Small Craft Advisory 407, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 PM AKDT Sat May 19 2018

.ANALYSIS AND UPPER LEVELS...
West of a blocking ridge over western Canada, there is a deep and
complex trough over southwest Alaska, the Aleutians and southern
Bering Sea. Within the trough, several upper lows are rotating
around one another; as one low decays, another moves into its
place. The main closed low brushes the Aleutians as it advances
toward the southwestern Gulf of Alaska. A section of this long
wave trough is attempting to work its way into the Gulf, but the
ridge in the region is stopping the low pressure in it's tracks.
The jet stream flows well south of the Aleutian chain but once it
reaches the ridge, it is forced northward. High pressure near
Russia, supporting an arm of the jet stream in the western Bering
Sea, assists in an intensification of a front that is approaching
the Western Aleutians.

Several short wave troughs just to the east of the Canadian ridge
are building over Southcentral Alaska creating cross barrier
flow. The result is a downsloping pattern which is allowing for a
short reprieve from the rain that has plagued the area for the
last week. While Southcentral is experiencing short periods of
drier weather, ultimately the overall pattern in southern Alaska
is a continuation of wet and cloudy conditions.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through the short term
portion of the forecast (Monday afternoon). The NAM does start to
diverge from the other models with a low in the southwest Gulf
beginning Monday morning, but this has only minimal impacts on the
sensible weather. Forecast confidence is a little above normal.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southerly wind this
afternoon will diminish this evening and remain light through
Sunday morning. Southeast winds may develop again Sunday
afternoon/evening, but they should not be as strong as today.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The broad upper level trough over mainland Alaska will keep a
familiar cloudy and showery pattern in place over Southcentral
into early next week and beyond. The bulk of the rain will be
confined to the Gulf coast, with another pair of fronts helping
to reinforce precipitation as they move inland Sunday afternoon
and again Monday morning. These fronts will also bring another
round of gusty southeast winds to favored gaps such as Turnagain
Arm and the Knik River Valley beginning Sunday evening. Meanwhile,
persistent cross barrier flow will keep inland areas limited to
sporadic shower activity and provide a few breaks of sunshine over
the course of the next few days.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

Showers (along with isolated evening thunderstorms over the Lower
Kuskokwim Valley) will continue to be a threat today through
Sunday evening as broad southeast flow aloft brings in enough dry
and warm air to aid in instability. A weak deformation zone over
the Kuskokwim Delta will enhance intermittent rain/showers into
the overnight hours tonight with more diurnal activity over
Bristol Bay and the Lower Kuskokwim Valley.

A shift in the pattern develops Monday as a large low forms over
the Gulf of Alaska. This will shift the low level flow to
northerly which will result in warming and increased instability
from the Lower Kuskokwim Valley to interior Bristol Bay. For this
forecast, have introduced isolated thunderstorms for that reason
on Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

A low is tracking south of the Aleutian chain and moving east this
evening. This is inducing broad cyclonic flow along with some
showers mainly over the Eastern Aleutians. High pressure will
build in behind this low Sunday evening and Monday ahead of a low
end gale force warm front which will move into the western Bering
Sea Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The overall progressive pattern will continue through much of
next week, as one upper low sets up over the Gulf of Alaska
through mid-week and is quickly replaced by the next upper level
low/trough tracking in from the west mid-week into next weekend.
The initial system will allow several systems to rotate into the
southern mainland through mid-week, bringing precipitation and
some gusty winds to the coastal regions of southcentral Alaska
over the weekend and into next week. However, the strong
southeasterly flow associated with these systems will favor
slightly drier conditions over the downsloped areas along the
lee-side of the Chugach range.

The low and associated front will begin to impact the western
Aleutians Monday morning and steadily track eastward through
Wednesday, covering much of the Bering and Aleutians with wet and
windy conditions. The low begins to stall over the central and
eastern Bering Wednesday night, and eventually begins to track
into the AKPen by Friday. Models are showing a triple point low
forming south of the AKPen Thursday morning, with an associated
front ahead of it tracking through the Gulf region Thursday
through Friday. This will bring a repeat scenario of wet and
windy conditions along the Gulf Coast with downsloping conditions
along the lee-side of the Chugach. The models continue to agree
that at least through next weekend, the persistent pattern of
cooler, cloudy, and unsettled weather will remain in place.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/BL
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...TP