Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 070230
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
530 PM AKST Fri Dec 6 2019


.ANALYSIS AND UPPER LEVELS...

The first in a series of warm fronts is pressing in along the West
Coast of AK. This will start the shift across much of the
southern half of AK to a warmer and much wetter pattern. First,
the weak shortwave that clipped through Southcentral AK last night
is moving off through the Copper River Basin today. In general,
it dropped anywhere from a trace to 1 inch of fresh snow. Out
west, radar and satellite are showing the incoming snow well. Snow
and blowing snow is already reducing visibilities a s low as one
quarter mile across parts of the YK Delta. A Winter Weather
Advisory is in place to address the snow and blowing snow out
there. Meanwhile, there are signs of the warm air coming as
observations across the Bristol Bay area are showing temperatures
already into the upper 30s.

An impressive jet is helping to amplify this pattern. From west to east,
we can see the 300mb (30,000) jet moving under the Western
Aleutians at 195 kts. This is helping to support the broad, cold
trough over the Bering Sea. Further east, we can start to see
signs of the jet strengthening over the North Pacific as
transverse banding is very evident on satellite. This jet is
currently around 115 kts. It is ushering much warmer air towards
the southern mainland and will serve to creating a strong area of
low pressure near Bristol Bay as we head into the overnight-Sat.

&&

.MODEL DISCUSSION...

Models have come into significantly better agreement in handling the
various surface low pressure centers over the next couple of
days. Generally, the American models have performed slightly
better to this point. However, there are still key differences in
the lows tracks and timing that will be key factors in the
sensible weather through Tue. The depth and track of the low
across Bristol Bay and the Western Capes tonight is much more
certain. The remaining area of uncertainty surrounds how much of a
coastal erosion/high surf impact this could be. Variables such as
sea ice and duration of the fetch will make this a challenging
forecast. Then for the next system moving towards Southcentral on
Sat afternoon, the NAM has recently come in much stronger and
further east. We have opted for a middle ground solution with that
feature. Then finally, the 3rd system late Sun into Mon is the
strongest. It will approach from the N Pacific into the Gulf of
AK. This is a common source region for high model uncertainty.

&&

.AVIATION...
PANC...VFR conditions should persist through the TAF period. Wind shear
will become a concern by early Sat morning. Down-inlet north
winds will hold at the surface, while southeast flow increases
aloft in response to the approaching front.

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

A large stream of subtropical moisture will begin to move into
the area this evening associated with strong southerly flow, east
of a low tracking across the Alaska Peninsula. Brief snow and/or
freezing rain is possible over the western Kenai Peninsula,
however it will be light and quickly change to rain as the
aforementioned southerly winds will warm surface temperatures
above freezing. Winds along Turnagain Arm are expected to peak
near 60 mph Saturday morning, but are not likely to affect the
Anchorage Bowl. As flow will have an easterly component, much of
the moisture will struggle to make it to the leeward side of the
mountains. However as the low moves over Southcentral, downsloping
will cease and snow/freezing rain is possible across Anchorage
and the southern Susitna Valley late Saturday night into Sunday
morning and temperatures briefly cool below freezing.
Precipitation will gradually end through Sunday as the system
departs to the north.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A very active weather pattern has moved into Southwest Alaska this
afternoon and evening as a strong Pacific warm front is lifting
north across Bristol Bay and into the Kusko Delta. Heavy snow and
blowing snow is impacting the Delta and will gradually turn to
rain or pockets of freezing rain in the late evening and early
overnight. Rain has already changed over along the coast of
Bristol Bay, although heavy snow will continue over higher
elevation interior mountains. Attention quickly turns to a fast
moving but strong gale force low moving north across Bristol Bay
to north of Nunivak Island overnight through Saturday. This low
will wrap up quickly and impact coastal locations with a strong
onshore S-SW wind possibly gusting as high as 45-50 knots. Cold
air wrapping around this low will likely result in a return to a
mix of rain and snow, but given the onshore flow off the warm
waters, no blowing snow is anticipated. A second area of enhanced
precipitation will develop across the Alaska Range Saturday and
Saturday night, bringing accumulating snow to that region and
locations near the Alaska Range. Yet another low barrels into the
Alaska Peninsula Sun eve, but the bulk of this system come
overnight and Monday (see extended discussion).

&&

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

A strong Pacific gale force warm front has reached the Southwest
coast with a secondary trailing low moving north in between
Dutch Harbor and Cold Bay, which will then quickly move into
Bristol Bay and track toward the Strait through the day Saturday.
This low will reach brief storm force over the marine waters south
of the Eastern Aleutians/AKPEN, but will likely remain a powerful
gale force low as it tracks along the coast as it moves northward.
Thereafter, a series of weaker and smaller lows will move across
the Western/Central Aleutians as broad cyclonic flow remains over
the Bering Sea. Yet another low barrels into the Alaska Peninsula
Sun eve, but the bulk of this system come overnight and Monday
(see extended discussion).

&&

.MARINE (Days 3 through 5)...
Monday there is a potent low near Bristol Bay which has the
potential to produce gale force winds and also produce light to
heavy freezing spray near Nunivak Island. A second low which will
be south of Adak also has the potential for producing small craft
to gale force winds. As time elapses, the Bristol Bay low
retrogrades towards NE Russia and the Adak low moves towards
Kodiak and phases with another low. Expect the gusty winds and
enhanced sea states to move into the Gulf Of Alaska. With this
cold pattern in place, expect the sea ice to grow for both the
Kuskokwim Delta and Bristol Bay.

&&

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

The vagaries of the models continue to influence the confidence
level in the long term forecast period. Looking at both
deterministic and ensemble guidance categorically the confidence
level is below average. There are a few items that can be
gleaned. Expect a highly progressive pattern with the longwave
pattern to retain residence over the Bering and the north Pacific.
There are multiple lows from Kamchatka to the southcentral Alaska
this forecast period. Although for the Bering and the NW Pacific
the placement and the intensities of the lows are not in sync.
The storm that will impact southcentral this weekend will break
the cold snap with temperatures in the Anchorage Metro Area
possibly getting back up into the 40s this Sunday. Also noteworthy
and very plausible, the GFS at this time is suggesting an
atmospheric river will move into southcentral on Monday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 155.
MARINE...Storm 155. Gales 119 125 130 139 150 160 165 170 172 177 178
                          180 181 185 414.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...PS


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