Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220051
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT Tue Aug 21 2018

.ANALYSIS AND UPPER LEVELS...
The low pressure system that has brought widespread rainfall,
which has been heavy at times, to much of the Gulf Coast and
Southcentral is beginning to make landfall near Prince William
Sound. Rain gauges along the Gulf Coast have already reported 2-3
inches of rain in the last 24 hours in the Valdez and Cordova
areas, while 1-2 inches have fallen near Seward and Whittier. This
difference was due to a nearly stationary southerly jet that
setup over Eastern Prince William Sound on Monday evening,
producing deep and moist southerly flow and allowed for
orographic enhancement. This abundant rainfall not only has
brought water level rises to the river and streams along the Gulf
Coast, but further north into the Copper River and Maclaren River
areas today. On the western edge of this low bands of moderate
rain pushed through the Anchorage Bowl and Mat-Su Valley`s today,
bringing nearly an inch of rain to those areas. Further west over
Cook Inlet, the deep layer moisture is diminishing behind the low,
which has allowed for some areas of stratus to develop around Cook
Inlet this afternoon. This stratus will be the main driver for
the development of fog and stratus throughout Cook Inlet
tonight.

The next weather maker for much of the state is moving very slowly
through the Central Bering, producing southerly gale force winds
and rain that has spread into the Pribilof Islands. This system
will continue tracking towards the southwest coast for Wednesday,
and then bring the next chance of rain to the South Mainland for
late in the work week.

&&

.MODEL DISCUSSION...
Model agreement remains generally good into Wednesday with both
the weakening low over the Gulf and incoming Bering lows. There
still remains some disagreement in how significant rainfall along
the western edge of the Gulf low will develop, as the GFS
continues to be a wet outlier with precipitation from the Kenai
Peninsula into the Anchorage Bowl/Mat-Su Valley`s. Based on radar
and satellite trends however, the GFS is initializing the best
with precipitation in this area and was preferred for forecast
updates today.

Later in the forecast model agreement remains very good both with
the track of the Bering low and a wave that will develop along its
southern edge. As a result forecast confidence is above average
into the end of the week.

&&

.AVIATION...
PANC...MVFR to periodically IFR ceilings will continue into early
this evening as bands of moderate rain push through. VFR
conditions should develop this evening as cloud cover and rainfall
depart the area. Overnight tonight the very moist low levels
around Cook Inlet will allow for the development of fog and low
stratus overnight tonight. At this point vicinity fog and low
clouds were introduced into the TAF, which may need to be updated
as confidence increases. After any fog/stratus burns on Wednesday
morning, VFR conditions are expected with light winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level low is situated over the southwest Gulf of Alaska
steering the atmospheric river into Prince William Sound this
evening. The upper low will move to the North Gulf Coast late this
evening then rapidly continue northeast into the southern Yukon
Territory by Wednesday Morning. A flat transient ridge moves in
behind this exiting low bringing in stable air that will result in
stratus and fog in the Inlet and across portions of south central
Alaska tomorrow morning. Following this ridge is a trough moving
in along the Bering Sea coast tonight transiting through Southcentral
Alaska late Wednesday. A broad southwest flow sets up late
Wednesday and Thursday with upper level troughs swinging through
Thursday afternoon and evening. This will keep the southern
mainland in the cloudy showery regime as we end this week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Friday)...

The pattern across Southwest Alaska the next several days will
feature mainly cloudy conditions with occasional fronts bringing
rounds of light rain to the area. The first and weakest of the
fronts moves in Wednesday morning, bringing scattered showers
mainly to coastal communities in Bristol Bay and the Lower
Kuskokwim Delta. The second front moves into the Lower Kuskokwim
Delta Wednesday evening, and drifts slowly eastward across
Southwest Alaska through much of the day on Thursday. Once again,
no heavy precipitation is expected, but this second front should
produce steadier rains than the first starting Wednesday night
and moving east across the region through the day. The third front
behaves very similarly to the second front, with rain starting
across the Lower Kuskokwim Delta Friday morning. Rain overspreads
across all of Southwest Alaska through the day Friday, then
tapers off Friday night. Throughout the next 3 days, steady
onshore southwesterly winds will keep conditions cool and wet
area-wide.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Friday)...

A large low currently centered over the western Bering will be the
primary weather-maker for all of the Bering through Thursday. The
low itself is expected to remain nearly stationary through
Thursday. Meanwhile, fronts and disturbances pinwheeling around
the low will lead to periods of deteriorated weather with somewhat
improved conditions in between fronts. The strongest associated
front moving into the eastern Bering now will move into mainland
Southwest Alaska Wednesday evening. The low will weaken more
quickly during the day on Thursday so no significant features are
expected to impact the area.

Another stronger low moves into the central Aleutians from the
south beginning Thursday night, It will become the dominant
weather feature across the Bering as the first low dissipates and
its moisture is shunted eastward into mainland Alaska late
Thursday night into Friday. The central Aleutians low itself will
slow down to a drift, while much of its moisture and precipitation
gets carried along with the jet into the Alaska Peninsula Friday
afternoon through the overnight.

&&

.LONG TERM FORECAST (Saturday through Tuesday)...
This weekend will start off with a large blocking high centered
over the North/Central Pacific extending north into the Gulf of
Alaska and the mainland. At the same time an upper level low will
be situated over the Western Bering Sea supported by a decent
southerly jet streak tapping into subtropical moisture. This
pattern will lead to improving weather over southcentral and the
Copper River Basin with continued cloudy and wet weather over the
Aleutians/Bering and the West Coast. A weak shortwave looks to
slide across the northern part of southcentral leading to clouds
and rain Friday night into Saturday morning, which should clear
out by Saturday afternoon/evening as the upper ridge amplifies and
moves east into the area. The ridge axis is expected to move over
the Gulf of Alaska by Saturday night/Sunday morning, potentially
leading to widespread fog across the area depending on how much
clearing happens.

By Sunday the low pressure system over the Bering moves northeast
over the West Coast bringing a wide swath of moderate to heavy
rain with it, enhanced by subtropical moisture. High pressure
looks to initially dominate southcentral and the Copper River
Basin on Sunday before rain pushes into the area by the late
afternoon/evening hours. The ridge looks to deamplify after this
system moves out of the area but models have recently been
struggling at resolving disturbances riding up and over the ridge.
Thus, forecast confidence remains uncertain heading into next
week as an active storm track tries to break down the North
Pacific ridge.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 135.
MARINE...None.

&&

$$

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



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