Summer finally arrives in Southcentral Alaska

Although the calendar reads mid June, Anchorage's weather so far this summer has been strikingly reminiscent of autumn. A chilly autumn. But Southcentral Alaska residents wishing for a blast of full-on summer should finally see their dreams come true this week.

Even though it's taken a while to arrive, it may be time to break out the shorts and sunglasses.

Summer didn't rush. March and April blessed us with many days of clear skies and normal or above-normal temperatures. May however was plagued with clouds transported northward from the Gulf of Alaska. Although rainfall in Anchorage was a little below normal for May, the monthly mean air temperature was 2.3 degrees below normal, substantial for this time of year.

Until this past weekend June weather has been a continuation of May, temperatures running well below normal with rain well above normal (0.93 inches so far this month in west Anchorage).

Saturday, however, was the first day of a break from the bleak, persistent onshore flow. Temperatures around Anchorage peaked in the mid-60s with abundant sunshine. Sunday was even warmer, peaking at 71 degrees at the National Weather Service office just south of the airport. Across town, temperatures soared into the mid-70s. That's the warmest temperature Anchorage has seen all year.

How unusual is that? Anchorage averages about four June days a year that hit or exceed 70 degrees, far fewer than Fairbanks, which averages 18 of them.

In July, by comparision, Anchorage averages seven days of 70 or above while Fairbanks averages a whopping 22 days that warm. Fairbanks' distance from the ocean is the biggest factor.


But for reasons not entirely clear, the frequency of 70-degree days has decreased in recent years. Perhaps more and more cloud cover transported over Cook Inlet from the Gulf of Alaska is contributing to cooler temperatures.

On Sunday, thunder cracked across East Anchorage during the late evening hours as some cumulus clouds formed over the Sustina Valley and moved over the Western Chugach Mountains, turning into thunderstorms.

Luckily, our sultry summer weather (by Alaska standards) should last a few days. The forecast through next weekend reflects the presence of an area of high pressure centered over the Bering Sea but extending into the western Gulf and most of Southcentral. The benefit to the Cook Inlet region is that this region of high pressure will produce weak flow from the Interior, meaning that the air near the ground is warmer compared to air moving from the Gulf.

The caveat however is that air in the middle atmosphere remains relatively cool, prompting the generation of large cumulus clouds in the afternoon and evening. When these clouds grow large in height they can produce hail and other ice crystals that frequently produce lightning.

Sunday was a more typical summer day, which may be repeated much of this week. Expect generally clear skies and rapid heating through the morning and early afternoon. The heating creates plumes of warm air called thermals, which rise thousands of feet above the ground. As air rises, it cools. If it cools enough, the moisture caught in the plume condenses into water droplets.

Continued ascent means even more cooling as some of the water droplets freeze. These ice crystals bump into each other and produce a separation of electrical charge within the cloud. Lighting forms when the charge from one part of the cloud is attracted to the opposite charge located in another part of the cloud or to the ground. The resulting thunder is the sound wave generated by the lighting as it vaporizes air molecules along its path.

Ingredients for thunderstorms include warm air near the ground, cooler air in the middle atmosphere (above 10,000 feet) and a modest supply of moisture.

Much of the moisture for these types of clouds is supplied locally, including evaporation of snow and surface water in the mountains. Mountain slopes also warm rapidly and generate the strongest thermals, hence large cumulus clouds and thunderstorms that typically form over the Chugach Mountains east of Anchorage and slowly move over the lowlands.

At the moment, the computer models indicate a return to a southerly flow late on Sunday June 23 -- heralding the return of substantial cloud cover and cooler air temperatures. In the meantime, enjoy.

John Papineau is a meteorologist who works at the Anchorage office of the National Weather Service.