Small Craft Advisory 410, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 PM AKDT Mon May 27 2019

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low continues to spin over the central
Aleutians this afternoon. Within the flow around the low, a short
wave is currently propagating northward over the Kuskokwim Delta.
This organized low has little upper air support as the jet stream
is zonal along 40N south the Aleutian Chain...extending northward
along the central Gulf to the AlCan boarder. The jet max is
positioned right over Prince William Sound where the winds are
reaching 110 kts. Deep southerly flow continues to stream clouds
northward through the Gulf into Southcentral.

At the surface, conditions are drier in the aftermath of the
front that brought heavy, record breaking rain to Southcentral
on Sunday. With broad instability and weak upper waves, isolated
to scattered showers are evident across the southern Alaska
mainland.

&&

.MODEL DISCUSSION...

There is good model agreement through mid-week as they all track
the low over the Aleutians to the North Pacific on Tuesday. The
first sign of even the slightest model inconsistency pops up on
Wednesday morning as a front associated with an incoming North
Pacific low moves over the central Aleutians. Wind speeds, while
similar, are slightly stronger with the American models than with
the ECMWF and Canadian.

&&

.AVIATION...
PANC...

VFR conditions are expected through Tuesday morning. With a
persistent surface ridge of high pressure along the coast,
confidence is high that the Turnagain Wind will bend into
the terminal each afternoon and evening, with lighter winds
overnight through about midday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Monday
afternoon through Wednesday afternoon)...

Southerly flow aloft will continue through midweek. An area of
low pressure over the Bering will generate several upper level
short-waves over Southcentral, which will provide enough lift to
produce isolated to scattered showers. A rumble of thunder or two
cannot be ruled out over the Susitna Valley this evening, and a
slight chance of thunderstorms is possible again over the Susitna
Valley for Tuesday afternoon and Tuesday evening. By Wednesday, a
front approaches the southern end of Kodiak Island, bringing more
widespread rain and areas of gale-force winds over the Barren
Islands and Shelikof Straight through Wednesday afternoon. As this
front nears Southcentral, gusty winds will pick up again
Wednesday afternoon through the usual gaps and passes including
Turnagain Arm, Knik River Valley, and along the Copper River.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today
through Wednesday evening)...

A broad upper level low centered over the Aleutians will continue
to bring scattered showers across the region through this evening
under its associated surface trough. Isolated thunderstorms are
possible early this evening over the Lower Kuskokwim Valley, though
the chance remains slight. Showers will become more isolated and
winds will diminish Tuesday as the aforementioned low tracks
southward into the North Pacific. A trough approaching from the
western Bering will merge with this low and from one larger low
which will stall south of the Aleutians Tuesday afternoon. This
will lead to increased southerly winds east of the circulation
which will cause an associated warm front to track northward on
Wednesday, thus bringing widespread rain and gusty easterly winds
to the region by the afternoon, particularly in gap areas.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Today
through Wednesday evening)...

A low stalled near Seguam Island is currently bringing scattered
showers to much of the central and eastern Aleutians. This will
also lead to gusty northerly winds west of its center through
early Tuesday morning, mainly in gap areas near Adak. Showers
will become more isolated Tuesday morning as the low tracks
southward. It will merge with a trough Tuesday afternoon, becoming
one large low which will subsequently bring gusty winds and rain
to much of the Aleutians west of Adak.

&&

.MARINE (Thursday through Saturday)...

Winds are expected to remain below gale force through Saturday.

&&

.LONG TERM FORECAST (Thursday through Sunday)...

The upper level pattern continues to show the longwave trough
positioned over the eastern Bering/western Mainland beginning
Thursday. General southerly flow is expected to continue in the
Gulf of Alaska, bringing another round of rainfall to the North
Gulf Coast at the end of the work week. Current timing puts this
next trough in the Gulf by Thursday evening and into Friday.
However, the forecast models are notoriously bad at maintaining
consistency on timing of these surface features so keep an eye on
the forecast for any changes in timing as Thursday approaches.

By Saturday, it looks like the main upper level low will move to
the Gulf of Alaska with a ridge building in over the northern
mainland. This will allow the surface ridge currently positioned
over the western Bering to slide eastward, bringing northerly flow
to the eastern Bering. In the western Bering, a storm will approach
the region late Sunday for the first time in a while. For the
southern mainland this change in the upper level pattern means
there is a possibility of a transition to sunnier weather starting
Saturday. However, this upper level pattern is a good set up for
a thermal trough in the Interior, which may lead to more afternoon
shower activity in the inland areas. Overall this would be a
transition to a pattern which is more what we would expect of
early June.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...LF