Small Craft Advisory 562, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKDT Sun Jul 22 2018

.ANALYSIS AND UPPER LEVELS...
The longwave pattern features a high amplitude ridge centered over
the eastern portion of the Gulf Of Alaska with the ridge axis
extending into the Yukon Territory. There is a negatively tilted
shortwave southwest of Kodiak associated with a frontal boundary
at the surface. Currently, there are some showers detected on the
radar at King Salmon. Over the Bering, the latest ASCAT/advanced
scatterometer pass has detected the remnants of a decaying low
over the Central Bering. There is already another new low just
east of Kamchatka

&&

.MODEL DISCUSSION...
Good run to run continuity with the models. Models remain in good
synoptic agreement through 72 hrs. The primary forecast challenge
at hand is again the progression of the marine layer over Cook
Inlet. There are a number of fine scale factors that influence the
development and the ebbing/flowing of the marine layer, and none
of the models have been resolving it particularly well. Expect the
marine layer kerfuffle for one more day/this morning. And then
the low that is southwest of Kodiak will move into the region and
mix the atmosphere, putting the kibosh on the marine layer
potential near Anchorage.

&&

.AVIATION...
There is a marine layer that is detectable via satellite imagery
over Cook Inlet. The current thinking is that the marine layer
will stay away from PANC. The progression of the stratus is
actively being monitored and updates/amendments will be made if
needed.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
After some fog and low stratus in a few areas this morning,
most of Southcentral can expect another warm and sunny day.
The upper level ridge responsible for this weather will continue
to shift slowly eastward, allowing for a high amplitude negatively
tilted trough to swing into the Gulf from the Bering Sea/Aleutians
today. A series of short-waves and surface fronts will then lift
northward across the western Gulf through Tuesday, providing
a cool rainy period for Kodiak Island and a trend toward cooler
ands cloudier conditions across all of Southcentral.

With deep southeast flow for much of the period, the northward
progression of these features into Southcentral will be slow.
The bulk of rain which spreads northward to Southcentral will
be confined to the southern to eastern Kenai Peninsula, with
downslope flow keeping most of the western Kenai Peninsula dry.
The surface high currently over the Gulf will shift to the north
Gulf coast as the surface fronts swing into the western Gulf,
leading to development of the typical gap winds on Monday and
Tuesday.

&&

.FIRE WEATHER...
The surface high over the Gulf will begin to shift north and
eastward today, with conditions becoming more favorable for
southerly winds along and near the Copper River. Winds won't be
real strong, but based on instability off the surface, think they
will reach critical thresholds. Thus, With temperatures and
humidity values expected to be similar to yesterday, have decided
to go ahead and issue a Red Flag Warning. The winds will be even
stronger on Monday, but there will be a slight cooling trend as
the ridge slides eastward. Still, there will not be much in the
way of low level moisture, so the winds could mix down drier air
from aloft and criteria could be met again. Will make a decision
by this afternoon on whether to extend the warning to Monday
afternoon/evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (through Wednesday morning)...
The mainland will be under the influence of a Bering Sea upper
trough through the short term forecast. A separate short-wave
trough will merge into the Bering system today as it comes over
the mainland, acting to increase easterly winds, mainly through
the gaps of the Aleutian Range. The disturbance will bring
steadier rain to the Wood River Mountains, and eastward toward
Dillingham. The lee-side of the Aleutians will remain downslope
dried, but a stray sprinkle or shower can not be ruled out. Rain
will be more showery for the Kuskokwim Delta, again with downslope
conditions closer to the mountains. A stronger disturbance rotates
through the trough on Monday, which will spread rain over the
Bristol Bay area as well as the Kuskokwim Valley while the
Kuskokwim Delta remains drier. The upper trough to the west
lingers through Wednesday, keeping cloudy and showery conditions
in the forecast. Temperatures will be near normal for most
locations. Winds will be generally out of the east at light to
moderate strength.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (through Wednesday
morning)... A low pressure system and front will keep the eastern
Bering Sea active with rain and rain showers for the next few
days. Winds will remain generally light across the area. For the
western Bering Sea/Aleutians, a good moisture stream and jet is
parked over the area, which will bring some decent rain from
Shemya through Atka today. Behind this front, a ridge will build
overhead, promoting a long period of low stratus and fog beginning
tonight.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...

Tuesday will feature a wave of moisture moving northwestward
around the upper level low centered near Sand Point moving into
mainland Southern Alaska. The heaviest rains will be along the
coast of the Kenai Peninsula, primarily staying south and west of
Prince William Sound until later in the day, and Kodiak. With
ongoing downsloping continuing through Turnagain Arm and the Knik
River Valley, most communities from Anchorage north should remain
dry until the evening hours. Tuesday night into Wednesday morning
will feature better chances for the precipitation to get into
Anchorage and the Matanuska Valley as a reinforcing wave of
moisture in the upper levels moves into Southcentral. Scattered
showers will be prevalent Tuesday and Wednesday across Southwest
Alaska, with the most favored locations for precipitation being
the east facing mountains. Precipitation chances will diminish the
farther north and west one goes as well. From midday onwards on
Wednesday, Anchorage and the Matanuska Valley should be drier
behind the wave as the downsloping winds continue.

Another round of rain moves into Southcentral, including Anchorage
and the Matanuska Valley on Wednesday Night. This wave moves north
into the Susitna Valley during the day Thursday, with scattered
showers continuing for much of the rest of the area. The passage
of the front will allow Kodiak to dry out relatively speaking as
the rain becomes much more intermittent there on Thursday.
Meanwhile that moisture will also spread westward into Southwest
Alaska during the day Thursday. Outside of some isolated showers,
Friday should be a generally drier day, though unfortunately still
mainly cloudy for most areas, as the upper level low and its
associated moisture retreats south and westward.

The models diverge greatly late Friday into Saturday area-wide,
due to different handling of a strong low and its associated
fronts moving into the western Bering on Thursday then pushing
eastward through the rest of the Bering and potentially Southwest
Alaska by Saturday. The models are generally agreeing on the
ridging over the Panhandle and western Canada slowly building back
to the west into the Copper River Basin late in the week. That
should help keep that area warmer and sunnier than the rest of
Southern Alaska this week with the exception of the front and
associated rain moving through there as well on Thursday. Breezy
conditions and relatively warmer temperatures may be cause for the
continuation of fire weather concerns Tuesday in the Copper River
Basin.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...JW