Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 161644

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 AM AKDT Fri Mar 16 2018


Overrunning precipitation has developed across portions of
Southcentral and Southwest Alaska this morning, resulting from an
influx of warmer air and moisture above a slightly colder air
mass extending from about 7500 feet elevation down to sea level.
The warmer air is somewhat deeper over Southwest Alaska, resulting
in more rain along the coast and even some freezing rain earlier
this evening near Dillingham. Water vapor satellite imagery shows
dry air rapidly moved over Southwest Alaska earlier this evening
which quickly shut off any brief heavier snow, which since has
become far more isolated rain and snow showers.

Most observations this morning from Kenai northward are just a
bit below the freezing mark, resulting in all snow through the
Cook Inlet Region. Stronger southeast winds through the Copper
River Basin resulted in Gulkana hitting 40 yesterday for the first
time in 2018. The southeast winds have also been gusting through
Turnagain Arm this evening, getting as high as 50 mph at McHugh
Creek. They have been diminishing snowfall rates through
Anchorage, but not enough to shut off the snow entirely.

The parent storm system causing the wintry precipitation around
the area continues to weaken and occlude this morning. Thus far,
storm accumulations have been unimpressive to say the least over
Southwest Alaska, with the only 2 stations to report more than
1/10 inch of liquid equivalent precipitation being Iliamna and
King Salmon.

Through the upper levels, ridging is building eastward over the
Copper River Basin and points east, while a weak upper level low
and some ridging north of it is creating a weak Rex Block over the
eastern Bering this morning and associated troughing moving into
Southwest Alaska. A deep upper level low currently over Kamchatka
looks poised to eventually make its way into Alaska for next week.



The models are in good agreement on the synoptic weather pattern
over the next couple days. One notable difference comes Sunday
morning, where the GEM and NAM have a shortwave of energy bringing a
quick hit of precipitation to Southcentral, where as the GFS and EC
do not. In keeping with previous forecasts with nothing to strongly
suggest the GEM/NAM solution is better, went with the GFS once again
for the forecast package. Thus, chances of snow Sunday remain for
now until there is better agreement.



PANC...Rain and snow showers have mostly pushed east of the
terminal, however low stratus may persist into the morning. After
sunrise, improving ceilings will return with VFR conditions with
light winds for the duration of the TAF period.


The remnants of the front that brought snowfall to much of
Southcentral and the North Gulf Coast overnight will continue
making landfall in Prince William Sound this morning.
Precipitation will be generally limited to the North Gulf Coast
and Susitna Valley, as downsloping over the Anchorage Bowl/Matanuska
Valley will prevent anything more than a few stray rain/snow showers
from passing through. The heaviest snowfall will likely fall over
the Northern Susitna Valley through this afternoon with deep and
persistent southwesterly flow and an additional 3-6 inches is
expected today. Strong winds that developed along Western Turnagain
Arm last night will continue for much of this morning, and will even
briefly bring a round of gusty SE winds to south/west Anchorage this
afternoon as the down inlet flow weakens, which will also likely
bring temperatures to near 40 degrees.

By late tonight, weak high pressure moving over the area will
bring a period of much quieter weather throughout the area that
will persist into the weekend. This will allow for clearing skies,
diminishing winds, with continued seasonably warm temperatures to
continue into Saturday.



The storm is in its waning phase this morning, but one last piece
of upper level energy is moving into Southwest this morning, and
this will keep accumulating snowfall from Dillingham north to
Sleetmute/Aniak, and west toward Bethel. Continued northeasterly
offshore flow over the Kuskokwim Delta will keep blowing snow with
visibilities near a half mile (even less in heavier bursts of
snow) into the late morning before snow tapers off quickly. The
entire shortwave trough lifts north and weakens this afternoon
with a dramatic reduction in precipitation intensity. This will
be followed by a more showery/intermittent light snow regime as
broad southerly flow and cold air advection moves over Southwest
Alaska through Saturday.

Another fast moving occluded front will move into Southwest on
Saturday afternoon and quickly move east Saturday night bringing
more snow chances to Southwest as the pattern remains active.



The occluded low over the Eastern Aleutians and Bering Sea will
fall apart and weaken by this evening. Attention turns to a strong
gale force low moving into the far western Bering Sea today and
tonight. The associated front will quickly race across the Bering
Sea through Saturday with a broad region of southwest flow behind
the low lasting through Saturday night.


.LONG TERM FORECAST (Days 3 through 7)...
Forecast confidence is fairly high beginning Saturday night with
a deep low crossing the northern Bering Sea in fast zonal flow
aloft and its associated front moving inland across Southwest
Alaska. While the low and most of the best dynamics will head
northward to northern Alaska, do expect the trough to cross
Southcentral Sunday night through Monday, bringing a shot of
precipitation. Warm air ahead of this storm system will lead to
a mix of precipitation types as it crosses the southern Mainland.

In the wake of this system, a digging trough moving out of Asia
will lead to rapid building of a ridge over the Bering Sea Tuesday
through Wednesday. This will usher Arctic air from Siberia
southward across mainland Alaska and into the Gulf, leading to
chilly temperatures and strong winds through the usual terrain
gaps along the Gulf coast and across the open waters of the Gulf.

Models diverge on whether this blocking ridge holds in place
through the remainder of next week or quickly evolves into
some form of a Rex Block. Thus, forecast confidence drops off
significantly late next week. In any case, a deep low will affect
areas west of the initial block (across the western Bering and
Aleutians) Tuesday through Thursday.


PUBLIC...Winter Weather Advisory 152 155 161.
MARINE...Gale 177 178 185.
         Heavy Freezing Spray 180 185.



LONG TERM...SEB is the U.S. government's official web portal to all federal, state and local government web resources and services.