Small Craft Advisory 420, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKDT Thu May 25 2017

.ANALYSIS AND UPPER LEVELS...
An upper level trough continues to swing through the Southern
Mainland this afternoon. This is evident on IR analysis with a
clearing line transiting to the east of the Anchorage Bowl. Across
the Chugach, puffy cu are developing. As of a few hours past
noon, no lightning was evident, but convection is becoming much
more widespread. West across the Bering is where the primary
dynamic forcing with a strong 140 kt jet exist. At the surface to
the north of the jet is an area of low pressure that is spinning
up, with thunderstorms noted on GLD analysis south of Shemya.
North across the Kuskokwim Delta is a weakening area of low
pressure, evident by the cyclonic flow of the clouds. Cold
temperatures aloft, with cold lower levels allowed areas around
Bethel to see a light dusting of snow this morning.

&&

.MODEL DISCUSSION...
Overall model guidance seems to be in good synoptic agreement
with each other for the present run, but do show some differences
compared to prior runs. The biggest change across the Anchorage
Bowl was to significantly decrease the amount of convergence
helping to aid in afternoon shower/thunderstorm development across
the Chugach. Beyond that frame, guidance maintains similar timing
and intensity of the upper level wave moving across the region
tonight. Larger differences occur as we move into the weekend and
watch the evolution of a strong wave moving out of the Bering.
American guidance seems to be charging the way and faster/deeper
than our Canadian and European counterparts. By Sunday afternoon,
model spread increases both at the upper and lower levels of the
atmosphere, decreasing overall confidence as we go into the
holiday weekend.

&&

.AVIATION...
PANC...Gusty southerly winds will continue through Friday
afternoon. Some showers are possible this evening and then again
Friday afternoon/evening, but VFR conditions should prevail.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Tonight, the area showers and thunderstorms will diminish with
the loss of daytime heating. The upper level wave bringing in
colder air in the upper levels will continue to swing from
southwest to northeast, so the only lingering thunderstorm threat
after sunset will be along the Alaska Range. Things will briefly
dry out and skies will clear out through the rest of the night for
most areas.

On Friday, most areas will do it all again regarding the showers.
During the afternoon, a similar amount of instability will be
present again. However, without the same intensity of an upper-
level wave and a lack of cold advection increasing lift,
thunderstorms are not expected and the showers will not be as
intense. The next storm system will move into Kodiak during the
day.

On Friday night, the showers will diminish in the evening once
again, but cloud cover will remain plentiful as the next front
works its way northward. This will turn the broad wind flow
southeasterly. The southeast flow will cause the usual upslope
areas along the Alaska Range and the Gulf coast to pick up steady
rain while the usual downslope areas, including Anchorage, likely
will see no precipitation at all.

For Saturday, the same frontal system will stall, so the same
upslope areas will continue to see steady rain all day while at
most the downslope areas see a passing shower.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Showers and isolated thunderstorms are expected to continue
across the region this evening as a strong upper wave lifts north
along Bristol Bay and the Kuskokwim Valley. Thunderstorm activity
will be confined to the Kuskokwim Valley and Western Alaska Range
where the best dynamic lift and highest instability sets up. The
wave moves across the interior after midnight ending thunderstorm
chances along the Kuskokwim.

The Southwest region will see continued southerly flow on Friday
and Saturday as the next surface low develops across the northern
Bering. This low moves slowly to the west coast through Saturday
morning and then stalls as a ridge building across the northern
gulf blocks its eastward progress. A second low moves just south
of the Alaska Peninsula early Saturday and stalls into Sunday.
Both of these systems are the focus for showers through the
weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A surface low races across the eastern Aleutians tonight bringing
light rain as it moves through. This low crosses to the Pacific by
Friday morning and stalls through Saturday just south of the
Alaska Peninsula. Increased shower activity across the Peninsula
Friday and Saturday as the low remains in the vicinity.
Precipitation trends go down on Sunday as the low lifts north
across the eastern Peninsula; however, this may need to be
adjusted as models are still not in agreement on how fast the
system moves. A ridge builds across the western Bering on Friday
and shifts east through Saturday. A stable air mass associated
with the ridge will bring patchy fog across to the central and
southern waters for the upcoming weekend.

The next front moves into the western Aleutians early Saturday
and diminishes as it slides across the central Aleutians with a ridge
holding over the central waters of the Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast begins on Monday with the cut off low that
is poised to develop near Kodiak Island this weekend lifting
northward through western Alaska. This will occur as part of an
amplification of the large scale pattern over the course of the
next week, with a sprawling longwave trough encompassing the
Bering Sea and western mainland as a stout ridge builds over
western Canada. Forecast confidence remains lower than normal as
models continue to diverge on the smaller scale details, but this
synoptic pattern will generally result in continued cloudy and
showery conditions across the southern mainland as broad onshore
flow remains focused along the gulf coast between the large scale
features. Benign showery conditions can also be expected over much
of the Bering beneath the upper trough. Temperatures will
gradually climb back to around normal through the week as the cut
off low departs into the arctic at the beginning of the week and
scours out what remains of the colder air aloft.

&&

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

&&

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