Small Craft Advisory 183, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
356 PM AKST Wed Feb 28 2018

.ANALYSIS AND UPPER LEVELS...
Clear skies prevail across much of the southern mainland this
evening, as the upper level ridge begins to build over the area
and shifts the long wave trough further eastward. Cold air
advection and a fairly tight pressure gradient continue to
produced a strong outflow wind regime across Prince William Sound
and northern Gulf coastal areas. Additional support from a 150 kt
jet streak sliding east across the eastern mainland tonight will
also help the warning level winds persist into Thursday morning
for Thompson Pass and Valdez. Quieter weather will prevail across
the southwest Mainland beneath a stout high amplitude ridge.
Further west, a powerful 957 mb low is tracking parallel to the
Kamchatka coast, with an occluded front moving into the Eastern
Aleutians and Bering tonight accompanied by rain and southerly
gale force winds.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement through the beginning of the
weekend as the Bering ridge shifts eastward over the mainland,
eventually bringing an end to the outflow winds across
Southcentral. This will allow the front to continue its eastward
push and reach the southwest coast by Thursday, with showery
southwest flow setting up in its wake. Primarily utilized higher
resolution NAM guidance to better capture localized outflow wind
effects.

&&

.AVIATION...
PANC...VFR conditions will prevail through the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Forecast continues to be on track through Southcentral, with
offshore winds continuing to be the main challenge. The cold air
dome has built in over the Copper River basin, and the encroaching
ridge continues to bring the strongest gap flow to Northeastern
Prince William Sound. Although the winds have tapered off slightly
this afternoon, they are expected to pick back up this evening as
the strongest portion of the northerly jet moves overhead. Models
indicate strong downward vertical motion through the Chugach
Range, and northerly oriented passes such as Mineral Creek Canyon
should see wind gusts above 70 mph. Thus, the high wind warning
has been extended until 7 a.m. and now includes Valdez as well as
Thompson Pass.

Colder temperatures are still expected to return tonight and
Thursday night, but by Friday, a front approaching from the west
should moderate temperatures a bit, in particular for the Susitna
Valley and western portion of the Alaska Range. Models are
indicating this front has slowed since yesterday, and is now
expected to weaken dramatically as it moves into the top of the
ridge. This forecast brings in precip to Kodiak on Friday but not
expecting it to move much further westward until the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Thursday and
Friday)...
High pressure over the Lower Kuskokwim Valley this evening will
slowly retreat into Interior Alaska through Thursday. Meanwhile, a
very slowly moving front will push into the coast, bringing
another round of rain and snow. With the front occluding, right
now it looks like most areas away from the immediate coast should
see all snow with it. The precipitation will begin Thursday
afternoon at the Kuskokwim Delta coast, a bit slower than previous
model solutions showed. As the front moves into Southwest Alaska,
Thursday night, it will quickly fall apart. Thus, the greatest
accumulations will be from Bethel east. However, it only looks to
amount to a couple inches. Associated with the front will be some
gusty winds on the order of 30-40 mph with higher gusts, which
will have the potential to cause additional visibility
restrictions due to blowing snow late Thursday afternoon and
evening. The worst effects of blowing snow should be confined to
the coast.

On Friday, the front will have largely dissipated, though remnant
precipitation activity will continue through the day through the
Lower Kuskokwim Valley and especially Bristol Bay as upper level
support moves over the area and the next low approaches from the
south. There remains significant disagreement as to how heavy and
widespread this activity will be, with the stronger solutions
suggesting a few inches of snow and lower ones suggesting
scattered snow showers with relatively little accumulation.
Nevertheless that models agree that to one degree or another, the
precipitation lingers straight through into Saturday morning,
with the likelihood of snow increasing the closer to the Alaska
and Aleutians Ranges you go.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Thursday
and Friday)...
The front over the central Bering is moving into the Pribilofs
and through the central Aleutians this evening. It will continue
to drift eastward into the eastern Bering on Thursday. Most areas
will see rain, though that likelihood decreases with increasing
latitude. The front moves into southwest Alaska Thursday
evening. Behind the front over the western half of the Bering, a
return to a showery regime is expected with a weak disturbance
traversing eastward through Friday. A North Pacific low makes a
glancing blow to the Alaska Peninsula and points south Friday into
Friday night, moving up along the dying front. However, its
northward progress stops as the front dissipates. Finally, a new
strong front moves into the western Aleutians and Bering Friday
afternoon and night, with more rain and gusty winds expected with
it. Weak surface ridging and high pressure builds into the eastern
Bering Friday night in between systems.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The operational run of the GFS continues to display good run to
run continuity. At the onset of the forecast period, the GFS
ensemble members are tightly clustered with the low near
Kamchatka, the high south of the Central Aleutians and a low south
of Prince William Sound. As the forecast period unfolds the
ensemble members remain grouped for the dome of high pressure in
the north Pacific, but the consensus quickly unravels in the
Bering. Very confident in the progressive nature of the pattern.
The GFS continues to depict 3 distinct frontal passages across the
Bering this forecast period with the remnants of these systems
moving into Southwest Alaska. East Domain (Kodiak, Anchorage and
Valdez) sensible weather highlights include a batch of wintry
precipitation for Kodiak and the southern Kenai Saturday March 3rd
that will move across the Gulf of Alaska later Saturday and
Sunday. Monday a new batch of wintry weather is anticipated to
impact Kodiak with moisture working its way up Cook Inlet later
Monday impacting the Anchorage Metro Area Tuesday and Wednesday.
Third batch of wintry weather is anticipated for Southcentral late
Thursday. In a nutshell, the Anchorage Bowl and the Mat-Su Valleys
can expect more snow! Stay tuned for more details.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131.
MARINE...Gale Warning 119>121 125 127 128 131 132 165 170 172 173
178>181 185.
 Heavy Freezing Spray 130 139 160.

&&

SYNOPSIS AND MODEL DISCUSSION...PEPE
SOUTHCENTRAL ALASKA...LF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...PJS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1043 PM AKST WED FEB 28 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0556 PM     NON-TSTM WND GST VALDEZ AIRPORT          61.13N 146.25W
02/28/2018  M78 MPH          VALDEZ-CORDOVA     AK   AWOS

            AWOS STATION PAVD, VALDEZ AIRPORT.

&&

RA