Small Craft Advisory 297, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKDT Sat Apr 8 2017

.ANALYSIS AND UPPER LEVELS...

Three separate upper level low pressure systems are affecting the
forecast area this morning. Two of them, the weakening but ongoing
low south of Kodiak and a new much stronger low near the western
Aleutians have surface reflections. The third upper level low is
associated with a weak shortwave quickly diving southwestward
towards the Kuskokwim Delta. Weak ridging is in place between the
Kodiak and western Aleutian lows. Ridging is also in place across
far eastern Alaska into Canada. Overall though, the two lows
nearest mainland Alaska are relatively weak, keeping broad upper
level flow weak.

The result of all this weak steering flow is slow movement of both of
the lows near mainland Alaska. Thus, the weather pattern area-wide
is slow to change. The atmosphere over southern Alaska is very
unstable, with the afternoon sounding nearly dry adiabatic
through nearly the entire lower half of the atmosphere.
Fortunately, thunderstorms are precluded for now by very dry air
in the lower levels. However, that didn't stop a few graupel
showers from occurring near Kenai this afternoon. Diminishing
forcing over the Gulf due to the filling of the low south of
Kodiak has also diminished the intensity and coverage of the
showers moving across the northern Gulf and the southern Kenai
Peninsula.

Things are mostly quiet across Southwest Alaska as well this
morning with the exception of some snow/graupel showers pushing
southward out of the Yukon Delta area towards the Kuskokwim Delta.
The trajectory of those showers on Bethel radar suggest any
activity reaching the ground should stay just west of Bethel
proper.

For the Bering, the next low racing eastward is bringing rain and
snow to the western Aleutians this morning, with winds near gale
force associated with the leading edge of the front. Elsewhere,
the deformation zone that set up near Cold Bay for nearly 24 hours
finally moved far enough west to end the snow there. However, the
squeeze play between the Gulf low and the skinny ridge over the
central Bering continues to keep conditions favorable for the
persistence of the deformation. Resultant snowfall has been fairly
minor in Cold Bay though, with an inch of new accumulation
reported for the day yesterday.

&&

.MODEL DISCUSSION...

Models continue to remain in good agreement owing to the
persistence of the current weather pattern. The major features
impacting the area are all of large enough scale that the models
are doing well at handling them. The weak and chaotic steering
flow in the upper levels is both suppressing significant shortwave
development across southern Alaska and keeping those that already
exist moving slow enough that multiple model runs tracking the
same waves of energy are further enhancing agreement. This
scenario keeps broad forecast confidence high.

A few forecast challenges to note: First is the evolution of any
shower activity across the Kuskokwim Delta through the day today
as the attending upper level low dips over the area then lifts
back to the north just as fast on Sunday. Second is the timing and
intensity of any rainfall near Kodiak as the low to their south
weakens. Third, one of the models with a solid background on
convection suggests more shower activity may develop late this
afternoon over the Kenai Peninsula. Despite the dry atmosphere,
graupel was reported in Kenai today so such a scenario is
plausible. With all of these unlikely to produce significant
impacts, these are issues that will continue to be monitored.

&&

.AVIATION...
VFR conditions will persist through the day today, with light
southerly flow developing by late this afternoon. Isolated showers
are possible along the mountains.

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

With the low continuing to diminish south of Kodiak Island, the
weather pattern will remain relatively quiet over the next several
days. Showers and gusts have already weakened across Kodiak Island
this morning as the low drifts to the south. Meanwhile, winds are
universally light across southcentral as pressure gradients
relax. light southerly winds are possible across the Anchorage
Bowl today as diurnal heating creates a weak mesoscale low.

The sounding this morning is nearly identical to the one
yesterday, with fairly steep lapse rates and still fairly dry air.
As a result, expecting shower activity across the western Kenai
this afternoon and evening to be similar as yesterday, if not a
bit more intense ahead of a shortwave. Steep lapse rates will
support shower development, while the continued dryness of the air
will limit shower coverage and intensity. Model data seems to
reaffirm this conceptual model. The HRRR model verified the most
accurately for showers in the northern Kenai Peninsula yesterday
afternoon, and the HRRR once more indicates convective development
this afternoon and is trending upward in shower coverage in each
run. Shower development is expected once more on Sunday evening.
An upper level wave will continue to provide more forcing for lift
on Sunday, however, cloud cover will limit surface support for
shower development.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Scattered snow and rain showers develop across the southwest today
as a strong shortwave slowly moves along the Kuskokwim Delta and
western areas of Bristol Bay, while at the same time, another
shortwave propagates westward from the Aleutian Range. Surface
flow will generally out of the north today and switch back out of
the southeast on Sunday. Hence, drier conditions ensue on Sunday
and Monday as downsloping occurs with easterly flow. The Alaska
Peninsula will even see a short break from precipitation on
Sunday, however a weak front passes on Sunday night mainly bring
rain chances along the Pacific side.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A gale force low across the western waters weakens tonight with
relatively benign weather on Sunday ahead of the next system. A
developing North Pacific low organizes south of the Kamchatka
Peninsula and moves the next front to the western and central
Aleutians early Monday. This warm system will bring easterly gales
and rain as it occludes on a northwesterly track. Adjustments
were made to speed up the front as models all resulted in faster
trends...the boundary pushes into the north Bering and across to
the eastern Aleutians late Monday night.

&&

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

The long term forecast will not be seeing drastic change from the
going forecast. Starting late Sunday and extending through early
into the week, the majority of the active weather will stay well
to the south of Alaska. The primary jet stream is zonal and at or
south of the 45th parallel. Low pressure that is presently to the
west of Shemya will become stacked and lose upper level support
through the Sunday and eventually move along the Aleutian Chain.
Behind that system is the next change.

A surface ridge that will build and traverse the AKPEN will
gradually give way to a cyclone that is progged to slide up and
through the Western chain and eventually impact Western AK. At
this stage guidance is stalling a front from this system across
western Alaska and extending through Kodiak. The longer term
forecast was trended towards the GFS and ECMWF through day 6 and
operated on a bigger ensemble blend through day 8.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...GALE...178.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...SS