Small Craft Advisory 547, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKDT Sat Jul 14 2018

.ANALYSIS AND UPPER LEVELS...
The jet stream is running parallel to the Aleutian Chain and
North Gulf Coast today. This pattern is shunting any deep
subtropical moisture just out of reach of southern Alaska. A jet
max is found moving into the Northern Panhandle with an associated
trough over the Copper River basin. Another, stronger jet max, is
moving into the Western Gulf of Alaska with an associated trough
over Bristol Bay. Stronger still is the digging 100kt jet exit
region over Eastern Russia seen heading for the Bering Strait.

A surface low and occluded front are sitting upon the northern
Gulf coast. Rain continues over Prince William Sound while showers
remain over the Cook Inlet region. Another shield of rain is
wrapping around a surface low moving near the AK Pen and being
aided by another surface low near Nunivak Island.

Low clouds and fog cover the southern Bering Sea as High pressure
tries to build in from the West following these disturbances.

&&

.MODEL DISCUSSION...
Models initialized well this morning, however, the evolution of
the low moving into the Gulf of Alaska by Sunday is trending away
from previous solutions. Expect changes from previous forecasts
to wind, precipitation, and cloud cover for the northern and
central Gulf and Prince William Sound.

Of additional interest is the phasing of the polar and subtropical
jet taking place Sunday into Monday creating a broad northwest
flow over western Alaska. With a progressive pattern in place,
this should allow a brief, but pleasant shortwave ridge to form
over the southern mainland midweek.

&&

.AVIATION...
VFR conditions will persist, but with breezy conditions off
Turnagain Arm expected to continue through the evening. Winds then
becoming light and variable into the early morning hours Sunday.
Some showers will move through the region Sunday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A broad trough over Southcentral will exit to the north tonight
while a weak, but strengthening low, south of Kodiak Island tracks
northeastward into the Gulf. After a brief break in steady rain
along the Gulf coast this afternoon, rain will spread northward
ahead of the low to the Kenai Peninsula and Prince William Sound
tonight and continue into Sunday. With the low on a more northerly
track than previous forecasts indicated, some light rain may also
spread inland to Anchorage and the Mat-Su. Much of the Copper
River Basin will also see some rain as the low passes to the south
on Sunday.

This storm will exit to the east on Sunday night as an upper level
short-wave dives southward across western Alaska then into the
Gulf on Monday. This will bring a weak trough across Southcentral
Monday afternoon through Tuesday. While instability will increase,
it looks like cloud cover will remain considerable through Monday
evening. Thus, it does not look favorable for thunderstorms and
have stuck with a forecast of scattered showers out ahead of the
trough.

Warmer and drier conditions are on tap for Tuesday through
Wednesday as an upper level ridge builds in from the west. This
does look like it will be short-lived as the upper level flow
pattern remains progressive.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The low near the Alaska Peninsula will continue to bring rain to
southwest Alaska this evening that will diminish through the
night. Weak ridging will move in Sunday morning out ahead of a
shortwave moving through the area from the northwest. This wave
will bring more rain to the area that will last into Monday
morning. Riding will then build in from the Bering and will clear
out precip through much of Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Ridging over the western Bering/Aleutians will track east through
the region clearing out most precip and bringing more cloud cover
as it passes. It looks to move over the eastern Bering Monday
night before pushing off further to the east. Behind the ridging a
system from the North Pacific will track in over the area bringing
rain with it. It will first impact the western Aleutians Sunday
evening and then make it to the eastern Aleutians Monday night.
Westerly, showery flow will follow behind this system.

&&

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

The run to run continuity is fairly good with the global models
this forecast period. For the third day in a row, the models
retain the idea of a progressive pattern. At the onset of the
period, Tuesday evening, the longwave trough is bisecting the Gulf
of Alaska and SE AK. Upstream there is a ridge axis over SW AK
and a new trough over the Central Bering. By Wednesday morning the
ridge axis will be east of the AK PEN over Kodiak, Cook Inlet and
north of the Alaska Range. At 500 mb both the GFS and the ECMWF
have the 570 DAM north of Anchorage which will translate into
numerous valley locations reaching 70F Wednesday afternoon which
will also keep a stiff southwest sea breeze moving up Cook Inlet.
By Thursday the ridge axis will shift eastward, bringing the
widespread subsidence and a warming trend to the Copper River
Basin. By Thursday, both the GFS and the ECMWF have a low north of
the AKPEN but the ECMWF is a little more aggressive with the
deepening of the low. The GFS and the ECMWF both depict an
expansive precipitation shield by Wednesday for the Western
portion of the state and the North Pacific, however the ECMWF is
slightly quicker bringing the QPF into Kodiak on Thursday. Its
also possible that the warm and dry weather may persist into
Thursday afternoon for the Anchorage Metro area and the Mat-Su
valleys.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...PS