Area Forecast Discussion
Issued by NWS Anchorage, AK

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712
FXAK68 PAFC 220146
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 PM AKST Sun Jan 21 2018

.ANALYSIS AND UPPER LEVELS...
A highly amplified pattern remains over the region. There is an
upper level trough over mainland Alaska extending into the Gulf.
An upper level closed low is at the southern end of this trough
over the southwest Gulf, and another closed low further north
along the Arctic coast of the state. There is a blocking ridge
over the central Bering Sea, with a trough over the southwest
Bering. Strong northerly flow is over the west coast of the
mainland and the eastern Bering Sea. Weak ridging is over the
Yukon extending into inland portions of south central Alaska. Some
moisture is over the northern Gulf that is bringing precipitation
to the Gulf coast and extending inland to some degree. There is a
weakening negatively tilted front over the western Aleutians and
southwest Bering.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through Tuesday
afternoon on the major synoptic features. Some differences are
evident over the Gulf with how the models handle the numerous weak
surface lows. There is also some disagreement on precipitation,
especially for inland areas of Southcentral. Therefore forecast
confidence is near normal.

&&

.AVIATION...
PANC...Some snow showers are possible during the TAF period. This
could briefly lower conditions to MVFR mainly on Monday afternoon,
otherwise VFR is expected to be predominant at this time.

&&

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

The weather is becoming increasingly active and interesting for
Southcentral over the next couple days. Here are some details:

Tonight:
Off-and-on snow showers and flurries are ongoing across much of
Southcentral this evening. The snow is being caused by a weak
disturbance progressing northwestward out of the Gulf towards a
digging upper level low currently over far northern Alaska. No
significant accumulations of snow are expected from this first
wave as much of the energy associated with it is being sheared
apart. The flurry threat, if you can call it such, will end after
midnight. Otherwise, outflow winds through the gaps continue this
evening with a weak surface low over the northern Gulf helping
drag the cold air now well entrenched over interior Alaska out
through the gaps. As this low weakens tonight, so too will the
winds. The cold, dry air moving over the relatively warmer waters
of the Gulf and Prince William Sound is making for good conditions
for mariners to experience heavy freezing spray in the windier
locations through Kamishak Bay, the Barren Islands region,
Shelikof Strait, and perhaps some local concerns in any narrow
gaps and fjords.

Monday:
During the day will be the calm between systems, though scattered
snow showers will continue through western Prince William Sound
and the north Gulf Coast through much of the day. Any small lows
over the northern Gulf will be absorbed into a stronger low over
the southern Gulf, so relaxing pressure gradients will continue to
have winds area-wide on the decline through the day.

Monday Night into Tuesday:
Upper level dynamics will be the forcing for the next snow event
for nearly all of Southcentral. An upper level low over the
southern Gulf will slowly track northward overnight. To its north,
a shortwave extending from the upper low will circulate westward
across the northern Gulf towards Southcentral. Meanwhile, the
aforementioned upper low over far northern Alaska will be moving
southbound as a trough over the Alaska Range slowly moves
southeastward. As a result, a zone of deformation and stronger
lift will develop over most of Southcentral and bring a long-
duration light snow event through those areas starting Monday
evening and persisting right through Tuesday morning.

Throughout this event, winds in the lowest levels will be light
with no downsloping expected, the current thinking is that the
entire area will get roughly equal amounts of snow. No one area
looks to be favored over any other with this event. Temperatures
throughout the event will be in the teens to low 20s, which is
plenty cold enough for the snow to be a light, powdery, fast-
accumulating consistency. Fortunately the light winds will
preclude there being any visibility restrictions from blowing
snow.

On Tuesday afternoon, the upper level trough over the Alaska Range
will have moved eastward over Southcentral, ending the snow.
Behind it Tuesday night, a sharp line of clearing skies will also
progress eastward, resulting in lots of sunshine by Wednesday.

&&

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

The main forecast challenge in the short term is how low will the
wind chill values drop tonight, mainly for the Kuskokwim Delta.
With abundant clear skies across the southwest Mainland,
radiational cooling won`t be a problem tonight. Wind speeds of 15
to 25 mph are anticipated for the upper parts of the region, when
combined with the low temperatures forecasted drops the wind
chill values to lower than -40 F. Thus, the wind chill advisory
issued earlier this afternoon looks good. Further upstream into
the Kuskokwim Valley, temperatures will be much colder. However,
wind speeds look to remain less than 15 mph, which is one of the
criteria needed for a advisory.

Otherwise, a persistent area of high pressure across the Bering
will continue to funnel colder air into the area in an offshore
flow regime, with no sensible change in the pattern seen through
the next few days.

&&

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

An area of high pressure will remain anchored across central
portions of the Bering Sea/Aleutians through the next few days.
This creates a rather stagnant pattern with a stalled gale force
front remaining in place across western portions of the chain, and
cold air continuing to pour into eastern portions of the Bering
and into the eastern portions of the Aleutians/AKpen. This will
bring continued freezing spray concerns to these eastern areas
through the period.

&&

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

Beginning the extended period forecast Tuesday night, there will
be a rather potent cold upper level vortex over the northwest part
of Alaska with associated troughing extending into the Gulf of
Alaska. Low level cyclonic flow will preferentially wrap around
the Alaska Range into the Bering Sea. Snow showers over parts of
Southcentral will quickly be end late Tuesday (although they may
linger longer over the Copper Basin) as strong offshore flow
increases in response to cold air advection. Temperatures will
continue to drop across all of Southern Alaska as 850 temperatures
plummet to -20 to -30C on Thursday as the upper vortex moves
south across the region. As the upper low digs into the North
Pacific, secondary cyclogenesis will develop in the Gulf with
increasing chances of coastal precipitation (in the form of snow)
Thursday into Saturday. Some locations such as Kodiak Island may
see the potential for good accumulations should the setup develop
as current global models depict...stay tuned.

Otherwise, the pattern looks to remain dominated by the eventual
large trough over the North Pacific into the Gulf of Alaska, along
with high latitude blocking over Eastern Siberia, for the
remainder of the weekend into early next week. This will likely
result in dry and continued colder than average inland weather as
the general storm track remains south of the forecast area.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155.
MARINE...Gale 128 130 131 138 150 155 165 172 180
         Heavy Freezing Spray 130 131 138 150 155 160 165 180 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JA



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