Flooding in Alaska, August 12 to 14, 2021

Name: Flooding in Alaska, August 12 to 14, 2021
Start: 2021-08-12T18:00
End: 2021-08-14T13:00
Location: Alaska

August 12 to 14, 2021 · NWS AJK · 2 events in 2 counties and zones

A strong atmospheric river with sub-tropical moisture moved out of the north Pacific and over Southeast Alaska. This impacted the northern inner channels with 24 to 36 hours of heavy precipitation from a deep upper level trough with a long fetch transporting a significant amount of precipitable water over the complex terrain of the Alaska panhandle.

Where it happened

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Tropical storms and surge
Floods
Wind
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Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
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The toll

Deaths: 0
Injuries: 0
Property damage: $346,000
Crop damage: None recorded
Events: 2

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS AJK wrote it up.

This atmospheric river produced very high freezing level with very warm air going all the way up to 14,000 feet. The warm rain melted snow that was left from the previous winter in the mountains along with ice melt from area glaciers to increase runoff into the Taiya and Mendenhall River drainage basins. Rainfall amounts from August 13th to the 14th in the Taiya River basin ranged from 0.50 inches at sea level to over 1.30 inches in the headwaters with more than 4 inches over the a 5 day period. The Mendenhall River basin say precipitation values from 3.34 inches near sea level to as much as 5.0 inches in the headwaters. Satellite rainfall estimates indicated there was a lot more rainfall over the Juneau icefield to increase the amount of runoff into the Mendenhall Lake and River system.

There was moderate flooding around the Mendenhall Lake area from the morning hours on August 13th through the afternoon hours of the 14th which fooded multiple roads near the lake and the Mendenhall Lake campground. This was the 3rd highest peak in its period of record going back to 1966 and levels not seen since the glacier dam release from 2016.

There was moderate flooding around the Taiya River from the early evening on August 12th to early afternoon on August 14th which included the the Chilkoot trail and the campground near Deya. This is the second highest crest on record behind the record crest of 19.89 feet from 2002 when a moraine dam broke free along West Creek a tributary of the Taiya River and this was the highest crest seen since 2013.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Juneau, AK wrote its last area forecast discussion before the first event at Aug 12, 2:04 PM AKST, 3 hours 56 minutes earlier.

Area forecast discussion, Aug 12, 2:04 PM AKST, 3 hours 56 minutes before

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
304 PM AKDT Thu Aug 12 2021

.SHORT TERM...A robust atmospheric river (AR) directs an
incredible amount of moisture towards the Northern half of
Southeast Alaska over the next 24 to 36 hours. Rainfall began
falling over Yakutat Wednesday morning and has since greatly
increased in intensity with 4 to 6 inches having already fallen
in that area. The 12Z PAYA raob shows a top ten PW value with an
IVT of 2nd all- time (734 kg/m/s). Farther east, rain is falling
across the northern Panhandle, but at much more manageable rates.
All of this changes tonight, as a shortwave lunges into the
Bristol Bay region and begins to shove the AR eastward into the
northern Panhandle later this evening and tonight. Heavy rain
rates reach the eastern gulf coast and Icy Strait corridor later
this evening and then Juneau by tonight. Heaviest rain is expected
between 10 pm Thursday and 10 am Friday for much of the Northern
Panhandle.

Model indicated recurrence intervals have not been hugely
impressive, expecting rainfall totals for this AR to be around a
10 year recurrence interval. But what gives us pause is the
performance of some of the higher res models, notably the ARW over
Yakutat and the under forecasting of global and regional models
like the GFS/ECMWF/Canadian and NAM. If one examines the QPF
forecast related to the ten year return interval levels, then the
high res models collectively pick out much of the Icy Strait
Corridor including Juneau as an area to watch tonight into Friday
for potential flooding issues. Did not make big changes to the
overall QPF forecast with today's forecast package. Nudged the QPF
forecast with some of the higher res models to increase totals
slightly along western and SW facing slopes and decrease QPF
slightly across the rain shadowed areas in Upper Lynn Canal. Still
expecting many daily rainfall records to be broken for today and
tomorrow. Through Saturday morning, expect an additional 3 to 6
inches of rain to fall across the Northern Panhandle with the
highest amounts in upslope favored areas (Downtown Juneau,
Douglas, Pelican, Mendenhall Valley, along the mountains in
Sitka). Slightly lower totals in Upper Lynn Canal around 1 to 3
inches due to rain shadowing from SW flow aloft.

