Small Craft Advisory 593, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 PM AKDT Mon Jul 3 2023

.ANALYSIS AND UPPER LEVELS...

A broad upper-level low is beginning to move from the eastern
Bering to the Southwest coast this afternoon. A series of
shortwave troughs continues to rotate around this low and are
lifting northward over Southcentral Alaska. The first of these
has moved north from the Kenai Peninsula to the Susitna Valley
this morning and continues to move farther inland this afternoon.
The second of these waves is approaching the Kenai Peninsula this
afternoon and is resulting in the redevelopment of rain showers
across Cook Inlet north to the Susitna Valley. Persistent
southeasterly ahead of an occluded front is also driving
precipitation over the eastern half of Prince William Sound.

Meanwhile, a surface low sitting at the base of the trough is
weakening as it moves through Shelikof Strait with another area
of rain and showers for Kodiak Island north across the Alaska
Peninsula. The combination of some daytime heating and the
approach of the first wave into the interior of the state is
also resulting in a few thunderstorms across the middle Kuskokwim
Valley this afternoon.

Farther west, a second upper-level low is moving southeast over
the western Aleutians and is phasing with a vigorous shortwave
lifting toward the central Aleutians. At the surface, a deep
984 mb low is moving east from Amchitka Island. An occluded front
associated with the low is stretched across the Adak and Atka
with the occlusion bending around the back side of the low.
Widespread rain and gusty southeasterly are occurring over this
area. A secondary area of circulation is also seen on satellite
at the triple point of the occlusion and the warm and cold fronts
just south of Atka.

&&

.MODEL DISCUSSION...

Models continue to be in good agreement with the overall synoptic
pattern through the next several days. The biggest difference is
in the progression of the next upper-level trough as the pattern
becomes more amplified over the next 24 to 36 hours. The NAM, and
to a lesser extent the EC, continue to move the trough for
Wednesday a bit faster over the western half of Southcentral than
the GFS and GEM. Nonetheless, the pattern is such that more
unsettled weather will impact the Southern Mainland, the question
is the exact timing.

&&

.AVIATION...

PANC...The passage of the occluded front has caused low level
cross mountain flow (and resulting downslope component) to weaken.
Mid to upper level flow has also become more southerly. This opens
up the terminal up to lower clouds moving up Cook Inlet. While
expect low level VFR conditions to prevail, there a chance of
brief periods of MVFR ceilings. Additional rain showers are likely
this afternoon/evening as the primary upper level short-wave
approaches, but expect nothing like the intensity of rain observed
last night/early this morning.

The wind forecast is quite tricky. The Turnagain jet (and associated
southeasterly winds) will quickly weaken this evening while
southwesterly winds move all the way up Cook Inlet. The terminal
will be in the vicinity of where these two meet. Thus, the
direction could shift back and forth from SSE to SW, with
irregular wind gusts late this afternoon through about mid evening
(05Z). After 05Z, it does look like SSE winds will win out as a
surface trough lifts northward across the western Gulf. There
might be a brief break sometime tomorrow morning, but largely
expect gusty SE winds to persist all the way through Tuesday
evening - strongest Tuesday afternoon/evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Thursday)...

A front extending across Southcentral Alaska this afternoon will
lift northward tonight, with a strong upper-level shortwave trough
quick on its heels. As the front lifts north of Anchorage this
afternoon, expect the Turnagain Arm wind to bend north, leading to
a period of gusty southeasterly to southerly winds across the
Bowl which will then diminish through the evening. This northward
bend, induced by southerly, up-inlet flow is also expected to
cause an area of convergence where it meets the Turnagain Arm jet,
leading to a quasi-stationary band of rain showers which should
set up over the northern Cook Inlet, but may impinge on West
Anchorage, leading to accumulations up to another two tenths of an
inch. Elsewhere across the mainland, expect the lift provided by
the upper trough to allow for continued scattered rain showers
through the overnight hours. Rain tapers off for inland locations
by Tuesday morning as upper level support departs northward,
though showers may persist along favored upslope locations with
continued moist, southerly flow.

