring its energy over to a new triple
point low centered over the central Gulf. Behind the old low, a
strong north-to-south oriented jet is pushing eastward across the
Bering. Meanwhile, a lobe of arctic air is also moving southward
across the Alaska Range. These features are all going to be
playing a role in the strong winds expected through portions of
Southcentral over the next 24 hours or so.

The lobe of arctic air will begin interacting with the new triple
point low over the Gulf before the more favorable upper level/jet
support moves into the area. As it races down the western side of
the low, it will move over Kodiak Island tonight. The combination
of the dense polar low and the deepening Gulf low will cause
winds to gust strongly through the passes in the mountains on
Kodiak Island, affecting both the airport and Kodiak City itself
with gusts up to 70 mph, and perhaps isolated gusts above that.
The cold air being pulled out of the northern Bays and passes of
Prince William Sound will also be reinforced, causing rapidly
strengthening winds through Thompson Pass and Valdez tonight as
well. Since Thompson Pass is higher and more exposed, it`s likely
that area sees the strongest winds of the event late tonight, with
gusts to 80 mph.

Following the lobe of cold air is the aforementioned strong jet.
This support from the jet stream will arrive in Kodiak first
during the day Friday followed by Valdez and Thompson Pass Friday
night. Since the cold air and the jet are not coincident, what
will result instead is a quick ramping up of the winds in both
places with the cold air, then a gradual diminishing of the winds
through the day Friday as the upper level jet tries to pick up the
slack for the ending of the cold advection. Once again Thompson
Pass looks to be ground zero for the strongest winds as both
features move through.

As of now it appears with the decoupling of the forcing features
and a lack of strong high pressure inland, the Matanuska Valley
wind is still expected to develop, but does not look to approach
hazardous levels from this event. Meanwhile, other than the winds,
precipitation chances remain mercifully low through the next
several days across the area.

&&

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

A broad area of cyclonic flow is present on satellite imagery over
southwest Alaska and the AKPEN. The surface low is now east of
Pilot Point. Cold air advection dominates the southwest with the
700mb temperatures ranging from -18C to -16C and are projected to
descend even further into negative territory tonight and Friday
morning. There is still a light band of snow on the King Salmon
radar but precipitation-wise this storm is on a glide path east.
The blustery winds will hold on a little longer tonight. The main
forecast challenge this forecast package was pinpointing the
temperatures for the next two days. Saturday, a dome of high
pressure will move into southwest and expect the temperatures
aloft to increase. Where there is subsidence, there is dry air.

&&

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

The latest ASCAT pass pings into both gales and storm force winds
for the eastern Aleutians which can be extrapolated to the north
also judging from observations out of St. Paul. Behind the low
center, widespread northwest flow and strong cold air advection
are happening. The tight pressure gradient will hold on for the
Eastern Aleutians tonight but expect the winds will gradually
diminish for all areas from west to east for the Bering and the
Aleutian Islands by Friday morning. Looking upstream, a high
pressure system will continue to build into the Western Aleutians
tonight with the ridge axis east of Atka by tomorrow afternoon.
Some places in the Central Aleutians will even see temperatures in
the mid 40s associated with the high. The longwave pattern
continues to be progressive and behind the transient ridge, a new
low will approach the Western Aleutians. The models have struggled
with run to run consistency and with consensus regarding this low
for the weekend. The pops and QPF in this forecast package for
this new storm reflect a blend of GFS and ECMWF. Stay tuned for
more details but generally expecting another round of gusty winds,
rough seas and wintry precipitation on Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Expect warmer temperatures than we have currently, across most of
southern Alaska. The pattern will be wetter than recent days,
especially for the north gulf coast. The extended pattern looks
much the same as the current flow pattern overhead, very
progressive and active. The biggest change will be a deviation
from the currently low- level outflow pattern to one where fronts
will begin to affect the area. Day three(Saturday evening) begins
with a strong shortwave under the jet entering the east Bering
Sea. Models are having trouble resolving different pieces of
energy along the jet/baroclinic zone and their
evolution/interaction with each other. The big takeaway is that
there will be a strong system, more possible high winds for the
eastern Bering/Aleutians, and a front eventually swinging into the
Gulf of Alaska early Monday. The pattern stays active into the
long term forecast with another strong system moving into the
eastern Bering Sea by the middle of next week. The system will
have implications over Southcentral Alaska as the front looks to
really enhance the pressure gradient over the coastal mountains.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131 and 181
MARINE...Heavy Freezing Spray 140
         Storm 132 150 155 165
         Gale 121-128, 130-131, 138-140, 160,170-185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...MTL



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.