Small Craft Advisory 507, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKDT Tue Jul 9 2019

.ANALYSIS AND UPPER LEVELS...

The upper level pattern continues to feature a trough-ridge
pattern over Southern Alaska. The ridge has been dominating the
weather over the mainland for several weeks now. The center of the
upper level high is over the Seward Peninsula, with the ridge
extending southwestward across much of Southern Alaska. This is
causing northeasterly upper level winds over the area. The trough
and associated vertically-stacked low is centered over Adak, with
associated shower activity ongoing across the central and western
Aleutians. The weakening ridging is allowing for isolated
thunderstorms to develop over the area mountains. Yesterday there
were a couple that formed over the Talkeetnas and the Alaska
Range, respectively.

Smoke from area wildfires has enveloped nearly all of Southern
Alaska this morning. A Dense Smoke Advisory remains in effect for
the Western Kenai Peninsula for the Swan Lake fire. Dense Smoke
Advisories have also been issued for the Lower Kuskokwim Valley
and the Kuskokwim Delta, with smoke from the Old Grouch Mountain,
Hurst Creek, and Foraker fires expected to reduce visibilities to
one quarter mile at times this morning.

&&

.MODEL DISCUSSION...

The models remain in very good synoptic agreement, with some
differences in the placement of the low approaching the central
Aleutians late Wednesday night into Thursday. However, since the
low is weak the difference in the weather regardless of model
choice is minimal. Forecast challenges abound in regards to the
local effects. Thunderstorm coverage and number remains a large
point of uncertainty over the coming days. As the atmosphere
destabilizes and the ridge weakens, the threat for storms will
increase, especially along the mountains and when any stronger
waves move through. Smoke density will also be another point of
uncertainty since hazy smoke now covers much of the area. Finally,
temperatures will be challenging, with sea breezes near the
coast, including Anchorage, likely playing a major role in how
warm temperatures get. Overall forecast confidence remains high,
especially as regards impactful weather.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist for the most
part. Smoke from the Swan Lake Fire will continue to be the only
weather concern, with the lowest visibilities expected in the
mornings, with periods of MVFR conditions.

&&

.FIRE WEATHER...

Southcentral Alaska... Starting with the good news: The record-
breaking heat that has been common across nearly all of
Southcentral is finally going to begin abating today. The upper
level ridge that has capped the atmosphere and prevented any
thunderstorms and associated cloud cover is weakening and moving
north and west away from Southern Alaska. Southerly onshore up-
Inlet windflow off the Gulf will also begin to cool coastal
locations, including Anchorage today. The cooler, marine air will
both lower temperatures and increase the humidity. Minimum
relative humidity values will also climb from the 20-40 percent
range today into the 50-60 percent range by Thursday.
Predominately southerly winds up the Inlet may not help air
quality concerns much in Anchorage and the Matanuska Valley with
the Swan Lake Fire, but overall smoke conditions will improve
since the wildfire smoke will move north into interior Alaska
instead of lingering in the skies above Southcentral.

Now the bad news: Besides air quality being unlikely to improve
much in Anchorage and the Mat-Su Valleys, thunderstorm potential
is expected to continue increasing compared with the weather
pattern the past few weeks. As usual, the greatest potential for
thunderstorms will be over the mountains, and the increasing
moisture will only add more fuel for any storms. While moisture
levels are high enough that most of the storms should produce rain
with them, cloud-to-ground lightning and associated gusty winds
can occur well away from the rainy part of the storm.

Southwest Alaska...The dominant upper level ridge is retreating
northward and weakening, and this will result in temperatures
trending back toward seasonal normals with increasing RH values.
Because of the weakening ridge, expect thunderstorms today to be a
little more widespread along and near the Alaska Range with some
isolated wet thunderstorms farther west along the
Kilbuck/Kuskokwim Mountains. Wednesday and Thursday, the flow
switches to onshore which will advect more moisture north into the
interior, so expect even more widespread shower/thunderstorm
activity and increasing chances for wetting rains. Smoke from
interior fires will be at their worst this morning, then lift and
move north the next couple days.

&&

.CLIMATE...

Numerous high temperature records were set across Southern Alaska
yet again yesterday. Here is a list of a number of them:

Location..................New Record...Old Record (Most Recent
Year)

Anchorage International...85 degrees...84 degrees (2003)
Anchorage Merrill Field...86 degrees...84 degrees (2003)
Kenai.....................83 degrees...82 degrees (2003)
Talkeetna.................91 degrees...89 degrees (2003)
Palmer....................84 degrees...84 degrees (2009)
Iliamna...................84 degrees...84 degrees (2003)
King Salmon...............83 degrees...82 degrees (2004)

Consecutive Days with daily record highs through July 8th:

Anchorage International...6 days
Anchorage Merrill Field...7 days (1915 records excluded as bad data)
Kenai.....................1 day
Talkeetna.................4 days
Palmer....................7 days
Iliamna...................5 days
King Salmon...............5 days

IMPORTANT NOTE: The thermometer at the airport in Bethel is
broken and its data are considered unreliable. Thus, all recent
temperature readings from there are not being included in any
official climate record. A technician will be going to fix the
thermometer there soon.

More climate records will be included in upcoming mornings for
Anchorage International.

