Small Craft Advisory 427, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKDT Tue Jun 4 2019

.ANALYSIS AND UPPER LEVELS...
A closed upper level is centered near Nunivak Island and continues
to retrograde westward into the Bering Sea. Ridging does extend
over Southwest Alaska today, which is helping to strengthen the
low level thermal trough and increase the level of instability.
Unlike yesterday, most of Southwest Alaska cleared out this
morning, with the exception of a portion of the Bristol Bay area.
Full sunshine will help to better realize potential instability
and lead to an increase in thunderstorm activity this afternoon
and evening.

To the east of the upper high, a trough extends across the eastern
portion of mainland Alaska, including Southcentral Alaska. A
well-defined trough axis is noted in satellite imagery from the
Copper River Basin southwestward to the Kenai Peninsula. Showers
have formed right out ahead of this axis. Elsewhere, sunny skies
prevail across most of the lowlands. As with Southwest Alaska,
the daytime heating will result in increasing showers and
thunderstorms as we head through the afternoon and evening hours -
similar to what occurred yesterday.

Out west, high pressure and stable conditions is leading to
widespread low clouds and areas of fog across the Bering Sea and
Aleutians. A weak low lifting up the west side of the upper ridge
is bring rain and a little bit of wind to the western Aleutians.

&&

.MODEL DISCUSSION...
Continue to see minor differences in details of upper trough
settling over Southcentral next few days, as numerous waves move
through the trough. However, overall models are in good agreement
with the large scale pattern. As has been the case for the last
couple days, the main forecast challenge is details of diurnal
convection as atmospheric conditions undergo subtle changes from
day to day.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. There will
once again be convection in the Mat-Su and Anchorage areas, but
expect the vicinity of Anchorage will remain mostly (if not
entirely) dry. The bulk of showers and thunderstorms will set
up late this afternoon and evening in a line from the Talkeetna
Mountains southward across the Matanuska Valley and continuing
down the front range of the Chugach Mountains. This would put
storms 10 miles or greater to the east of the airfield.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main challenges in the short range forecast are defining
coverage of thunderstorms across across the region and intensity
of storms expected to develop each afternoon and evening through
Thursday. For today, the highest certainty for thunderstorms is
across the Susitna and Copper River Valleys along a thermal ridge
that settles across from the interior. An east to west boundary
across these locations provides a broad area for storms to
initiate. A significant disturbance that rotates southward from
the low across the Alcan border will bring scattered thunderstorms
around the Talkeetna Mountains.

A persistence pattern can be expected through Thursday as the
upper low over the Alcan slow drifts southward keeping chances for
afternoon showers and thunderstorms in the forecast and benign
weather across the Gulf with a ridge extending from the North
Pacific.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
With the thermal trough continuing to strengthen across Southwest
Alaska and significantly clearer skies as compared to yesterday,
conditions look more favorable for more abundant thunderstorm
development today. Storm motion from the N-NE to S-SW will
continue to spread the scattered thunderstorms that have formed
over the Kuskokwim Valley this afternoon, south into the Greater
Bristol Bay area through the evening. These thunderstorms have the
potential to produce frequent lightning, small hail and brief
heavy downpours.

The persistent upper level ridging over the eastern Bering will
shift slowly to the southeast over the next several days, with
the associated surface ridge strengthening and moving eastward
towards the coast. As the upper ridge starts to build inland it
will push the area most favorable for thunderstorm development
further to to the east for Wednesday. On Thursday, the thermal
trough begins to weaken noticeably and shifts east over the
Western Alaska Range. As the ridge over the Bering moves eastward
as well conditions will start to stabilize from west to east with
convective activity mainly confined to the western slopes of the
Alaska Range for Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure situated over the Bering will keep conditions rather
calm and quiet across the central and eastern Bering and
Aleutians over the next few days. This persistent pattern will
continue to bring stratus across the Bering through Thursday
afternoon. The western Aleutians are experiencing a more somewhat
more active pattern. A front stretched out across Adak and the
western Bering will retreat westward through Thursday afternoon. A
North Pacific low south of the Aleutian Chain will lift northward
Wednesday afternoon and bring slightly stronger winds to the
central Aleutians marine zones.

&&

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

Quiet seas continue for all Marine areas due to high pressure.
Forecast confidence remains high.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday through Tuesday)...

High pressure continues into the longterm, meaning more of the
same weather that we have been seeing. The low pressure system
forecasted to enter the Gulf of Alaska has slipped back in time
and is now being shown to enter the gulf early next week. There is
also considerable difference between the major models in this
system, with the GFS and ECMWF being markedly different. We chose
to go with the GFS as it seems the more realistic solution.

In the past couple days, there was increasing confidence in the
movement of this low. However, model guidance has now actually
diverged more and gotten worse. Therefore, confidence is low of
what is going to happen to start next week. It actually appears
the weather we currently have may set up a bit of a blocking pattern
and keep our current weather into early next week. This is not
surprising, as models do have tendency to move high pressure out
of a given area too quickly. Right now it's really a coin flip
(between the GFS and ECMWF) as to what will happen in the long term.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH/JR
MARINE/LONG TERM...BB