Area Forecast Discussion
Issued by NWS Anchorage, AK

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834
FXAK68 PAFC 241322
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Sun Jun 24 2018

.ANALYSIS AND UPPER LEVELS...
Southwestward propagating scattered showers linger from the
Copper River Basin to the Talkeetnas as the first of two upper-
level shortwaves continues its westward progression over southern
Alaska. Scattered showers are also lingering over northern
portions of the AKpen aided in part by orographic enhancement from
a weak northerly flow and a weak trough extending along the
Alaska Range. Farther west, IR satellite shows an increase in mid-
and upper-level cloud cover across the central Aleutians as the
next storm system begins to take shape.

Any well-organized storm activity remains south of our region as
the longwave trough over the Bering keeps the jet over the North
Pacific. Instead, our weather continues to be dictated by
shortwave troughs in the mid-levels ejecting from two broad area
of circulation. The first is a 545dm low in the northeastern Gulf
pinwheeling easterly waves across southern Alaska. This will
continue to bring the threat for scattered showers and
thunderstorms through Monday. The second, a broad 540dm low in
the northern Bering, is ejecting shortwaves along the Aleutians.
This is helping spin up a weak surface low and steer a front
across the Aleutians.

&&

.MODEL DISCUSSION...
The models are all still in good agreement with the large-scale
synoptic features. The NAM and GFS are also in better agreement as
to the timing and placement of the second shortwave moving east
to west across southern Alaska today. Models keep lingering
showers over interior Southcentral Alaska this morning with
another round of showers and isolated thunderstorms firing up over
the Copper River Basin, Talkeetnas and interior portions of
southwest Alaska this afternoon and evening.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Conditions
could approach MVFR briefly this morning depending on how much
shower activity approaches the airport.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Tuesday)...
There is a distinct difference between the relatively benign
weather over the Gulf from weak high pressure and the much more
active and showery weather over the South Mainland from a series
of easterly waves pushing through the region. The first of which
has brought widespread rainfall to the Copper River Basin
overnight, and will continue spreading westward into the Susitna
Valley this morning where it will persist into the evening. There
remains uncertainty in how far south these showers will develop,
leading to lower confidence in the POP forecasts for the Anchorage
Bowl and Matanuska Valley today. Instability looks slightly
stronger than what occurred yesterday afternoon, which should
allow for more lightning strikes over the Northern Copper River
Basin and Talkeetnas this afternoon. While most of the
thunderstorm activity will dissipate by late this evening, the
continued track of weak easterly waves moving over the area will
keep showers in the forecast from the Kenai Peninsula northward
into Monday.

Starting Monday, model agreement begins to break down as a
shortwave trough moving northward though the Gulf begins to
interact with the easterly waves over the Mainland. This
disagreement continues to impact forecast confidence and finer
scale details in the POP forecast. In general the forecast is for
cloudy and showery weather will persisting through Monday,
especially for the Copper River Basin/Susitna Valley which should
continue to see the most widespread shower activity.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Sunday and Monday)...
Generally quiet conditions are in place across much of Southwest
Alaska to start out this morning. The exception are an area of
scattered to numerous showers generally north of King Salmon and
east of Dillingham. The rest of the area is under mostly cloudy
skies. During the day Sunday, a thermal trough building
southwestward out of interior Alaska will increase the instability
for much of the area north of the Nushagak Hills. Widespread
showers with isolated embedded thunderstorms are expected across
the Lower Kuskokwim Valley late this morning through the
afternoon. Convective activity that largely initiates along the
Nushagak Hills, Kuskokwim Mountains the the Alaska Range north of
Lake Iliamna will drift northwestward with time, extending the
threat of isolated thunderstorms to Bethel and points north and
east this evening.

A northward moving atmospheric wave on Monday will confine the
thunderstorm activity largely to the immediate Kuskokwim Valley
and points north, with significant initiation expected off the
Alaska Range midday as well. The wave will bring in a more stable,
cool, moist air mass to Southwest Alaska, making Tuesday a
noticeably cooler day with considerable cloud cover expected
area-wide.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Sunday and Monday)...
A large, elongated low currently over the western Aleutians and
its associated front is causing a widespread area of precipitation
from Adak eastward through almost to Dutch Harbor this morning.
This feature will be the primary weather maker across the Bering
for the first half of the week. Tonight, the triple point low
developing east of the parent low will take over as the primary
low near Adak. That low will slowly drift eastward through Monday,
weakening as it does so. Thus, the steady heavier precipitation
will diminish with time into a scattered shower regime by Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A long wave pattern across the state will bring a persistent
pattern of diurnal showers inland and periods of light rain across
the Bering and Gulf of Alaska. The low pressure center will
reside near the Alaska Peninsula during the long range forecast. A
surface low organizes Tuesday night as two strong waves of energy
merge together as they wrap around the core of the low. This will
send a relatively weak front to the Southern Mainland on
Wednesday with lingering showers through the end of the week as
the organized low slowly fills while it spins near the Alaska
Peninsula. The to the west of this system, low clouds and patchy
fog will be the main theme for the western Aleutians and Bering as
a ridge builds from the North Pacific. Only minor changes were
made for updates as models are in relatively good agreement with
the synoptic pattern through the end of next week.

&&


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

&&

$$

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



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