Flooding in Alaska, August 21 to 25, 2019

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKDT Wed Aug 21 2019

.ANALYSIS AND UPPER LEVELS...

An upper level trough is departing Southcentral to the east over
the Copper River Basin this morning. Weak ridging and an
associated just streak is moving into the Gulf at the base of the
trough. An upper level shortwave trough is helping move the jet
streak into the eastern Gulf. Ridging is building over the
northern Bering, with a vertically stacked low located south of
the central Aleutians. This pattern with broad northerly flow over
Southern mainland Alaska continues to support dry conditions.
Scattered light rain that occurred over Kodiak yesterday is moving
off with the upper level jet streak and associated upper level
shortwave.

A front at the leading edge of the vertically stacked low south of
the Aleutians is producing the only consistent rainfall in
Southern Alaska. This is occurring from around Cold Bay westward
through Dutch Harbor. A weakening meso-low over the Susitna Valley
is moving wildfire smoke northward up the Parks Highway, and
southward down the western Susitna Valley. Temperatures across
much of Southern Alaska are dropping through the 40s for most
areas away from Anchorage and the coast, as skies clear out.

&&

.MODEL DISCUSSION...

Model agreement continues to be very good regarding the track of
the low south of the central Aleutians tracking east and
developing triple-point lows as it moves along. There is some
minor disagreement on how far north the rain shield goes, namely
as regards how much rain Kodiak gets out of this system. Outside
of Kodiak and maybe the immediate Gulf coast, the low track is
simply too far south to bring much-needed rains to Southcentral.

The next feature of interest is a passing upper level trough that
will dig into Southwest Alaska late Thursday night, and then pivot
eastward into Southcentral sometime during the day on Friday.
Minor timing issues persist, but this feature remains
Southcentral's best chance of precipitation in the short-term. The
models are very different in their depiction of how much
precipitation can be expected, but the theme even among the
wettest ones is to not expect very much out of it, but the
question of whether any one area sees any rain at all remains
unanswered.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. Smoke from the
Swan Lake and other area fires may briefly lower visibilities, and
will otherwise be in the area. However, between low confidence on
when that would happen and a generally unfavorable wind flow to
transport much thicker smoke into the area, it was left out of the
TAF. The Turnagain Arm wind is expected to develop south of the
airport tonight. It is not forecast to impact the terminal
directly, but may affect approach and departure routes. It may
eventually move into the airport Thursday afternoon, beyond the
current TAF period.

&&

.FIRE WEATHER (Southcentral)...

Fire weather concerns remain largely unchanged this morning.
Relative humidity values are expected to gradually increase over
the next few days. Temperatures are expected to remain in the
range of normal to a few degrees above normal for the next few
days as well. This translates to highs remaining in the 65 to 72
degree range, roughly, for most areas. There remains very little
chance of precipitation over any of the area fires, as any chance
of rain through Thursday remains over the mountains. Precipitation
chances increase a bit on Friday as a mostly dry cold front moves
over the area. This may be able to collect enough moisture for
isolated to scattered showers to develop, mainly Friday afternoon.
Unfortunately only small rainfall amounts of up to a few
hundredths of an inch in any showers can be expected.

Winds will be the primary concern in the short-term. Light winds
will persist over the area fires today, but this evening, the
winds will turn to east to southeast. The winds won't be
particularly strong, up to 20 mph through Turnagain Arm. Winds
through Kenai Lake and Cooper Landing may get to 10 mph with
higher gusts, which may impact fire conditions at the Swan Lake
fire. The easterly winds are expected to persist through the Kenai
Peninsula and Turnagain Arm through much of the day Thursday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...

Expect a dry day across Southcentral as weak ridging moves
overhead. Clouds associated with an upper low over the Interior
will affect mainly the far northern Susitna Valley and much of the
Copper River Basin. Otherwise, expect hazy sunshine as smoke from
the many ongoing wildfires continues to blanket the region. A
Dense Smoke Advisory is in effect for the Interior Kenai Peninsula
south to Seward. Light winds and stable conditions will keep
smoked from the Swan Lake Fire trapped near the surface. Would
expect some improvement in visibilities during peak heating this
afternoon and evening, though there was not a whole lot of
improvement yesterday. This will largely depend on fire activity
and how much new smoke is being generated.

A wind shift will ensue across the Kenai Peninsula this evening in
response to a front entering the southern Gulf. Low level winds
will shift from northwesterly to easterly and increasing overnight
tonight through Thursday. This should help clear out some of the
smoke on the eastern Kenai and push smoke westward across the
western Kenai and possibly back into Anchorage. A robust Turnagain
Arm wind will develop which should keep visibilities from dropping
very low. Also expect some localized gusty winds through the Knik
River Valley and along the Copper River, but these will be
relatively weak.

The front moving into the Gulf tonight will be followed by the
parent low moving in from the Aleutians on Thursday. This low will
track quickly to the eastern Gulf, with the bulk of rain
remaining over Kodiak Island and the Gulf. Thus, Thursday looks
like another dry day across Southcentral. The next chance of rain
will come Friday as a trough drops out of the northwest. While the
trough does not look very strong, cold air aloft will produce
enough instability to generate some showers, especially along the
mountains.

&&

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

Quiet weather is expected today...with generally clear skies and
behind the influence of an upper trough, temperatures will trend
several degrees warmer than yesterday in most of Southwest. A
shortwave trough will dive southward from the Chukchi Sea
Thursday, strengthening north flow and bringing cooler conditions
Thursday and Friday. While some precipitation is possible along
the associated front, this looks to be a fairly dry system and
it's likely that precipitation will be limited to the mountains.

&&

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

A triple point low developing roughly 200 miles south of Dutch
Harbor will continue to influence the eastern Aleutians today,
bringing precipitation and small craft advisory level winds and a
small swath of gales south of the AKPEN tonight as a barrier jet
develops against the Aleutian Range. This system will exit to the
east Thursday, and broad north flow will prevail over the Bering.
Expect widespread fog over the Bering by Friday as cold air with
Arctic origins passes over warmer waters.

&&

.FIRE WEATHER (Southwest)...

Several fires continue in the greater Bristol Bay area, including
the Levelock Fire and a fire near Port Alsworth. Smoke will
continue at the surface in the vicinity and downwind of active
fires. That said, based on satellite-derived fire temperature,
fires look to be less active than yesterday. Thus, expect smoke
to be less widespread than we've seen the past few days, and a
general haze over Southwest is more likely. In any case, a dry
cold front should bring some good mixing Friday.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

A low in the Gulf will produce easterly small craft winds and
possible low-end gales along the northern Gulf coast as it tracks
eastward through Friday. Behind the low, westerly winds return to
the Gulf through the weekend. Meanwhile out west, northerly small
craft winds are expected over the eastern Bering while a ridge of
high pressure persists over the western Bering.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

There is increasing confidence that the long term forecast
includes an active pattern over the southern Mainland while more
stable conditions will persist over the Bering. A longwave trough
in place over much of the state will allow several upper level
waves to propagate across the southern Mainland. Northerly flow is
expected along the western coast of the state through the weekend.
There is increasing confidence regarding the arrival of a North
Pacific low entering the western Gulf by Monday afternoon, however
models solutions continue to disagree on the strength and track
of this low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Smoke Advisory: 121 125.
MARINE...Gale Warning: 155.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...KO