Gale Warning 329, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 AM AKDT Fri Apr 20 2018

.ANALYSIS AND UPPER LEVELS...
The satellite picture shows a transitional picture, with southern
Alaska between systems in a generally weak flow. The stacked
system that had set up shop over the Bering Sea is being pushed into
the Gulf of Alaska, while the compact system over the northern
Gulf is rapidly weakening. The more active features aloft are to
the northeast and southwest. A depression over the Canadian Arctic
works in tandem with a Chukchi high to bring a northerly jet and
trough southward over the Great Land. The other is rapidly
developing shortwave within the sub-tropical jet, south of the
western Aleutians.

The compact system over the Gulf is quickly losing steam as it's
forcing has moved off into Canada. The deformation and
precipitation shield associated with it is too falling apart
quickly as the flow slackens. Mainly weak showers are left in the
northern Gulf and Prince William Sound area. Middle and high
clouds associated with the system are spilling into the Copper
River Basin and over the Chugach. The progression of that area of
moisture will be the short-term focus while the dropping Arctic
trough will be the longer-term focus of sensible weather.

&&

.MODEL DISCUSSION...
Guidance remains in lockstep, bringing the Arctic trough
southwestward over the central Bering Sea by the weekend. The same
goes for the low pressure system currently south of the western
Aleutians, as models are in great agreement with position and
strength as the system moves eastward, south of the Alaska
Peninsula. The interaction of these two systems and one over
Kamchatka will set the stage for the long-wave pattern of the next
week or longer.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through early
afternoon. The Turnagain wind should develop and make it's way
into the airport, along with some convective showers in the
afternoon and evening hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The surface low near the entrance of Prince William Sound has spun
up rather nicely from late yesterday through overnight and is
bringing widespread rain showers around the Sound.

A cold, upper level trough is also digging southward over the
interior which is causing the atmosphere to become less stable and
will increase the shower activity over inland areas of
Southcentral. Areas close to sea level will mostly remain all
rain. The northern Susitna valley near the Alaska Range is the
area which will see the most snow with a few inches likely north
of Talkeetna today through tonight.

Winds will increase today through Turnagain Arm, the Knik River
valley and along the Copper River as the low currently near Prince
William Sound rapidly weakens and a nose of high pressure builds
along the north Gulf Coast. These gap winds generally help
diminish precipitation inland, but the instability associated with
the cold temperatures aloft and the lifting from the digging
trough will likely be enough to bring some showers into the
inland areas anyway.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Fri and Sat)...
A change is in store to start the weekend. Much of the day will be
dry and mild for nearly all of SW AK. But an upper level trough
will be digging from the Interior of AK towards the region. This
trough has enough cold air and upper level energy to make it
through the AK Range. The trough will pivot across the YK-Delta
tonight into Sat. While it will start out rather starved for
moisture, it will eventually start to pull some up from the Gulf
of AK as flow turns S-SE over the AK and Aleutian Ranges. There
should be plenty of lift and instability associated with the
potent trough, so once the moisture is in place, expect widespread
showers to develop. They will first build along the Alaska Range
Fri eve, but will then build westward through Sat. Some of the
showers could be heavy enough to produce a quick burst of snow and
some isolated gusty winds. Temperatures will continue to drop
below freezing overnight and climb to near 40 during the day
Saturday. Thus, expect a mix of rain and snow with this system.
The best chance for any snow accumulation will be confined to the
higher elevations and across the colder, interior portions of the
Lower Yukon Valley. Those locations could see a few inches into
Sat morning.

Later on Sat, the system continues to shift into the Eastern
Bering. As it does so, southeasterly flow will tap into some
lingering moisture from a remnant low near the Alaska Peninsula.
This should serve to reinforce the wet weather, but should also
allow temperatures to rebound a few degrees.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Fri and
Sat)...
Weather will remain quiet across the Bering, as per climatology
for late April. An elongating front will keep wet
weather and some patchy fog over the Western Aleutians into
tonight. A "run-of-the-mill" 980 mb low will skirt south of the
Central Aleutians today. This will keep some small-craft level
easterly winds on the Pacific side of the chain. As the low
"reemerges" from the North Pacific, it will bring some small-
craft easterly winds to the Alaska Peninsula by Sat afternoon. It
will also bring some moderate rain to the AK Peninsula, favoring the
south side. Weather will remain relatively quiet across the rest
of the area until a deepening low approaches the Western Aleutians
on Sun.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A low will head out into the Eastern Bering on Sunday, bringing
precipitation to the western portions of the Bristol Bay and
Kuskokwim Delta zones. Most Bristol Bay locations should be warm
enough for rain, with the areas from Dillingham north and west
seeing some light snow. The Kuskokwim Delta coast will stay cold
enough for a few inches of snow accumulation.

Meanwhile a progressive pattern yielding gusty, cloudy, and cool
conditions across southcentral, and will remain in place from
Sunday through late next week. Models are showing a series of
fronts entering the Gulf region originating from a north Pacific
low south of Kodiak (Sunday into Monday) and eventually a large
low over the Bering (Monday through Thursday). This will continue
the pattern of southeast flow downsloping the areas west of the
Chugach and Alaska/Aleutian ranges, bringing cloudy, cool, but
drier conditions to these areas. Whereas along the Gulf coast,
steadier precipitation is expected with each frontal passage,
along with gusty winds along the northern Gulf and through favored
channeled terrain. By Friday, there is a chance that precipitation
over the area could begin to dissipate as high pressure starts
moving back over the southern Mainland. However, there is very
little model agreement on this solution, lowering confidence in
the forecast for the later part of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...TP