Termination dust waits for no one

When I moved to Alaska in 1982, I was a real cheechako. I did not even know what termination dust was. The term was never mentioned in meteorology school. It does not appear in the meteorology glossaries of the NWS, American Meteorological Society or the Weather Channel, nor is it in Wikipedia. Back then it was a bit of an initiation for new arrivals to figure out what the others were talking about. Here are a couple recent examples, taken Tuesday evening, August 28th. (I back-dated this post a few days to that date.)

Termination dust, the new snow on the mountains that signals the termination of summer, is a favorite weather chat for Alaskans. It is an especially “hot” topic when it comes earlier than usual, and late August is quite early to see it in Southeast Alaska, even in Haines, near the northern end. But this cold summer, termination dust has been spotted in early August and even in July around Anchorage, and I’m sure many other places and times. (Have you seen or, better yet, photographed any? Leave a report in the comments, or send me a photo.)

From a meteorological standpoint there is nothing much to termination dust besides precipitation and cold enough air to keep it in the solid phase long enough to be seen lying on a mountain. Since mountains in Alaska come in various sizes, and the larger ones have their heads and shoulders in subfreezing air virtually all the time,  there could be termination dust whenever. You just have to look at a big enough mountain. (By the way, snow stays frozen for quite a distance below the freezing level. For an explanation, see my post on What’s the snow level.) Of course, on the north coast of Alaska, snow commonly descends to sea level at any time of year. But there is some interesting meteorology behind this example.

Here in mild Southeast most storms that bring large amounts of precipitation ride a southerly or southwesterly flow aloft and the surface low brings most of the moisture and precipitation around its south side as well. This sort of fetch usually means warm advection, a surge of wet warmth pulled into the area by the low. Many times there is colder air following the warm air around the low (cold advection, deliniated by a cold front), but for southeast Alaska, that cold air has traveled far from its arctic source over the relatively warm North Pacific, thus is still mild by Alaska standards. That’s why the southern half or so of the panhandle is roughly as warm as Boston in the winter.

Offshore flow: warm in summer, cold in winter. What about fall?

On the north side of oceanic lows, especially with a distinct high over the interior, the air is usually drawn off the continent and onto the water…the reverse of what I described above. This is what gives northern SE Alaska more 80+ F (27+ C) summer days and more subzero (<-18 C) winter ones than the southern part.  It’s a taste of the interior’s extremes, blunted a bit, but with wind added as the air rushes down valleys or over ridges. In June, Haines had a couple days of strong offshore flow and the temperature hit 88 F (31 C). Other examples in September link offshore flow with warm, sunny days (well into the 70s F–low 20s C). Why this time heavy rain and a high of only 54 F (12 C) at 8 am that fell during the day to 46 F (8 C) in the afternoon? The difference is that this low came down from the northwest, fed and pushed by a pool of cold air aloft. Summer’s over. Here’s the surface map:

The state outline is hard to see if you are not used to it, so you might want to click on it for a larger version. The 1004 mb low in the eastern part of the map is the one of interest, at that time just off the coast of the northern panhandle, having arrived via Prince William Sound. A tongue of high pressure is helping to push cooler air from the interior though the mountains and onto the waters of Southeast. Northwest winds picked up early in the morning and blew strong all day at Haines, while at Skagway the usual onshore (southerly) winds hung on longer, gradually going calm in the afternoon then blew from the north at only around 10 knots into the evening. These differing winds show the tight nature of the low, as the two cities are only about 15 miles apart. Rainfall was heavier for Haines as well, around one inch (25 mm), or about twice that of Skagway. As the low slid southeast, Juneau got roughly half again more than Haines. Here’s the 500 mb map, giving the pressure pattern, winds and temperatures for roughly 18,000 feet (the contours are labeled in meters).

You can see the pocket of cold air, around -25 C, (-13 F) drawn by the computer over the western Yukon, and moving down from the northwest just behind the trough passing Yakutat. The sharp trough and the cold air aloft made for a dynamic system and heavy, if brief, precipitation, possibly with embedded convection. These factors are perfect for dropping the snow level well below the 5,900 ft (1,800 m) freezing level reported on the Yakutat balloon sounding. Clouds began to lift once the trough passed, allowing the new snow to be seen while fresh the same evening.

The next day the strong ridge over the west coast of Alaska moved over Southeast, quickly clearing out the skies and allowing the sun to bring the temperature back to  a very nice 72 F (22 C), aided by offshore winds (downslope winds warm the air). Meanwhile, in Alaska’s interior, the clearing skies and the cooler airmass combined to drop most areas into the  20s F (-3 to -5 C) overnight, with Eagle recording the cold spot in the state at 23 F (-5 C).

All-in-all a perfect setup for termination dust…just a little earlier than we are used to or perhaps were hoping for after such a cool summer. The next storm had the more usual southerly fetch and wiped out any termination dust still visible from town. Don’t worry… it will be back.