&&

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

We are beginning to transition away from the extreme (for Alaska)
heat that the region has been experiencing the past week.
Temperatures over most of the area should be noticeably cooler
than yesterday and tomorrow even more so. Temperatures today would
normally be referred to as hot except compared to the past week
they will be cooler! This is all due to the ridge continuing to
weaken and move away form Southcentral Alaska. What this also
does is increase the potential for afternoon and evening
thunderstorms over the mountains. Today and tomorrow will see an
uptick these storms as the subsidence from the ridge is no longer
around to keep them at bay. However the warm air at the surface
and cooling temperatures aloft are making the atmosphere more
unstable.

The stratus is still hanging out at the southern end of Cook Inlet
and has been unable to make much headway up the Inlet due to the
warm airmass. This may change as the week progresses and the
advance of the marine layer up the Inlet could become an important
weather event for the Western Kenai Peninsula and perhaps as far
north as Anchorage.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)... The
dominant and strong closed blocking high which has been in place
across Southern Alaska for the last several days will finally
retreat north and weaken, allowing for a gradual downtrend in
daytime high temps. Thunderstorm activity will also increase,
first along the mountains today, then inland Wednesday and
Thursday, especially across the Lower Kuskokwim Valley and
interior mountains and into the Kuskokwim Delta. Uncertainty rises
both days as varying models solutions are depicting more
organized showers/thunderstorms moving farther west into the
Kuskokwim Delta in association with an "easterly wave", especially
Wednesday night. broad onshore flow will develop across coastal
regions as the interior low strengthens, so coastal areas will be
thunderstorm free (especially across Bristol Bay) as cooler air
moves north.

Wildfire smoke will be at its worst this morning, then it will
gradually improve as daytime heating commences and the steering
flow moves northward. Since the worst fires are northward, this
should allow dense smoke to end by midday today.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A broad and diffuse low over the western/central Bering Sea and
Aleutians will remain in place, keep cyclonic winds and
intermittent light rain over most of the Bering Sea west and south
of the Pribilof Islands. Sea stratus and fog will also be ever
present with the weather pattern remaining stagnant through
Thursday night as another low and associated front move in and
replace the current system. The quietest weather will be over the
eastern Bering where the main weather challenge will be fog and
stratus as winds will remain weakly onshore along the Southwest
mainland coast.

&&

.MARINE (Days 3 through 5)...
(Saturday through Monday)

Relatively benign conditions will persist across the Bering Sea,
Aleutian coastal waters, and the Gulf of Alaska through the
period. Look for sustained winds and wave heights to remain below
small craft conditions.

&&

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

The persistent weather pattern that's been in place for the better
part of two weeks now will undergo some modification for the
weekend into next week. This will occur as a low, currently
centered near Adak, sends a few ripples of energy eastward into
the Gulf. Meanwhile, another area of low pressure will reposition
to the Arctic coast of the Northwest Territories/Yukon area of
Canada. Several disturbances will dive southeast then east across
the northern third of the state.

The net effect is for the ridge of high pressure that's stretched
from the Gulf to eastern Siberia gets undercut by the Aleutians
low sending vort maxes (small areas of increased lift and
ascent...or essentially weak mid level areas of low pressure) into
the southern Gulf, while the Canadian low flattens the ridge from
the northeast as several disturbances round its base. This in
turn stretches the high pressure ridge axis somewhat, kind of
like a rubber band) as the Gulf high moves somewhat southeast,
with the Siberian high retrograding west some.

With this increased separation of distance between the two high
pressure centers, the thermal trough also weakens, with 850 mb
temperatures cooling some 6 to 10 degrees Celsius, with a
corresponding decrease in afternoon high temperatures likely for
all of southwest and southcentral Alaska. However, this will only
knock highs back into the lower to mid 70s, which is still well
above average for this time of year.

The other thing this separation does is create the likelihood of
increased convective coverage, even if just scattered in coverage
at best, given less subsidence and disturbances glancing the area
as the ridge the edges of the ridge of high pressure. These
disturbances in turn help cool the atmosphere aloft while
increasing wind speeds up higher up, helping to allow the air to
rise more freely. As a result, this could potentially open the
door for better thunderstorm coverage, and given the recent heat
dome thats entrenched the state, a few strong storms would be
possible.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Smoke Advisory 121 152 155.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
FIRE WEATHER...JPW & JA
CLIMATE...JPW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...PD

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
303 PM AKDT Tue Jul 9 2019

AKZ101-111-121-125-141-145-112315-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass,
Glennallen, Eureka, McCarthy, Paxson, Slana, Talkeetna, Willow,
and Cantwell
303 PM AKDT Tue Jul 9 2019

...ELEVATED WATER LEVELS ON RIVERS AND STREAMS ACROSS
SOUTHCENTRAL...

Many rivers across southcentral continue to be at or near bankfull
stages due to the warm weather and glacial melt.

The Kenai River below Skilak Lake is at minor flood stage with
water in yards and in one garage.

Elevated water levels persist for Eagle River. For those near or
attempting to cross Eagle River as part of the Crow Pass Trail, be
prepared for very high and swift water.

Along the Richardson Highway, the Klutina and Tazlina Rivers
remain near bankfull stages.

Water levels across the region should decrease with a change to
cooler temperatures over the next few days.