Area Forecast Discussion
Issued by NWS Anchorage, AK

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998
FXAK68 PAFC 180134
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKST Sun Feb 17 2019

.ANALYSIS AND UPPER LEVELS...
The progressive upper level pattern continues. Two main upper
troughs are the dominant features today. The first extends across
western Alaska to over the western Gulf. It has set up broad
southerly flow over Southcentral Alaska. This upper feature is
supporting a weather front which is producing widespread snow
across the Cook Inlet/Kenai Peninsula to MAT-SU Valley corridor.
Coastal areas of the Kenai Peninsula(south and east) and the north
Gulf had or were transitioning to a mix of rain and snow. Very
strong easterly winds were evident across Turnagain Arm and
Portage Valley with a few gusts in the 60-70 mph range. A mix of
rain and snow was evident over Bristol Bay with some snow and
blowing snow over the Delta.

The second upper wave extending from the Kamchatka Peninsula to
over the western Aleutians was supporting a develop surface low over
the central Aleutians and it`s associated front. The surface low
is continuing to intensify and is moving rapidly east. Storm force
southerly winds and rain are associated with this feature. The
front is moving rapidly ahead of the low with the cirrus cloud
shield just making it the the Pribilof islands and western this
afternoon.

&&

.MODEL DISCUSSION...
Models are in fair agreement with the general synoptic pattern.
They have been struggling with the positioning of surface lows
past 48 hrs, as well as the QPF pattern and timing. Very
confident that the active nature of the pattern for the Bering and
northern Pacific, but realistically expect some fine tuning of
the forecasts the next couple of days.

&&

.AVIATION...
PANC...VFR conditions will develop this evening after frontal
passage and should stay into Monday. LLWS will end early this
evening as gusty southeasterly winds develop across the airport
complex. This gusty southeast winds will continue into the
overnight hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Widespread precipitation will slowly diminish and give way to
more showery precipitation across the southcentral mainland
overnight as a low south of the Kenai Peninsula and a trough
extending north across Cook Inlet continue to push to the east.
Strong southeasterly winds across Turnagain Arm will bend into
Anchorage and up Knik Arm this evening as the trough lifts through
the area. It is this transition that remains the biggest forecast
challenge this evening as warmer air aloft will likely mix down
this evening, briefly warming temperatures and changing
precipitation type to a wintry mix from northern Cook Inlet into
the lower MAT-SU valleys. As the trough lifts north, colder air
will begin to advect back into the region, changing any liquid
precipitation back to snow, with light accumulations are possible
overnight.

A second frontal system will then move into Southcentral on the
heels of the first with another round of inland snow and coastal
rain beginning early Monday. The bulk of the precipitation
quickly slides east with the surface low Monday night, so
precipitation amounts and snowfall accumulations are not expected
to be as significant as the previous system. However, a shortwave
and weak surface cold front will trail this second feature,
dropping south from the interior mainland. This shortwave will
provide enough upper-level support and lift to allow snow showers
to persist from the Susitna Valley south across the Anchorage Bowl
and northern half of the Kenai Peninsula Monday evening through
Tuesday morning. Accumulations across valley locations should once
again remain light (1 to 3 inches), but a prolonged westerly
upslope flow could bring higher amounts to portions of the
Talkeetna/Chugach/Kenai Mountains. Northerly outflow wind swill
then begin to increase late Tuesday as the trough slides south and
high pressure builds over the interior.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sunday night
through Tuesday)...
Lingering snow showers across the Lower Kuskokwim Valley will
continue to diminish as the next system arrives over the Southwest
coast tonight. As a Bering low nears the Pribilof Islands
tonight, its leading front will bring another round of
precipitation to Southwest. The forecast was particularly
challenging regarding precipitation types across the Bristol Bay
area. While snow is likely across the Kuskokwim Delta and Lower
Kuskokwim Valley, a mix of rain and snow is likely over the
Bristol Bay area, and the AKPEN will likely be warm enough for
rain. Southerly flow associated with the front will initially warm
temperatures overnight, however as cold air continues to be
pulled in on the backside of the low, a change to rain/snow mix is
likely Monday morning across the Bristol Bay area and the AKPEN.
Gusty winds will also accompany this precipitation, as the low
moves inland on Monday leading to brief periods of low visibility
in areas receiving snowfall.

Given the long duration of westerly fetch, the coastal flood watch
previously issued has been upgraded to a coastal flood advisory
along the coast from Goodnews Bay to Port Heiden for Monday. While
there are areas of shorefast ice along the northern coast of
Bristol Bay, there is uncertainty with how storm surge will
interact with the ice. As the low continues to track inland,
conditions will generally improve for Tuesday before the next
front associated with a North Pacific low nears the coast Tues
night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The active weather pattern over the Bering will persist through
the week. A strong low continues to move across the southwestern
Bering. Its leading front continues to push across the central
Aleutians, and will approach the Southwest coast overnight.
Precipitation associated with this front has begun to fall mostly
as rain over the western and central Aleutians, however a mix of
rain and snow or a change to all snow will be possible as
northerly flow wrapping around the back side of the low allows for
ample cold air advection. Gale force winds accompany this leading
front through the evening while an area of storm force winds has
folded in on the south side of the low. These westerly storm force
winds are expected to persist through the evening over the
southern Bering before diminishing to gales early Mon morning. As
the low passes over the Pribilofs, colder northerly flow will wrap
around the backside, which when combined with lingering snow
showers, may cause a brief period of blowing snow. As the low
makes landfall late Mon morning, westerlies will continue across
the Bering through Mon afternoon. Meanwhile, another front briefly
clips the western Aleutians Mon morning before lifting north Mon
evening. By Tue morning, a front associated with a North Pacific
low moves over the central Aleutians, bringing another round of
southerly flow and relatively warmer air.

&&

.MARINE (Days 3 through 5)...
On Wednesday a 990 mb low tracks north along the eastern Bering
bringing gusty gales as it slightly deepens before it crosses over
the Bering Strait and fills. During this same time frame, the
next low organizes across the North Pacific and tracks to the
southern Bering by early Thursday morning. There is uncertainty on
the strength and track of this storm as a more powerful system
develops south of it and races toward the western Bering on
Friday. Both systems have the potential to bring storm force winds
across the southern Bering.

&&

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

A wetter pattern is expected to continue through the end of the
work week across the southern mainland and surrounding waters. A
drier trend is still expected for the southern mainland at the end
of the week as a ridge amplifies across the mainland. Models are
in good agreement with the general synoptic pattern across the
Bering as low pressure systems continue to develop across the
North Pacific and track toward Alaska. However, models are lacking
in run to run consistency on the individual track and strength of
several low pressure systems that will move to the area.

Only minor changes were made for the Wednesday forecast with
the main concern surrounding a low tracking north across the
eastern Bering. This system is expected to bring gusty winds and
snow across the Kuskokwim Delta with snow spreading eastward
across to Southcentral and the Gulf for Thursday. This solution is
highly dependent on the development and evolution of a strong
wave at the base of the Bering trough, developing near the Alaska
Peninsula.


&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145.
 Coastal Flood Advisory 161.
MARINE...Storm Warning 170 172>176.
 Gale Warning 119 120 130>132 137 138 150 155 160 165 171 177 178
             180.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...KH



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