After coordinating with the RFC, A Flood Advisory has been issued
for Taiya River and Mendenhall River through Saturday afternoon
and the Situk River through Friday morning for rising river levels
reaching Minor Flood Stage. The threat for flooding continues
across much of the Northern Panhandle through Friday evening. The
risk for landslides still exists, especially during the peak
period of heavy rain from 10 pm Thursday through 10 am Friday.

Regarding winds, breezy conditions will continue for much of the
northern Panhandle with lighter winds dominating the south. Given
anticipated strengthening of gradients through Lynn Canal, kept wind
speeds to gale force through this evening. This goes well with
the strong wind gusts in Skagway and Haines through tonight. Gusts
of 50+ mph have already been observed in Skagway this afternoon
and should continue through the overnight hours. Expect another
enhancement of both land and marine winds early Friday morning as
another wave of low pressure pushes into the NE gulf Coast. Expect
small craft conditions to develop in much of the Central and
Northern Inner Channels, peaking late Friday morning then
diminishing Friday night. Due to such a stable atmosphere and WAA
aloft, it will be difficult for strong low level jet winds (45
kt) to mix down to the surface. Kept strong wind gusts of 40 mph
in Skagway as the gradient will remain tight up there; however do
not think other areas across the Northern Panhandle will see
winds that high Friday. Expect a wind shift from SE flow to SW
flow Friday evening as the main front pushes SE through the
region. A brief burst of stronger winds is likely for both land
and marine areas as the front passes through and winds shift
Friday evening. Expect winds to dramatically decrease late Friday
night through Saturday as the gradient and flow aloft weakens.

Overall confidence is high through the period.

.LONG TERM.../ Saturday to Thursday, as of 9 PM Wednesday /
A short wave aloft from the northern stream moves from western
Russia into the Yukon Delta and then into the northwest Gulf of
Alaska Saturday morning. This merges with the southern stream south
of the Aleutians, and the entire moderate long wave trough will
move across the panhandle Saturday evening.

There is a trailing low over the north Pacific that chases after
the out-going frontal band and moves into the British Columbia
coast. The northern edge of the moisture on this wave will be far
enough north to impact the weather of at least the southern
portion of the panhandle Saturday. Depending on any shifts in
tracks it could reach the south central panhandle.

A flat ridge over the western gulf will move slowly east across
the gulf followed by a short wave from a trough over the central
Aleutians. The trough exiting and the start of the ridge should
limit amount of shower activity for maybe 24 hours late Sunday to
Monday. The westerly flow that follows over the flat ridge into
southeast Alaska Panhandle will have rain spreading back in by
Tuesday.

Overall a continued damp weather pattern for SE Alaska, with no
large significant storms anticipated for the week.