On Tuesday, the overall synoptic pattern amplifies as the trough
over the Bering digs into the north Pacific and ridging builds
over the central and eastern Gulf of Alaska. Subsidence behind the
aforementioned shortwave Monday night may allow for a period of
sunshine Tuesday morning before the next surge of upper level
moisture moves over the region. There is some uncertainty Tuesday
into Wednesday as to where the southerly moisture surge will set
up as the pattern amplifies and how quickly it will progress
eastward. Current guidance has the plume setting up over Kodiak
Island, the Aleutian/Western Alaska Ranges, and the western
Susitna Valley. The ridge is expected to move slowly east on
Wednesday and Thursday, so some of this rain will likely make it
into Southcentral, but the timing and magnitude remains uncertain.
Some guidance hints at an initial surge on Wednesday which may
lead to rain from the Kenai Peninsula, through Anchorage, and into
the Mat-Su Valleys. However there is more confidence that the main
trough axis shifts eastward on Thursday, leading to more
significant rainfall for these areas with a south to southwesterly
moisture fetch. Further east, the Copper River Basin will remain
more under the influence of the building ridge, which will allow
temperatures to climb into the upper 70s in the Copper Valley by
mid-week under mostly clear skies.

&&

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

An upper level low sitting off the Southwestern Alaska coast is
helping to bring showers across the area today. Lightning strikes
over the interior Middle Kuskokwim Valley occurred earlier this
afternoon and while this activity is moving north, any clearing
that would enable low level warming through this evening, could
develop additional thunderstorms toward the Western Alaska Range.
Given the position of the low, southeasterly flow across Southwest
will persist. As this trough lifts further inland, a front
extending from another low further west will follow soon after on
Tuesday into Wednesday. This next system has more energy than the
latter - ramping up southeast gap wind flow through Kamishak Bay
and the Alaska Peninsula. Relatively more subtropical moisture
will also be incorporated and widespread stratiform rainfall is
likely Tuesday and Wednesday. Given the southeast orientation of
the overall flow, downsloping on the lee of the mountain ranges
could inhibit chances for precipitation and QPF. Conversely, the
windward slopes will have higher rainfall totals. Winds begin to
gradually decrease by Wednesday night along with precipitation
coverage and QPF, but overall flow pattern will not change much as
the the low center catches up to the frontal passage this week.

&&

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

The Bering Sea has had an ongoing pattern of troughiness with a
couple of embedded lows incorporated into the complex. The less
vigorous system is lifting north into Southwest Alaska, while the
next system, bringing gales to the Western Bering and Aleutians,
is strengthening and occluding. As its front progresses east, gap
flow could become enhanced to gale force, especially as the front
approaches the eastern Aleutians and Alaska Peninsula. Plenty of
moisture is being tapped from the subtropical Pacific, so
expecting widespread rainfall. The pattern is not too unusual as
of late. Brief periods of high pressure, from the eastern
Aleutians and Alaska Peninsula to Saint Paul, are experiencing
fog. Fog could continue to be a possible issue at the Pribilofs
as they are in a bit of a col. They could see precipitation to
clear out some of the fog, but it will depend on the northward
extent of the front on Tuesday.

&&

.LONG TERM FORECAST (Days 4 though 7: Friday through Monday)...

The long-term continues to be dominated by a broad trough and its
associated low. Models are giving strong indications of southerly
and southeasterly gap flow winds persisting throughout the long-
term as each wave of energy passes through. However, model spread
grows in relation to where the exact track of the aforementioned
trough/low will be and their strength, impacting the precipitation
and wind forecast. Expect windy and wet conditions to persist due
to the trough and associated low becoming more negatively tilted,
but a brief break is possible Friday between rounds of precipitation
getting funneled in by the aforementioned trough. Overall, forecast
confidence is moderately low given the discrepancies on the low
placement/track and its strength.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...CQ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...VR
LONG TERM...NS