September 2012 Synoptic Summary

Statewide Extremes
Highest Temperature 79°F at Bolio in the southeastern Interior on the 14th
Lowest Temperature 12°F at Birch Creek in the eastern Interior on the 12th
Highest Average 54.9°F at Bolio in the southeastern Interior
Lowest Average 30.2°F at the Chigmit Mountains station, elevation 4,500 feet, west of Cook Inlet
Most Precipitation 55.39 inches at the McArthur Pass, elevation 1,300 feet, on the southern Kenai Peninsula

Stronger than normal high pressure aloft held over the northeast Pacific for a good part of September. This displaced the storm track – the path along which north Pacific low pressure systems move from the western north Pacific toward North America – northward over the Aleutians and the Gulf of Alaska...and at times over the mainland of Alaska. Although there were some periods of quiet weather during September, these came between storms, mainly during its first and third weeks.

Had some factors in this scene been typical, these storms would not have had a large impact. The first and unfortunate factor was the focus of the energy and rainfall coming with the storms on the state's most populous regions – particularly Anchorage and South Central Alaska. The other factor was the amount of rainfall and the strength of winds which accompanied the storms.

The storms delivered an excess of two ingredients: rain and wind. From a look at conventional weather maps alone, this would have seemed to be an unlikely outcome, but so it was.

First, the rain and the rivers. At the start of September, some rivers in the central north Interior will running high, mostly near bank full, but not above flood stage. There was some loss of shoreline on the Koyukuk River at Bettles. Ironically, one of the casualties of this was the loss of some of the river gauging apparatus.

On September 15, a north Pacific low pressure system moved northeastward and reached the Alaska Peninsula. It strengthened to 970 millibars (28.64 inches) and reached Kuskokwim Bay at mid day on September 16. Torrential rains fell in much of South Central Alaska for the next five days. Rivers from the coast of Prince William Sound, and nearly everywhere on the Kenai Peninsula, already running at relatively high stages, responded smartly, rising well above flood stage in a number of major river basins.

Both the large geographic extent and the magnitude of the flooding were remarkable, in many places more so than at any time since record keeping began 20 to 40 years prior. There was extensive water damage to riverside structures and roads. By dint of good preparation, and a certain amount of good luck, there were no serious injuries. A fast-acting rescue crew narrowly managed to pull a hapless kayaker out of the Eagle River, near Anchorage, on September 16.

The real impact came in the ensuing week. Flooding became extensive and serious from the Susitna Valley north of Anchorage over most of the Kenai Peninsula down to Seward. Landslides came down over some highways out of Seward. By September 20, the rain and high water had caused road closures north of Valdez, and on most of the Denali Highway, from McKinley Park east to Paxon.

The Susitna River flooded most of the town of Talkeetna on September 20. Part of the town was evacuated and a disaster declaration was issued for Talkeetna and Seward. The populace responded good naturedly. Newspaper pictures showed residents patiently strolling through floodwaters as they carried away possessions. One picture showed a retired sled dog resting on a windsurfing board as her owners gently pushed the board over floodwaters to safety.

The Susitna did more serious mischief On September 21, a 500-foot section of the Alaska Railroad was washed out north of Talkeetna. The roadbed was completely washed away, leaving the tracks and ties hanging like a hammock from one end of the washout to the other. Crews worked aggressively on a 24-hour schedule to repair it. The job was finished in a little less than a week.

The Kenai River was the scene of the most serious flooding in South Central Alaska during the fourth week of September. Hundreds of homes in the Kenai River basin had varying degrees of flooding. Rivers in the Matanuska and Susitna Valleys began to recede at the end of September. The floods on the Kenai River finally ended in the last week of September.

Consensus was that the high river discharge on rivers of the southern mainland hurt the fishing season. Fast water resisted upstream travel of fish, and then scooped up eggs in shallow spawning areas. The outcome varied considerably from one river to the next.

The heavy rain caused high water north of the Alaska Range too. On the weekend of September 22 and 23, the Nenana River north of McKinley Park reached record high stages. Parts of the town of Nenana, some of the employee housing at Mount McKinley Park and part of the town of Healy were flooded. Sections of the road from Fairbanks to Anchorage were partly washed away, but not ever closed

Just as the waters crested in the Nenana Canyon on September 22, seven experienced whitewater guides put in for a trip down the roughest sections of the rapids. Three of the rapids were conservatively rated Class V. One of the participants allowed that he was "a little nervous" as they headed into the Canyon. All of them emerged unhurt.

Then there were the wind storms. A North Pacific low pressure system moved along the Aleutians and reached Kuskokwim Bay on September 4. Although this system appeared to be of only medium strength on the maps, the direction of the wind on its leading perimeter was set in a narrow range of directions from which it could descend into Anchorage at maximum possible strength.

