Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 241744 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
844 AM AKST Fri Nov 24 2017

Corrected for typos.

.ANALYSIS AND UPPER LEVELS...
The high winds that have prevailed across most of the Alaska
Peninsula are gradually diminishing as a ridge of high pressure
over the Central Aleutians builds into the Bering. This ridge,
however, is being shifted east as a strong front swiftly advances
toward the Western Aleutians. A short wave trough propagating
through the northerly flow aloft over western Alaska is now
pushing south across interior Southwest Alaska headed towards
Kodiak Island. This trough will be a focus for strong winds due to
its coupled nature with the jet and northerly wind direction.

A trough rotating around the deep low in the Southern Gulf has
started to move into the Central Gulf bringing with it a weak
frontal band carrying some light snow to the Northern Gulf.
Additionally the trough is tightening the offshore pressure
gradient helping to intensify the northerly flow over the area.
The combination of the strong pressure gradient along the coast
and cold air pooling inland in the Copper River Basin is bringing
strong gusty outflow winds to the typical gaps such as Thompson
Pass and the Valdez, the northern bays of Prince William Sound,
Resurrection Bay, Passage Canal and the Copper River Delta
favored for the strongest winds.

&&

.MODEL DISCUSSION...
Models remain in fair agreement through the short term with some
differences in the handling of a trough rotating around the low in
the Gulf and swinging through the eastern Gulf Friday night. The
primary impact of this feature will be to the Southeast Alaska
Panhandle, the winds in the Gulf waters and perhaps to strengthen
of coastal outflow winds if the trough brushes past the northern
Gulf coast Saturday. For the western domain, models have come into
somewhat better agreement with regards to the rapidly developing
surface low tracking into the southern Bering Saturday and
Saturday night with consensus building for this to be another
strong storm. Confidence regarding finer details of the track and
timing of the system however remain low.

&&

.AVIATION...
PANC...Dry offshore flow with breezy north winds today and tonight
will keep conditions VFR. Winds will diminishing somewhat early
Saturday morning as northerly winds aloft and the offshore
pressure gradient slacken.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Expect widespread gusty winds across Southcentral, Kodiak Island,
and the Gulf today thanks to a deep surface low over the eastern
Gulf, strong low level offshore flow and cold advection. Some of
the strongest winds will be in the Shelikof Strait/Kodiak Island
region with enhanced subsidence as an upper level jet streak
passes just to the west and upper level impulses dive down the
front side of the upstream ridge. The more northerly component to
the mean low level flow is not great for really high winds for
most Kodiak Island communities, but the winds will be felt in
bordering marine areas. Winds will also be quite strong in the
usual Prince William Sound communities of Seward, Whittier, and
Valdez thanks to the strong low level forcing. A high wind warning
remains in effect for the Valdez to Thompson Pass area until 6pm.
However, it now looks like mid to upper level flow will reverse
and become southerly this afternoon, so warning level winds (75
mph) may come to a close earlier than thought.

Winds will persists in most locations through this evening. Then
expect inland locations to begins to drop down significantly
overnight followed by coastal locations on Saturday as the low
exits the Gulf and the upper level ridge approaches from the west.
Temperatures will plummet where the wind does die down, with
Sunday morning shaping up to be one of the coldest thus far this
winter.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 though 4)...
(Today through Monday morning)

Relatively quiet weather is in store for the region through
Saturday, before another strong storm approaches the region. For
today, a speed max currently located over Norton Sound will dive
southeast across the region, with an associated ribbon of
vorticity co-located just to its east. Light snow showers are
currently developing across portions of the Kuskokwim Delta on the
leading edge of these features. Given limited moisture depth and
the area being in a neutrally favorable jet position, we don`t
expect anything more than a light dusting of snow today, as all of
this heads south through the remainder of southwest Alaska during
the afternoon, with the highest chances near the coast.

A lull in activity is expected on Saturday, as a ridge of high
pressure aloft crosses the region, bringing a period of
increasing subsidence (sinking air) and clearing skies. This will
be short-lived though, as the next system moves into the region
by Sunday morning. The models have all trended much stronger with
this system as it moves towards the central Bering, with an
associated occluded front reaching the coast during this time.
This storm will be much stronger than the previous few. As such,
increasing surface convergence and lift aloft will provide
another bout of widespread precipitation, some of which may be
moderate in intensity at times. As for precipitation type, given
the amount of cold air in place ahead of it with an over-running
setup forecast, early thoughts are the precipitation will remain
all snow for virtually all of the area with a few inches
accumulating. The exception being some of the outermost capes
where a brief mix of rain and snow may occur.

&&

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

Another strong storm looks to impact the region towards the end of
the weekend. A ridge of high pressure aloft this morning over the
central Aleutians will continue to shift eastward through tonight.
A weak cold front stretching south from a Siberian low will cross
western portions of the region tonight with some light rain
crossing the region. A few locations will see some strong wind
gusts approaching 50 mph as the front moves through.

Our attention then turns to a rapidly strengthening low that
undergoes bombogenesis (a low that deepens at least 20 mb or more
in 24 hours). Some discrepancies still exist in the 00z model
suite (similar to the past few model runs) with the NAM being the
slowest/weakest, the ECMWF (European) the strongest, and the GFS
in between them. The 06z GFS and NAM did trend towards the
stronger European solution, and overall the models are slowly
converging on the stronger low. For now, we used the GFS for this
forecast package as it was a "good middle ground approach" to the
forecast.

That being said, here`s what it means for you. As this low
strengthens from 983 mb around sunrise Saturday morning south of
Shemya to ~945 mb over St. George Sunday morning. The atmosphere
will respond accordingly with winds rapidly increasing at all
levels of the atmosphere. An associated cold front will interact
with strong dynamics aloft and convergence at the surface to
produce widespread precipitation along and ahead of the front,
some of which will be moderate to heavy at times. This front will
also begin an occlusion process as the low reaches the Pribilof
Islands. Although strong to near hurricane force gusts may develop
along the initial front, it`s not uncommon for winds to peak
behind an occluding front when you often get the best combination
of a push of air from high levels of the atmosphere (PV anomaly),
with very strong cold and dry air advection. This in turns help
maximize wind gusts which will likely reach hurricane strength if
the models don`t change their forecast.

In short, another strong storm will impact virtually all of the
Aleutians and AKPEN during the latter half of the weekend if the
model trends hold. Hurricane force gusts will be possible. A
moderate level of model uncertainty still exists, so please
monitor later forecasts.

&&

.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 well as the front
looks to really enhance the pressure gradient over the coastal
mountains.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131.
MARINE...Heavy Freezing Spray 140
         Storm Warning 132 150
         Gale Warning 119 120 121 125 126 127 128 130 131 136 137 138
                  139 140 155 160 165 170 173 174 175 176 177 178 185

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JER/RA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...MTL



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