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000
FXAK68 PAFC 201205
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 AM AKDT Thu Sep 20 2018

.ANALYSIS AND UPPER LEVELS...
A persistent and slow moving closed upper level low is now
centered over the North Slope this morning with a series of very
low amplitude shortwave troughs undercutting the low. Clouds and
rain associated with this pattern are moving into Southwest Alaska
in association with weak warm air advection. Over the Bering Sea,
transient shortwave ridging is moving eastward ahead of a
substantial subtropical warm front (gale force) which is moving
into the southwest Bering Sea. Over Southcentral, the pressure
gradient and upper flow are weak, but plenty of low and mid level
stratus/altostratus inhabit the region.

&&

.MODEL DISCUSSION...
The numerical weather models are now in very good agreement and
are quite stable for the past couple iterations with respect to a
strong gale force low moving into the Bering Sea and a closed low
over the North Slope. Both features will have an increasing impact
on the sensible weather across all of Southern Alaska headed into
the late week. Model consensus remains great through 72 hours
until things begin to fall apart with a series of lows tracking
into the Gulf of Alaska as the general trough pattern shifts east.

For this forecast, the GFS was favored for the strong gale force
low in the Bering with the NAM over Southcentral.

&&

.AVIATION...
PANC...MVFR CIGS this morning will last through late morning
before afternoon heating helps lift CIGS to VFR. Extensive high
clouds is expected to keep any fog at bay this morning. CIGS are
expected to drop below 5000 feet again tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low over northern Alaska will slowly move
northeastward through Friday afternoon as westerly flow continues
over southern Alaska. Considerable clouds will remain over the
area, but showers will be minimal, and mainly confined to the
Susitna and Copper River valleys. On Friday afternoon a fairly
vigorous low in the Bering will send a front into Southcentral
from the west. This will switch the flow aloft to southwest Friday
night. This direction is favorable to give measurable rain to
much of Southcentral. Conditions will also get a bit breezy
through Turnagain Arm and the Copper River basin as the surface
gradient increases due to the low in the Bering and a triple point
low developing near Kodiak Island.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (through Saturday)...
Low pressure across the northwest moves a weak front across the
Southwest today spreading rain showers from west to east. The next
organized front moves to the coast tonight, with gusty southerly
winds and rain spreading inland through Friday. The surface low
driving the front nears Nunivak Island early Saturday morning,
keeping ongoing rain through Saturday night. Gusty conditions
taper down for much of the Southwest on Saturday, however gusts
continue for the coastal locations as the low center lifts north
of the Kuskokwim Delta late day. The winds shift to the south to
southwest as the low moves just north, which will bring higher
waves to the coast and thus, the threat for coastal erosion.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Winds are expected to increase rapidly this morning across the
Central Aleutians as the next North Pacific low nears approaches
Bering. This front spreads moderate to heavy rain across the
Bering along with max gale winds. The associated surface low is
expected to cross over near Kiska this morning, then continues to
deepen on a northward track to the northern waters late tonight. A
cold air mass pulls around the backside of this occluded system
later on Friday increasing westerly flow across the central and
eastern waters through Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7/Saturday night through
Wednesday night)...
At the start of the extended period (Saturday night) a mature low
will be sitting along the west coast of Alaska, with widespread
rain across Southwest Alaska and areas of rain lingering over
Southcentral. Expect slow improvement Sunday through Sunday night
as this system weakens and lifts northward.

Beyond that (for the first half of next week - and beyond) the
latest model guidance (including ensembles) is all over the place
with details of specific storm systems. Thus, forecast confidence
is about as low as it gets in the ability to say what might
happen on any specific day. However, there is quite a bit of
agreement in the overall large scale pattern, with a long-wave
trough over the Bering Sea/Aleutians occasionally extending right
across mainland Alaska and a ridge centered over the northeast
Pacific/western North America. Key differences are in the
interface between the trough and ridge (in vicinity of the Gulf
of Alaska on up to Southcentral Alaska). The overall amplified
pattern supports development of deep surface lows with a general
storm track from the North Pacific northward toward mainland
Alaska, with additional storms moving through the trough out
in the Bering Sea/Aleutians. In this type of pattern would
generally expect wet conditions across most of Alaska.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 155 165 170 172>177 179.

&&

$$

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