Small Craft Advisory 576, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Fri Jul 27 2018

.ANALYSIS AND UPPER LEVELS...
Water Vapor Satellite imagery shows a parent low spinning over
Bristol Bay early this morning with a ridge to the east centered
near the Dixon Entrance. On the backside of the low, a front is
powering across the central Aleutians. The low associated with the
front is moving southeastward across Kamchatka.

The low over Bristol Bay is currently a weakening mature cyclone;
therefore, the surface lacks any organized support. Short waves
rotating around the low core are bringing ongoing scattered
showers across the southern mainland and near shore waters.

The one important change in the current synoptic pattern is that
the nose of subtropical jet is sharpening up this morning. This
jet has been stationary between the Akpen low and the ridge to the
east and has been the responsible dynamic for pumping abundant
moisture to the southern mainland from the Gulf over the last few
days.

&&

.MODEL DISCUSSION...
Models are in good agreement with the general synoptic changes
through the upcoming weekend; however, there are slight differences
in the development and track of the Bering low which drives the
weather for the upcoming weekend. The NAM and NAMnest were the
main influences for this morning's package. The main challenges
through the weekend are strengthening winds around the front
moving up Friday night from the North Pacific and the coverage of
rain.

&&

.AVIATION...
PANC...VFR conditions are expected to prevail through Saturday
morning; although, brief lower ceilings are possible this morning
as lower clouds and fog have developed in the vicinity. Light
winds are expected to increase out of the southeast during the
afternoon and evening before tapering off overnight.

&&

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

The upper level low remains just west of Bristol Bay which is
keeping the flow over southcentral Alaska from a southerly
direction today. This southerly flow aloft will continue through
the weekend making the position of the jet stream and timing of
upper level waves the main drivers of the periods of rain.
A weak disturbance will move up Cook Inlet today bringing in some
showers with it but should move through the Susitna Valley by
late evening.

A jet streak can be seen on satellite imagery over the western
Gulf of Alaska and heading into Prince William Sound. This will be
the area that will see most of the persistent rainfall today. This
jet streak will also be over the Copper River Basin today which
will increase the chances for some light rainfall. However the
gusty southerly gap winds will develop again and that should
produce some downsloping which will limit the amount of
precipitation there.

Early Saturday morning the next wave will move over Kodiak Island
and continue to progress northward through the day which will
once again bring some rain/showers into the inland areas of
Southcentral.

&&

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

There has not been much change in the pattern in the past 24 hrs
with the longwave trough influencing the region. At the surface
there is a low west of Bristol Bay which is decaying. Broad
cyclonic flow from the low will continue to steer a plume of
moisture into the region. As time elapses, the low in the Bering
will push eastward and increase the pressure gradient over the
AKPEN, resulting in southeasterly gap winds Saturday and Sunday.
The moisture from the second system will be a multiple day event
with the moisture surge anticipated Sunday and Monday.

&&

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

To gage what is going on in the Bering, the best place to look is
at 500 mb where we see a low west of Bristol Bay, a low north of
Adak and a third low near Kamchatka. With each low there is a
contingent of shortwaves. Expect an active pattern for the Bering
and the Chain through Monday and beyond.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There is little change to the long term outlook today which
features a fairly simple and stagnant pattern over the region.
Beginning Sunday night, a large upper low will be situated over
most of the Bering while an upper ridge will be centered over
Southeast Alaska. This will promote broad southwest flow over most
of the southern mainland leading to a continuation of the
unsettled weather for next week. There are some model differences
in the ridge axis placement over southcentral which will
determine how far east precipitation will spread. The Euro and
GFS models slightly amplify the ridge axis west of Anchorage
Monday night through Tuesday, which would keep most of
southcentral dry. By Wednesday there is decent agreement that the
ridge slides east allowing a short wave trough to swing up from
the Gulf of Alaska into southcentral bringing another round of
rain to the area. The low center then moves east over the
southwest mainland as a ridge builds in over the western Bering
by Thursday bringing cooler temperatures and overall cloudy and
wet conditions to the mainland.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...KVP