Small Craft Advisory 560, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKDT Sun Jul 30 2017

.ANALYSIS AND UPPER LEVELS...
Upper level pattern has the persistent high pressure centered
south of the Aleutians and protruding into the Bering Sea with
the low centered south of the Gulf. A weak trough extends
northward into the mainland part of the state causing weak upper
level flow. The jet stream is running through the northern Bering
Sea and turning sharply to the south and heading between the
ridge and the low. Instability remains today over the Copper River
Basin and northern Susitna Valley, extending into the Kuskokwim
Valley and northern Bristol Bay zone. Steering flow is light from
the east so showers and storms that develop will drift westward.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement through the middle of the
week. There is a difference in the weak low moving into the Gulf
in the mid-to-late week timeframe as well as a low in the eastern
Bering Sea at during the same period. These difference will likely
be resolved by the time we get closer and should not make a large
difference in the current forecasts but just some small refinements.

&&

.AVIATION...
PANC...VFR conditions and relatively light winds will persist
into Monday afternoon. Turnagain Arm winds will likely produce
gusty Southeast winds beginning mid to late afternoon on Monday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The thermal trough draped over Interior Alaska this evening will
provide for a chance of showers from Anchorage northward while
thunderstorms will be primarily to the higher elevations of the
Copper River Basin and Talkeetna Mountains. This is a result of
very weak storm motion over the mountains where the strongest
instability exists, which will help keep any lightning strikes
from moving into the valley's. The thermal trough will then shift
northward for Monday, limiting any thunder activity to the eastern
Alaska Range and Wrangell Mountains. Elsewhere southerly flow
will begin to bring cooler temperatures and increasing clouds that
will spread throughout the area into Tuesday. Most areas away
from the mountains should remain dry until Tuesday when a frontal
system approaching from the Gulf will bring widespread rains back
into the Gulf Coast while a separate frontal system moves through
the southwest. This setup will bring a chance of rain and cooler
temperatures back into Southcentral for mid-week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There is an area of instability over a corridor from Bristol Bay
through the Middle Kuskokwim Valley that is bringing showers and
the potential for thunderstorms that will last through this
evening. The front that has been tracking east across the Bering
will make it to the Kuskokwim Delta on Monday and then to the
rest of southwest Alaska Monday night. This front will bring more
widespread rain to the area that will linger through Tuesday.
However, rainfall intensity will be reduced on Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure remains over the Bering and Aleutians with a front
skirting the north side of the ridge riding east through the
northern Bering. This first front will make it to southwest Alaska
Monday with weak ridging filling in behind it. Another system
will approach the Western Aleutians Monday night from the North
Pacific bringing another push of rain and warm air to the islands.
This system will make it over the Central Aleutians and Bering on
Tuesday.

&&

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

In the long term a zonal pattern will maintain hold over the
region. There will be a weather pattern change, as the ridge that
currently dominates the Bering is replaced by an upper level
trough. This will result in more unstable weather across the
Bering as a low progresses from west to east. While this low will
try to push eastward into Southcentral by the middle of the week,
some models indicate that ridging will be potent enough to weaken
the low and associated front, and to keep it from moving too far
eastward. However, the ECMWF instead indicates a much stronger
low, and more favorable orientation for moderate rain in western
portions of Southcentral by mid-week.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...EN