Small Craft Advisory 283, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
402 AM AKDT Wed Apr 4 2018

.ANALYSIS AND UPPER LEVELS...
Water Vapor Satellite imagery shows a broad low over Alaska with
a closed low circulating over the southern tier of the state. A
ridge axis extends across the Bering from a ridge center north of
Kamchatka. Two other closed low centers are located to the west
and south of the Aleutian chain, but are little influence in the
synoptic pattern as the low over the mainland spreads its
dominance westward. At the surface, a center of low pressure is
developing over the western Gulf of Alaska with snow showers
circulating along the northern gulf across to Bristol Bay.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement with the synoptic pattern
through the end of the week, however there is still uncertainty
in the forthcoming snow event as the upper level low deepens on
an eastward track while the developing surface low is pulled
north. Generally kept the inherited forecast trends with slight
upward trends in precipitation coverage and amounts.

&&

.AVIATION...
PANC...
VFR ceilings and visibilities are expected to diminish to MVFR
late this morning, or possibly even brief periods of IFR due to
snow showers, as a front moves through from the gulf. Northerly
flow is expected to change quickly to the southeast with the
frontal passage this afternoon. Wind shear was not added to the
current TAF as there is still some uncertainty on the timing for
the front.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Our spring snow event is beginning to show it's face this morning.
However, the forecast challenges remain the same. Snow showers
will rapidly increase in coverage and intensity over the western
Prince William Sound region this morning. The unstable air mass
being thrust against the mountains will produce convective snow
showers, with periods of heavy snow reducing visibility. The
Portage Valley will also see some moderate easterly winds that
will blow snow around, but with surface temperatures near or above
freezing, falling snow should be the predominant low-visibility
threat. While the snowfall will largely be to the east and toward
the coast as the event starts, the shifting low-level flow to the
southeast will get the majority of the Seward highway in the
action as well. Expect heavy snowfall from Seward north to
Turnagain Pass with over a foot of snow expected along this
corridor. Snow showers will rapidly diminish in intensity and
coverage late tonight into Thursday.

The uncertainty remains for the Cook Inlet region. The Matanuska
wind will continue to reinforce dry air at the surface through the
day, as well as increasing downslope drying from cross-barrier
flow this afternoon. The X-factor is instability. Instability will
be released as the air mass is driven against the coastal
mountains, so snow showers should gradually try and moisten the
column. The best chance to see accumulating snow today will be the
upper elevations of the Anchorage area and Chugach Mountains. As
the upper level trough brings a favorable area of lift overhead
this afternoon, some showers should be strong enough to saturate
the column, even at lower elevations. However, downslope drying
will still take a bite out of the moisture, so accumulations
should be minimal.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A cold upper level low will continue spreading south and west
over southwest Alaska bringing a noticeable change to the weather
for the next couple of days. For today, cloud cover will increase
and temperatures will cool off some as a few snow showers slowly
drift west across the Kuskokwim Delta, Bristol Bay coast, and the Northern
AKPEN. As the low center moves further west over Bristol Bay
tonight, the mid and upper level flow will become southwesterly
allowing more moisture to move up into the region. Mid and upper
level cold air will be dragged south underneath the low steepening
lapse rates and increasing atmospheric instability over the
southwest mainland. This will lead to numerous snow showers
developing over the lower and upper Kuskokwim River Valley tonight
through Friday afternoon. Models are keying in on a narrow snow
band setting up along a corridor from roughly King Salmon to
Sleetmute by Thursday afternoon as the dynamic setup becomes more
favorable. This increases the chances for more significant
accumulations within this region between Thursday afternoon and
Friday morning. Exact snowfall amounts cannot be pinned down until
models come into better agreement regarding the placement of this
feature, as small differences in the location can mean the
difference between no snowfall and possible advisory level
amounts.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A weak frontal boundary will continue to decay over the
southwestern Aleutians and Bering while an upper level high will
be replaced by an upper level low over eastern half of the Bering
today. The upper low will bring increasing cloud cover and
eventual snow shower activity through Thursday in cold northeast
flow as the mainland low retrogrades offshore of the Kuskokwim
Delta. This will also bring heavy freezing spray to the coastal
waters north of Cape Newenham as colder air temperatures return to
the region. Another front will then move over the Western
Aleutians by Thursday afternoon, bringing a large swath of
precipitation and a period of gale force winds to the area.

&&

.LONG TERM FORECAST (Days 4 through 7)...

A progressive pattern is on tap. The forecast period commences
with an upper level low in the Gulf Of Alaska and a second low
near the Central Aleutians. Even early in this period, there is
some significant divergence in solutions. For the low near SE AK,
the GFS and the Canadian GEM have the mid-level low near 52N and
140W while the ECMWF and the NAM are further west. The NCEP RMOP
(Relative Measure Of Predicatility) for the ensemble mean is in
low confidence territory by 96 HRS for the Gulf of Alaska. But
when, the western low was evaluated the divergence at 500 MB was
more significant but the RMOP was substantially more confident
with this new closed low. All the while there is another
disturbance influencing Southwest AK and all of the major models
depicting the negatively- tilted shortwave in roughly the same
place with a robust impulse of energy. However, that feature will
rapidly push north of the Alaska Range and out of the AOR.
Even with the confidence issues due to the lack of consensus, the
tangible sensible weather high-lights include gale force winds
with both the low in the Gulf Of Alaska and the Central Aleutians.
Expecting moisture to wrap around the Gulf Low and across
portions of Prince William Sound Saturday. The frontal boundary
that is bisecting the Bering will bring another round of
precipitation to the Pribilof Islands and the Chain this weekend.
By Sunday, both the GFS and the ECMWF have another surface low
south of Kamchatka which will continue to push towards Attu Island
and then take a more easterly track pushing across the Aleutians.
This system too will yield another round of blustery weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 180 181 185.
         Gale Warning 119 130 139 130.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KV
LONG TERM...PJS