Small Craft Advisory 495, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKDT Wed Jun 24 2020

.ANALYSIS AND UPPER LEVELS...

A weakening upper level low was present near Kodiak this morning
continuing to move to the northeast slowly. As a result, radar
imagery was showing steady rain falling across the Prince William
Sound area including Cordova, Seward, Chenega, and Whittier.
Further to the west over the western Kenai Peninsula, some light
rain showers were observed. The predominant flow across the Kenai
Peninsula is cross barrier leading to a drying of the low levels
on the leeward side due to downsloping. Not much in the way of any
synoptic scale forcing is present from the Copper River Basin to
the Susitna Valley to the Lower Kuskokwim Valley as the region
remains in between weather systems. As a result, weak upper level
ridging was present over the region leading to a partly cloudy to
mostly cloudy sky.

A second upper level low was present to the south of the Aleutian
Chain that will be the main weather maker later in the week for
Southcentral. This will lead to continued unsettled weather for
the Gulf Coast and Kodiak Island. A frontal system was present
over the Kuskokwim Delta extending southward into the Bering Sea.
Over the Bering and the Aleutian Chain, subsidence under an upper
level high is keeping moisture trapped beneath an inversion. This
is leading to the typical widespread low stratus deck/patchy fog
areas with pockets of drizzle/sprinkles.

&&

.MODEL DISCUSSION...

Model guidance through day three continues to remain in really
good agreement on the synoptic scale features. The typical subtle
differences emerge with the timing and strength of the upper level
trough that moves through Southwest Thu night into Friday with the
NAM continuing to be the more progressive model and keeping an
open wave. The GFS is slower, closes the low off, and is further
south with this feature. The Canadian shows a more elongated feature
during this timeframe. The ECMWF is weaker as well, closer to the
GFS solution at this time. The exact timing, orientation, and
strength of this feature will affect the sensible weather at the
surface.

&&

.AVIATION...
PANC...VFR conditions are expected to persist throughout the
forecast package. Gusty southeast winds will diminish for a period
of time this morning before picking back up again Wednesday
afternoon.

&&

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

A cold core upper level low is centered over the southern tip of
the Kenai Peninsula this morning with an upper level trough
extending over the north Gulf coast. The combination of the trough
and cold air aloft is keeping showers across Southcentral this
morning. The atmosphere will be unstable enough this afternoon
for some thunderstorms over the western Kenai Peninsula as well as
the Susitna Valley and Copper River Basin.

This trough will move north and across the Alaska Range tonight
bringing in more stable air for Thursday which should keep any
thunderstorms from developing. Showers will still be possible
especially in the afternoon and evening closer to the Talkeetna
Mountains and Alaska Range but it should be a drier day overall
for Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Satellite shows the cold front is pushing into Southwest Alaska
as expected. It is bringing cloud cover and rain with it. Enough
instability exists along this boundary that isolated thunderstorms
are possible again today, this time shifting north to zones 155
and 152. Behind the front, we are still expecting cooler than
average temperatures. Temperatures will start to cool tonight,
with colder temperatures really settling in tomorrow night. Minimum
temperatures look to be around 40, with a few spots dipping into the
upper 30s. Highs are expected to be in the 50s. This cool, stable
airmass will remain in place into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

The Bering continues to remain generally quiet. The cold extending
south in the eastern Bering is now pushing into southwest Alaska,
with northerly flow in the Bering behind it. Ridging moves into the
central Bering Friday and sea stratus and/or fog can be expected
with it, as per usual. Otherwise, weak flow exists with no
significant weather systems in the Bering.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Gulf of Alaska: An eastern Gulf low moves inland for Saturday
morning. Confidence is good that coastal easterly small craft winds
and waves will subside. High pressure moves across the Gulf through
Sun. A new front enters the western Gulf Saturday afternoon with
good confidence that southerly winds and waves remain below small
craft levels.

Aleutians and Bering: An Arctic front swings over the Bering
through Sun. Confidence is good that small craft winds and waves
to 10 feet spread into the eastern Bering for Sunday. A North
Pacific low and front sweeps east to the south of the Aleutians
through Sun. Confidence is good for easterly small craft winds
and waves with low end gales possible Friday along the Aleutians
to become to northwesterly after the low passes.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

Models have come into somewhat better agreement in the
positioning of the long wave trough over the mainland and
timing/positioning of the front moving north toward the mainland
this weekend.

On Saturday, the upper longwave trough sets up broad
southwesterly flow over western Alaska. At the surface, a cold front
moving across the northeast Bering Sea pushes south as an occluded
front moves north across the eastern Bering Sea/Alaska Peninsula. At
that time there is still a weakening thermal trough over the
interior. So expect showery conditions over the southern Mainland
with rain over the eastern Bering Sea and Alaska Peninsula. By
Saturday night, the upper longwave moves east some and sets up broad
southwest flow across much of the mainland. This spreads widespread
cloud cover and increased chances of rain, associated with the
aforementioned fronts across much of the southern Mainland. Models
however differ on the timing of the rain.

Rain/showers look to hang around through Monday as the upper trough
lingers over the mainland. Models differ significantly of the
orientation and progression of the trough Tuesday so confidence
would be fairly low then. However, some shower activity is still
possible.

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

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BJB
MARINE/LONG TERM...MK/RMC

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
353 PM AKDT Wed Jun 24 2020

AKZ121-250100-
Western Kenai Peninsula-
Including the cities of Kenai, Soldotna, Homer,
and Cooper Landing
353 PM AKDT Wed Jun 24 2020

An area of strong thunderstorms, with dangerous cloud to ground
lightning and small hail, was located 19 miles east of Nikiski at
350 PM. These thunderstorms will continue to move to the west at
10 miles per hour through 5 PM.

Areas affected include Captain Cook State Recreation area.