Small Craft Advisory 426, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 PM AKDT Sun May 28 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low centered just west of Kodiak Island
moving to the north. A negatively tilted associated surface front
with plenty of moisture is situated over the Gulf and pushing
into south central Alaska. Most of the rain for this system is
falling along the coastal Mountains, with most inland areas being
"down-sloped" and thus fairly dry. There is a Large vertically
stacked low over the western Bering Sea, centered just north of
the western Aleutians. There is a ridge of high pressure between
these two lows over the eastern Bering Sea. There is a rather
strong and large ridge over northwest Canada extending southward.

&&

.MODEL DISCUSSION...
The numerical models are still struggling a bit with the pattern
over the Gulf and mainland Alaska. There is decent agreement on
the surface pressure pattern, but there are still some decent
differences in the timing and amounts of precipitation expected in
the next 24 hours. A middle ground (a "poor mans ensemble")
approach was used to handle the uncertainty.

&&

.AVIATION...
PANC...VFR conditions are expected, although there is a small
chance of some MVFR conditions in showers Monday morning through
Monday afternoon. Some brief gusty south winds are expected late
this afternoon, then winds should be fairly light.

&&

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

The regional weather continues to remain dominated by the
amplified and slow moving upper level pattern. The closed low
near Kodiak Island continues to keep broad south-southeast flow
aloft and extensive coastal precipitation. Rather efficient
downslope drying is keeping most inland locations dry, but rather
cloudy and continued well below average. The warm front has moved
onshore and will quickly weaken, allowing for the pressure
gradient along the coast to relax. Gusty southeast winds along the
coast and down Turnagain Arm and Knik Arm will quickly weaken
overnight as the parent trough begins to move up Cook Inlet. While
low level forcing for ascent is rather weak, there should be
enough lift aloft along the leading edge of the shortwave trough
to support light rain/showers along the Kenai Peninsula and
northward into Anchorage and the Susitna Valley Monday morning
into the afternoon. Afternoon heating in the Susitna Valley and
along the Alaska Range/Talkeetna Mountains should be sufficient to
support isolated thunderstorms, as well.

Monday night, expect drying conditions as the upper low moves
northeast into the interior of Alaska, leaving clearing skies and
rather cool overnight temperatures. Tuesday looks to be the first
rather widespread dry day (with the exception of scattered showers
along the mountains surrounding the Copper Basin) with a slow
warming trend expected heading into mid week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The upper level low near Kodiak Island will continue to slowly
track north through southwest Alaska helping to produce isolated
thunderstorms over southwest Alaska this afternoon/evening and
Monday afternoon/evening. As the low pushes off to the north,
southwest Alaska will dry out Monday night into Tuesday as weak
ridging moves in.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Ridging over the central Bering will slowly track east making it
over the mainland Monday night. This ridging will stall the
western Bering low from progressing to the east with its front
only making it to the Eastern Aleutians on Tuesday. The low will
keep rain over the Western and Central Aleutians into next week.

&&

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

The long range forecast beginning Tuesday night through Thursday
is expected to trend toward dry conditions inland with periodic
breaks in the clouds as a transitional ridge pattern builds behind
the exiting upper low. Meanwhile, a weakly organized low over the
southern Bering tracks along the Aleutians to the Alaska
Peninsula bringing light rain as it moves through. Northerly flow
along the Bering on the backside of this low cools the air mass in
its wake.

At the end of the week, there is high uncertainty in how models
evolve the low pressure system moving near the Alaska Peninsula.
This is largely due to the models currently struggling with how
to handle energy that races ahead of the low center, rounding the
base of the trough near the eastern Gulf. Models typically
struggle with how fast they can break down a ridge, which in this
case, will be stretched along and east to west axis from the Yukon
territory to Southcentral. The inherited forecast was updated
with WPCs ensemble blend approach with a higher percentage of the
ECMWF used mid week as it had better consistency during the
latest runs.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...KH