Area Forecast Discussion - SouthCentral and SouthWest


FXAK68 PAFC 251414
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKST Sun Feb 25 2018
.ANALYSIS AND UPPER LEVELS...
Skies are in the process of clearing out across much of 
Southcentral Alaska this morning as dry northwest flow and weak 
cold air advection are established behind a departing trough over
the panhandle. Patchy fog has formed over Upper Cook Inlet and
into the Copper River Basin as a result of the clearing skies and
falling temperatures. The northwest flow pattern is also supporting
widespread gale force winds across the western Gulf including 
Kodiak Island. A transient upper level ridge is moving over the 
southwest mainland, promoting widespread stratus development with 
some areas of fog across Bristol Bay supported by onshore flow at 
the surface. Meanwhile, the next trough poised to impact the 
mainland is moving into the western Bering Sea. A strong warm 
front extends across the central and eastern Bering from a surface
low north of Attu, bringing widespread rain and southeast winds 
to the Pribilofs and Eastern Aleutians as well as the Alaska 
Peninsula. A wide swath of storm force winds is moving into the
western Bering aided by strong cold advection on the back side of
the low. 

&&
.MODEL DISCUSSION...
Models continue to show good synoptic agreement in the short term
as a progressive pattern remains in place over the region through
the early part of next week. The current weak outflow pattern 
over Southcentral will quickly be replaced by the Bering system as
it occludes and then tracks eastward across the mainland Sunday 
night into Monday, bringing another round of snow to much of the 
region. This then looks to be followed by another round of outflow
winds by Tuesday as that system departs to the east.

&&
.AVIATION...
PANC...Patchy fog over the Upper Cook Inlet region will keep the
potential for IFR/LIFR conditions at the terminal through the
morning hours, with VFR conditions and light winds expected for
the rest of the day once the fog lifts. Snow will then spread into
the terminal beginning around 08z tonight, bringing a return of
IFR conditions to coincide with the most intense snowfall through
early Monday morning. Conditions may improve to MVFR towards the
end of the TAF period as snow begins to diminish in intensity. 

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Outflow winds along the North Gulf Coast and some patchy fog 
around Northern Cook Inlet will be the primary forecast concerns
today as a ridge of high pressure slides over the South Mainland.
The most significant winds will continue to blow from the Barren
Islands southward into Kodiak Island, however a brief uptick in
the outflow winds in Seward, Whittier, and Valdez will continue
until late this morning. As far as the fog/stratus around Cook
Inlet, there is enough mixing from the offshore flow to prevent it
from becoming too widespread, however a few patchy areas will
continue drifting around the area until late this morning. 

The focus then shifts to the incoming front that will bring a
quick shot of snow to Kodiak Island this evening, and then spread
northward into the Gulf Coast and Southcentral for late tonight
into Monday. With no downsloping and temperatures remaining well
cold enough to support snow for the entire event, confidence has
increased enough in the potential significant accumulations in 
Anchorage and the Matanuska Valley for Winter Weather Advisories
to be issued this morning. The biggest concern from this event
will with the heaviest snowfall that is expected to fall during
the Monday morning commute, with additional snowfall that should
persist into the afternoon commute. The Matanuska Valley and
Hatcher Pass will likely see the highest accumulations from this 
event due to its southwest-west flow that tends to allow snowfall 
to linger longer than other areas. This event will also likely 
not have the distinct back edge than the last 2 recent snow events
had due to broad cyclonic westerly flow that will persist into 
Tuesday. As a result scattered snow shower activity will likely 
linger through Monday night, though most of the accumulating 
snowfall should end in most areas of Southcentral by early Monday 
evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Sunday and
Monday)...

A short lived period of quiet weather will grace Southwest Alaska
this morning before the next system comes charging across the
Bering Sea towards the western coast. A frontal system will make
landfall this afternoon bringing another round of snow to most 
locations, with 2-4" accumulating by the end of the day on Monday.
Over a foot of snow is expected for upslope locations on the 
western side of the Kuskokwim and Alaska Ranges as strong westerly
flow will help promote good orographic lifting. Farther south 
over the AKPEN, a nose of warmer air will push up with the system 
Sunday afternoon leading to precipitation falling mostly as rain 
over the southern AKPEN while rain will mix with snow further 
north towards King Salmon before changing over to all snow in the 
evening. 

Colder air moves in behind the front causing temperatures to fall
through the day on Monday, with high temps expected in the
early morning. The cold air advection will also lead to strong 
west/northwesterly winds Sunday night into Monday across Bristol 
Bay and into the lower Kuskokwim River Valley. Winds could gust 
as high as 50-60 mph along the Bristol Bay coast with even higher 
gusts possible through channeled terrain over the AKPEN. Because 
of the strong northwest flow, snow showers will continue in the 
mountains through much of the day, potentially continuing through 
much of Monday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Sunday
and Monday)...
An upper level trough will reassert itself over the Bering Sea
today replacing a transient upper level ridge, continuing the 
progressive pattern seen across the area this week. A strong 
surface low will move west across the central Bering today with an
associated frontal system sweeping across the Bering and 
Aleutians bringing rain which will change to snow showers on the 
backside of the front. A 140 kt jet max on the bottom of the upper
level trough will support the development of this system bringing
a swath of storm force surface winds (50 kt) and building seas to
the Bering Sea between the Aleutians and Pribilofs Sunday 
afternoon through Monday morning. The long fetch of the westerly 
winds will build seas to over 35 ft by Sunday night.

Once the low moves into Southwest Alaska, conditions will calm
down somewhat, with a much weaker low moving into the western
Aleutians late Sunday night into Monday morning. This weak low
will slowly track eastward through the central Bering through
Monday night, but no hazardous weather is expected from it.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The progressive pattern with zonal westerly flow will continue 
Monday night into Tuesday with a couple more cold troughs tracking
across southern Alaska. West to northwesterly outflow will 
increase across Southcentral Alaska and the western Gulf Monday 
night following the passage of one trough. The next trough, 
bringing additional cold air aloft with it, will swing into 
Southwest Alaska Monday night and continue into Southcentral and 
the Gulf Tuesday with another small shortwave following through 
the northwesterly flow aloft late Tuesday night through Wednesday 
morning. 

Further out to the west, a rapidly deepening north Pacific low
will track north towards the western Aleutians on Tuesday and 
then cross into the western Bering Tuesday night and Wednesday. 
With the low still strengthening as it passes not far to the west 
of Shemya, storm force or stronger winds are likely. In response 
to the deepening storm, a strong upper level ridge will build over
the central Bering Tuesday and then continue to amplify as it 
shifts east over the eastern Bering Tuesday Night and west coast 
of Alaska Wednesday.

Forecast confidence decreases rapidly Friday through the weekend
due to uncertainty in how fast the large ridge will push east and
in what manner it will break down. Models, however, do agree on 
general trend back towards active zonal flow for the first half of
next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101, 111.
MARINE...Storm 132, 138, 165, 170, 175, 177, 178, 179, 352, 411, 413, 414.
Gale 119, 120, 130, 131, 136, 137, 139, 141, 150, 155, 160, 171, 172, 173, 
     174, 176, 180, 185, 351, 412.
Heavy Freezing Spray 130, 181, 185.

&&

$$

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