Small Craft Advisory 373, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 AM AKDT Tue May 9 2017

.ANALYSIS AND UPPER LEVELS...
The broad low that has resided near the Alaska Peninsula during
the last several days is now starting to move east. An upper ridge
has amplified behind this system, centering over the central
Bering early this morning. A North Pacific low nearing the western
Aleutians is slowing its momentum as the ridge blocks its
northeastward progression. At the surface, winds are tapering off
this morning through channeled terrain along the coasts of
Southcentral. Showers along the northern Gulf are diminishing as the
surface low near Kodiak drifts east. Light rain developed across the
Matanuska Valley yesterday evening along a weak upper wave and
continues to initiate along a narrow corridor from the Talkeetna
Mountains to northern Cook Inlet early this morning.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement with the synoptic pattern
along the southern mainland and the Gulf, but differ in their
solutions with a North Pacific low moving up to the central
Aleutians on Wednesday. The model choice for this low was the GFS
which was the middle range solution on the 00Z model run. This slow
moving system is expected to have relatively low impacts with small
craft winds and rain as it takes and eastward track; however, there
is still some uncertainty in its strength. The high resolution NAM
was used to make minor updates across Southcentral with slight
increases in wind speeds along coastal gaps.

&&

.AVIATION...
PANC...VFR conditions will persist through Wednesday. Rain
developing along a shortwave moving south over the Matanuska
Valley is not expected to move into West Anchorage as a surface
ridge remains along Prince William Sound. Southeasterly flow this
morning should shift to the west this afternoon with an afternoon
sea breeze, then taper off tonight and remain light through
Wednesday morning.

&&

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

In general, northeasterly flow aloft is expected across Southcentral
through tonight. Weak upper impulses will track south in the flow
and help to maintain periods of showers across the mainland.
Stability indices across the Alaska Range bordering the Copper River
Basin indicate the potential for wet thunderstorms. The upper
steering flow would tend to push them south off the range this
afternoon and evening. There is potential for thunderstorms again in
the Copper River Basin on Wednesday afternoon. Elsewhere, Wednesday
will bring an overall drying trend, especially along the coasts.
However, there will still be potential for showers inland into
Wednesday night.

Surface pressure gradients will be fairly light today so expect
sea breezes to be the main wind regime this afternoon and
evening. Outflow winds along the Gulf coast/eastern Kenai
Peninsula will start ramping up Wednesday as pressure gradients
tighten in response to the intensification of the north Gulf
surface trough.

&&

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

An upper level wave digging southward will help provide the
forcing for isolated afternoon thunderstorms along the Alaska
Range today. Increased moisture in the mid-levels ahead of it should
provide just enough fuel for a few thunderstorms, though some
limiting factors remain: dry air in the low levels and weak forcing.
Winds in most areas should remain light and variable.

For both Wednesday and especially Thursday afternoons,
diminishing cloud cover with building high pressure will allow for
ever-increasing sunshine across Southwest Alaska. Temperatures are
expected to spike into the upper 60s in the Lower Kuskokwim Valley
on Wednesday afternoon and around 70 on Thursday afternoon. The
warm temperatures will promote low relative humidity levels as
well. Fortunately, high pressure will keep winds nearly calm.

&&

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

A front over the central Bering Sea will continue to slowly press
eastward today into tonight, but will weaken with time as well.
The next big weather maker begins making its move into the western
Aleutians on Wednesday. A very favorable jet streak pattern will
lead to rapid cyclogenesis, as the surface low pressure system
makes full utilization of the left exit region of the subtropical
jet. Strong baroclinicity will also favor the development of the
low. Thus, over the course of the day Wednesday, look for rapid
strengthening of the winds across nearly the entire Aleutian
chain. The low itself will slow its movement to a crawl on
Wednesday night as the upper level low moves overhead, making the
system vertically stacked. The strongest winds will then be found
on the southwest quadrant of the low as cool air races along the
southern periphery of the low.

&&

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

A lower than normal confidence extended forecast is shaping up for
this forecast package. The first of systems to impact the
forecast area will begin in the Bering mid week as low pressure is
set to advance across the Western Aleutian chain Weds night/Thur
morning. The movement of this low is what is causing the
confidence to falter this afternoon. Earlier global guidance was
suggesting that a weak jet streak would develop across the chain
and shift this low south of the Gulf of Alaska and south to the
Southeast panhandle. As later guidance begins to roll in, the
thought is now shifting towards a weakening stacked low that will
slide across the Southwest mainland this weekend. National went
with an ensemble approach that seems to favor the more progressive
movement to SE. Further clarity should show as we get closer to
midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

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