Small Craft Advisory 538, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKDT Mon Jul 13 2020

.ANALYSIS AND UPPER LEVELS...

An upper level low over southwest Alaska is tracking eastward toward
the Gulf. A second upper level low is positioned south of the
Aleutian Chain and slowly moving toward the southeast. Farther west,
a ridge of high pressure over the Bering continues to keep a broad
area of marine stratus and fog in place.

&&

.MODEL DISCUSSION...

Model solutions are generally in good agreement, and there has
been improving agreement regarding the track and placement of the
low in the Gulf on Tuesday. Farther west, there is disagreement on
the placement of the low center over the western Aleutians Tuesday
night into Wednesday however all solutions do indicate a slight
strengthening of this system as it nears the Chain midweek.
Overall, the NAM solution was preferred for the morning forecast
package.

&&

.AVIATION...
PANC...

A brief period of gusty southeasterly winds returns to the
terminal this afternoon before diminishing by the evening. Expect
VFR conditions.

&&

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

The primary weather feature affecting the area over the next
couple days is a low that is developing over the western Gulf. This
low will affect a pattern change across south central Alaska, with
the help of a favorable jet stream. Over the past week, nearly every
day featured a good chance of thunderstorms over the mountains, as
an unstable air mass persisted across the area. This morning, all of
that has changed as a stable marine air mass is now entrenched over
south central Alaska. That said, the stagnant air mass won't be
nearly as conducive for rain shower activity today as all the lift
will be with the low in the Gulf. Thus, while most areas will
struggle to get rid of the clouds for a good portion of the day,
chances of rain for inland locations will be substantially lower
today, with no thunderstorms expected anywhere in the area.

Meanwhile, the low in the Gulf is increasing chances of rain for
coastal locations today, as southeasterly flow hits the mountains.
The low is likely to strengthen enough to cause gale force winds
to develop north and west of the low center this afternoon into
tonight. The northerly winds behind the low will begin the
process of scouring out the stable air mass over Southcentral, as
the offshore winds begin drying out the area. Thus, look for
decreasing cloudiness as the day goes on. While the low is
causing easterly winds into the coast today, the Turnagain Arm and
Knik River Valley winds are expected to redevelop this afternoon
and evening. These winds are not expected to be quite as strong as
previous days, and only the southern and western most portions of
Anchorage could see breezy conditions later today. As the low
consolidates and tracks east tonight, the gap winds will rapidly
weaken.

On Tuesday, south central Alaska will enjoy the results of building
upper level ridging over the entire area. Northwesterly offshore
winds will cause the gap winds to shift to Resurrection Bay and
Passage Canal, where winds of 20 to 30 mph with higher gusts are
expected.
A gorgeous, sunny day is expected for nearly all of south central
Alaska as the drying out of the atmosphere continues. The ridging
will allow the air mass to warm quite a bit as well. Thus, 70s for
highs will be the norm for most everyone, with upper 70s possible in
the Susitna Valley. As far as thunderstorms go, the atmosphere
should be sufficiently dried out already. This, coupled with
continuing downslope northerly flow over the Alaska Range, will lead
to all except a few isolated areas of the mountains staying
thunderstorm free again on Tuesday, though typical mountain showers
in the afternoon are still possible.

Warm, dry, and breezy conditions are likely across portions of the
Copper River Basin Tuesday afternoon. Northerly winds out of the
Alaska Range will lower the afternoon relative humidity values
down to the 25 to 35 percent range with winds of 10 to 20 mph
expected.

&&

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

An upper level ridge is expected to nudge its way into the region
beginning today and will remain across southwest Alaska throughout
the first half of the new work week. Since the flow is off the
Bering, especially for the Bethel to Kipnuk to Hooper Bay area, this
is bringing the marine influence from the Bering onshore. This is
being observed this morning especially in Hooper Bay where fog has
been present a majority of the night. As a result, temperatures were
brought down a touch due to the marine influence expected across the
aforementioned areas, especially the coastal areas. Further east,
for Aniak and Sleetmute down to Bristol Bay, the flow will mainly be
downsloping off the mountains which is a drying flow. This is
anticipated to lead to mostly sunny to partly cloudy skies
especially for Tuesday and Wednesday this week.

One other thing to note is as the upper level ridge builds into
the region, some weak upper level waves in the flow may touch off
an isolated thunderstorm this afternoon into this evening.
However, the instability indices are marginal at best with the
most likely scenario being just the run of the mill pulse type
downpours that form with one maybe tapping into just enough
instability to produce a lightning strike. The best chance for
thunderstorms will be across the Kuskokwim Mountains, including Aniak
over to Sleetmute.

&&

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

An upper level ridge and corresponding surface high pressure
remains over much of the Bering through Tuesday afternoon. This
will continue the low stratus/patchy fog in areas. An upper level
low with its associated surface low will continue to slowly
meander eastward along the central to eastern Aleutian Chain
today through tonight. As a result, periods of rain showers are
expected especially along the south facing slopes of the central
and eastern Aleutian Chain. Dutch Harbor will be on the drier
side of this system as the flow through a good depth of the
atmosphere is from the southeast which will create some cross
barrier flow. However, even with this flow a few showers cannot be
ruled out this afternoon into this evening. A brief respite is
expected across Nikolski and Dutch Harbor as some weak upper level
ridging builds over the region Tuesday morning through early
Wednesday. However, the low stratus is expected to linger as moisture
remains trapped beneath a subsidence inversion.

Another low and its associated front is expected to impact the
Aleutian Chain beginning Tuesday morning. As a result, small craft
advisory level winds are expected between Shemya and Amchitka
between Tuesday afternoon and early Wednesday morning. Periods of
rain will also accompany the front affecting the western Aleutian
Chain first (including Adak and Amchitka) before pushing eastward
reaching Dutch Harbor by Wednesday morning. This low will
progress slowly eastward as the week progresses.

&&

.MARINE (Days 3-5: Wednesday through Friday)...

Models are in better agreement today with a low lifting over the
central Aleutians by Wednesday afternoon, but some uncertainty
still remains with the strength of this low. This system will
drift southward on Friday, with some models trying to bring high
pressure back into the Bering as this system weakens. The next
North Pacific low approaches the region late Friday into Saturday,
and models continue to have a difficult time resolving the
strength and placement of this system.

Over the Gulf, winds will shift on Wednesday once a low pressure
system moves into the eastern Gulf. Models diverge a bit on
Thursday in whether upper level ridging tries to build in, which
could bring strong winds through Shelikof Strait and around Kodiak
Island through Friday.

&&

.LONG TERM FORECAST (Days 4-7: Thursday through Sunday)...

An upper level ridge over the eastern Bering and southwest Alaska
continues to move into south central Alaska spreading farther
eastward through the middle of the week. This is a favorable pattern
that will allow for low stratus to move into southwest Alaska under
the high pressure, keeping the area relatively dry through mid-week,
aside from afternoon showers over higher terrain. Chances for
precipitation increase across the region towards the end of the week
as an upper level wave begins move southward from northern interior
Alaska.

The weekend becomes a bit more uncertain as a front associated
with a North Pacific low begins to move over the Alaska Peninsula
and spread farther northward across southwest and south central
Alaska through Sunday. Models are struggling on the placement and
timing of the associated low center as it moves across the eastern
Bering and into the mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 141 (Kennicott River).
MARINE...Gales...119, 120, 131, 351.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...AH