The only portion of the interior that has stayed well below freezing is the far southeast, where the proximity of the upper ridge prevented winds from mixing down the warmth aloft; Northway has not exceeded 14°F.
Fairbanks airport reached 45°F on Wednesday, as the southerly chinook winds managed to break through the surface-based inversion for just a brief time in the afternoon; the temperature rose from 34°F at 1pm to 45°F at 2pm, and then dropped back to 30°F at 3pm and to 15°F by mid-evening as the stagnant colder air rolled back in. A more extended period of gusty warm winds yesterday afternoon took the temperature back up to 40°F with a bit of mixed rain and snow.
Wednesday's fluctuations in temperature and wind speed in Fairbanks are evident in the half-hourly observations from UAF's Smith Lake site, see below. The second temperature spike, with the larger wind speed spike, is the one that produced the high temperature at the airport, but at Smith Lake the temperature jump occurred between 12 and 12:30 (presumably AKST), i.e. at least 30 minutes earlier than at the airport. The elevation difference between the two sites is only about 100 feet, but this might explain the delay in warming at the airport as the wind burst took time to mix down.
The relative humidity plot from Smith Lake shows the low humidity of the chinook air - see below. The humidity fluctuations nicely highlight the contrast between the two air masses, one of them cold, humid, dense, and stagnant, and the other warm, dry, less dense, and moving quickly northward. The density contrast between two such air masses is so great that they might be thought of as immiscible, like oil and water: they simply do not mix. The warm air tends to slide over the cold air, and in the Fairbanks area the chinook flow doesn't often make it to the valley floor unless the pressure gradient is large or winds become unusually strong.
Similar charts from the Poker Flat Research Range, about 30 miles north of Fairbanks at ~700' elevation on the Chatanika River, show that temperatures were above freezing for two lengthy periods on Wednesday - see below. The early afternoon spike in wind speed was also observed at this location (note that the time axis appears to be UTC), but the temperature rise was more gradual as there was less stagnant, stable cold air to displace at the higher elevation.
Good analysis Richard, especially the contrast in mixing as a function of density. There's lots of sources of humidity in our urban zone...some man-made and the rest probably due to sublimation of the snow. The degree of air pollution (http://co.fairbanks.ak.us/airquality/) is an indicator of the local ground bubble density caused by inversion.ReplyDelete
In contrast to Fairbanks, have a look at Blair Lakes' WX station sitting in the valley to the south. It's exposed to and reflects quickly any changes in wind, temp, or humidity. It takes some time for that to be measured (if ever) in town. It's one of our first weather canaries:
Here's a better link for the Blair Lakes RAWS:Delete
Note from the map it's located in relatively flat terrain about +350' above Fairbanks adjacent to facilities used to monitor military firing exercises.
As the cloud shield moved in from the south today the Tanana Jet's winds and temperature rose and the humidity dropped as Richard describes.
The warmth aloft is impressive. Even over the Yukon and further east over the Canadian Shield, it's very widespread.ReplyDelete
As long as the ridge over NW Canada remains stationary Alaska will be warm. When it moves SW with no forcing influx from the oceanic south/southwest then cold coming with clear skies and widespread isobars.ReplyDelete
I agree, and unfortunately for me that means being trapped in Juneau due to fog and no flight home to Fairbanks. Gotta love Alaska inversions in winter.Delete
I can't believe I just now found your blog. I gearing up to do my Nenana Ice study for the year. The surface of the ice at Nenana looks a bit slushy.ReplyDelete
Welcome! I'd be glad to learn more about your study - have you done it for many years? Judging from the webcam, I wouldn't want to be out there this year.Delete