Small Craft Advisory 408, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
413 AM AKDT Sun May 21 2017

.ANALYSIS AND UPPER LEVELS...
A highly amplified pattern has developed. Widespread cyclonic
flow will continue to influence the region...sustaining a
disturbance over the Alaska Peninsula and another across the North
Pacific. Ahead of these features is robust southeast flow. The
most notable area of moisture flux is across Southeast Alaska
where an atmospheric river has set up.

&&

.MODEL DISCUSSION...
All of the deterministic models depict a highly dynamic pattern
almost reminiscent of a winter type pattern. The GFS and the
ECMWF are in good agreement and appear to be in sync with the
prominent features through Thursday. Of note, the RMOP or
Relative Measure Of Predictability for the GFS ensemble mean is in
low confidence territory with the trough at 500 MB and the low
that enters the Bering later in the forecast period. Expect more
fine tuning of the forecast package with each issuance.

&&

.AVIATION...
PANC...Generally expecting VFR conditions this morning. By 18z
light showers will move into the area. Cigs will drop later in the
day possibly reaching 030 by 3z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A couple days of steadier light rain is in store for most places.
A very winter-like pattern is setting up, despite the lack of cold
air. A low pressure system is backing into the area from the east
as a deformation zone/warm front forces a rain shield from east to
west across the area. Light rain will spread from western Prince
William Sound across the Chugach into the Cook Inlet region today.
While the deformation band will move over the Kenai Peninsula and
park itself late this morning, the Anchorage area should see a
short break in the rain before the next front moves overhead this
afternoon and evening. Though, some light rain should make it over
the mountains, especially for east Anchorage and the Hillside.

Steady rain will envelop the north gulf coast for most of the next
24 hours. Late tonight into Monday, the system pushes inland
leaving a showery regime in place, mostly along the coast.
Turnagain Arm convergence looks to play a role in convection
around the Anchorage Bowl while standard afternoon showers off of
the mountains are in store elsewhere to the north. Tuesday begins
a short dry period.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
As an upper level wave pushes through the Lower Kuskokwim Valley
this afternoon there is potential for isolated thunderstorms to
develop in a corridor from the Lower Kuskokwim Valley towards
Bristol Bay. Otherwise, showers are expected across southwest
Alaska this afternoon/evening. Overnight, a dissipating front
will push in from the Bering bringing light, widespread rain. The
rain will then gradually diminish during the day Monday before a
stronger system pushes in late Monday night bringing heavier rain
to southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The front over the central Bering and Aleutians will continue to
track east making it to the Eastern Aleutians late this morning
and over southwest Alaska tonight. Weak ridging will follow behind
the front bringing a brief reprieve to the rain before a stronger
system moves in over the Central Aleutians Monday
afternoon/evening. This system will make it to the eastern Bering
Monday night. It will leave behind weak westerly flow and light
rain showers over the Bering and Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...An amplifying trough
over the Bering Sea will bring a wet and windy period of weather
to Southwest Alaska starting late Tuesday morning. Models have
been slowly coming into better agreement bringing an unseasonably
moist airmass (based on progged PWAT values) to mainland Alaska.
Very impressive jet dynamics along the west side of the trough
will act as a lifting mechanism which will create some very
favorable rain processes. While rainfall amounts vary from model
to model, it looks quite likely that coastal locations of
Southwest Alaska could see over an inch of rain while inland
locations could see a quarter of an inch to three quarters of an
inch.

One of the biggest struggles in the extended forecast is just how
far east the rainfall makes it. Latest model runs suggest a
further eastward track which would allow Southcentral Alaska to be
impacted by the southwesterly flow aloft. This would allow
rainfall to begin possibly as early as Wednesday morning and
persist through Friday. However, it appears that this wet pattern
will begin to dissipate during the Memorial Day weekend, as models
are now in quite good agreement with bringing a ridge of high
pressure over the Northern gulf from Pacific Northwest coast.
This would potentially bring warmer and drier conditions to
Southcentral Alaska over the weekend and into Monday. The big
question is: will the rain end on Saturday or on Sunday? Stay
tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...MC/TP