&&

.HYDROLOGY...With around 4.5 inches of rainfall forecasted today
in Yakutat, we will have to watch the Situk. There are signs that
our forecasted amounts are lower than what will fall today (see
above), so flooding is still a risk here. Further east, Taiya has
fallen below action stage, but we fully expect its waters to rise
past flood stage sometime tonight or Friday. Should confidence
rise, a flood advisory or warning will be issued. Juneau area
rivers, while somewhat high, have not responded as much to light
rains lately. But again, given saturated conditions, and expected
heavy rainfall rates tonight, especially in the higher elevations,
we do expect Jordan, Montana, Mendenhall Lake among others to rise
quickly toward Friday. The Salmon at Gustavus needs to be watched
tonight as well. We have less concern for the Chilkat in Haines
and rivers around Sitka, as recurrence intervals are not as
impressive in these locations. But for now, the flood watch is
justified.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Areal Flood Advisory until 4 PM AKDT Saturday for AKZ018-025.
     Flood Watch through Friday evening for AKZ019>023.
     Strong Wind until 7 PM AKDT this evening for AKZ019.
     Strong Wind until 1 AM AKDT Friday for AKZ018.
     Strong Wind from Friday afternoon through late Friday night for
     AKZ018.
     Areal Flood Advisory until 10 AM AKDT Friday for AKZ017.
MARINE...Gale Warning for PKZ012.
     Small Craft Advisory for PKZ013-022-031>035-041>043-051>053.

&&

CM/Bezenek

Visit us at http://www.weather.gov/Juneau

The radar

The national radar mosaic for the moment the first event began.

Open the radar archive at this moment

Event by event

2 flood. Local standard time, as filed.

Flood, Taiya Inlet

When: August 12, 6:00 PM AKST to August 14, 5:00 AM AKST
Where: Taiya Inlet County, 1.75 mi NW of Skagway to 3.11 mi N of Skagway
Cause: Heavy Rain
Property damage: $345,000
Reported by: Official NWS Observations
Coordinates: 59.469, -135.332

The Taiya River was steadily rising through August 12th from persistent precipitation and ice melt from glaciers higher up in the headwaters and the river water levels went over minor flood stage of 16.5 feet around 1500AKDT. The precipitation continued through the evening hours of August 12th and the Taiya River went over moderate flood stage of 17 feet around 1900AKDT. The precipitation intensity increased through the early morning hours of August 13th and combined with the high magnitude of warm air associated with the atmospheric river the amount of runoff from glacier/snow melt elevated the rate of rise on the Taiya River significantly. The amount of precipitation and the additional runoff produced rapid river rises on the Taiya river through the morning hours of August 13th. The heavy rainfall begin to diminish and the river crested in the early afternoon on August 13th around 1430AKDT at a level of 19.19 feet before falling below flood stage through the morning hours of August 14th. There was significant damage to National Park infrustruce at Sheep Camp from flooding and from a large rock/landslide. Also, the Chilkoot trail was closed from significant flooding and areas around the campground and Deya were flooded.

Flood, Juneau Borough

When: August 13, 8:30 AM AKST to August 14, 1:00 PM AKST
Where: Juneau Borough County, 11.68 mi NW of Douglas to 12.03 mi NW of Douglas
Cause: Heavy Rain
Property damage: $740
Reported by: River/Stream Gage
Coordinates: 58.402, -134.581

The Mendenhall Lake rose sharply through early morning on August 13th and went above minor flood stage of 9.0 feet by 545AKDT. The precipitation intensity continued to increased through the late morning of August 13th and combined with the high magnitude of warm air associated with the atmospheric river the amount of runoff from glacier/snow melt elevated the rate of rise on Mendenhall Lake significantly. The lake rose rapidly through the morning to go over moderate flood stage of 10.0 feet by 945AKDT from. As the heavy rain taper off in the afternoon of the 13th the lake continue to rise but not at a steep rate from all the runoff in the mountains and water funneling through the Mendenhall Glacier and surrounding Juneau Icefield. The lake did not peak until the early morning hours around 545AKDT on August 14th at 11.55 feet and was not was not below flood stage until 1815AKDT.

The Mendenhall Lake campground was evacuated from significant flooding along with road closures of View drive and Skaters Cabin road. It should also be known that some locations along the Mendenhall River below the back loop bridge had significant amount of water in their yards.

What these are

Flood: Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in Alaska in 2021 Every state in 2021 This day in weather history Warnings in Alaska, August 2021

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 979384, 979439.