A cooperative station weather observer in Glen Alps, in the Chugach Mountain foothills east of Anchorage, reported a peak gust to 131 mph on the night of September 4 to 5. Countless large trees were blown down, and there was other wind damage to structures. The damage from this storm was augmented by two factors. First, the ground was still wet and soft from rains in August. Second, the summer's growth of leaves remained on the trees as the wind increased. Combined, these factors strengthened the wind's grip on the forest canopy and weakened the ground that held the trees.

At least 50,000 homes and businesses lost power as the storm hit. The Anchorage airport was closed until mid-day September 5. The absence of the city's lights and some clear spaces between clouds gave residents on unaccustomedly good look at the stars and night sky, and by coincidence, a fine aurora. Accounts of life during the storm noted that two commodities were in particular demand: electric power generators, to keep food from spoiling in freezers, and chainsaws for clearing trees from buildings, lawns and roads.

Some of the storm made it into the southeastern Interior around Delta Junction and Tok. There were downed trees and power outages there, but on a much smaller extent than around Anchorage. The strongest reported wind was 76 mph at Delta Junction.

By September 6, the weather system that caused the wind storm had traveled into the Beaufort Sea, about 300 miles northeast of Prudhoe Bay. Brisk west winds behind the low brought high surf to the Arctic coast east of Barrow on September 7 and 8. Half of the runway at Kaktovik was flooded by the sea on September 7.

The next important low pressure system, arriving in Kuskokwim Bay on the night of September 15 to 16, appeared to be very much like its predecessor. This was true to some extent. This was the same storm that was to bring heavy rain and major floods to South Central Alaska. There were strong winds again in Anchorage, but not as strong as the winds on the night of September 5 to 6. Since the first storm had already blown down most of the leaves from the trees and cleared the weak stands as well, this second blow had relatively little impact in South Central Alaska.

They weren't so lucky farther inland. On the night of September 16 to 17, a tightly focused and violent portion of the storm in South Central Alaska the night before reached the towns of Tanacross and Tok, in the southeastern Interior. Local residents estimated peak winds up to 100 mph at the height of the event, between midnight and 5 AM. The wind damage from this storm was considerably greater than that done by past storms in the area. The soil was relatively dry and firm as the wind increased. The trees taken down in the storm were healthy and strong; one was two feet in diameter. All of the blown down trees fell in nearly the same direction. About 20 percent of the forest was downed at Dry Creek.

There were some effects felt from this storm as far west as McKinley Park. On the evening of September 16, massive snow devils up to 1,000 feet high were seen blowing off from the mountains near the Parks Highway.

The thunder and lightning season in Alaska came to a gradual end in September. On the afternoon of September 9, thunder and lightning were observed around Fairbanks. One quarter inch hail fell southeast of town, covering the ground. A bystander said that it looked like winter. Temperatures moderated for awhile, and on the afternoon of September 23, an unusually late thunderstorm occurred near Fairbanks. Hours later, a cold front blew in from the west and light snow fell briefly. Snowfall at valley elevations was not significant anywhere in Alaska south of the Brookr Range.

The summer wildfire season briefly stirred to life on September 16. Strong winds developed over the Tanana flats at mid-day and caused the Dry Creek fire to rapidly flare. On that night, a large area of flames in the distance was visible from Fairbanks. The dark autumn night sky gave the flames greater relative brightness than they would have had in the twilight skies of the usual fire season. The 2012 fire season in Alaska was relatively small. A total of 260,288 acres burned, even less than the season total of 293,018 acres burnt in 2011.

Overall, temperatures in Alaska were above normal in the northeast, and below normal on the Bering Sea coast. This reflected the persistent westerly flow over Alaska during September. Onshore flow off the Bering Sea kept temperatures there cool, and when the wind had time to reach northeast Alaska, it had lost some moisture and warmed. Precipitation was from twice to three times normal in South Central Alaska and on the central Arctic plains. Only the southeastern Interior had notably below normal precipitation – less than 40 percent of normal in some places.

At sea, gales blew somewhere over Alaska coastal waters on twenty-six days in September. The strongest storm of the month was a 952 millibar (28.11 inches) low which moved northeastward through the Gulf of Alaska on September 26, and weakened just south of the Kenai Peninsula on September 27. The storm built up high seas throughout the Gulf of Alaska, but did little damage ashore.

Sea surface temperatures in Alaskan waters were close to normal in the Gulf of Alaska. Surface water temperatures in the eastern Bering Sea and Alaska Peninsula waters were from 2°F to 5°F cooler than normal. The extent of these cooler waters spread well south of the Aleutians over the western Gulf of Alaska at the end of September.

The total extent of sea ice coverage in the Arctic Ocean shrank to 1.3 million square miles – by far the smallest ice pack observed since records began in 1979, and smaller than the previous minimum ice cover by an area about the size of Texas. The entire Chukchi and Beaufort Seas were clear of ice by mid September, with the exception of a very persistent area of ice west of Barrow, which did not clear until September 25.