Wednesday, April 1, 2015

Thaw Days

First off, my apologies to regular readers for the extended absence - I've been out of commission for some time.  Spring has been making an early appearance across interior Alaska, with temperatures reaching 50 °F on 4 days in late March in Fairbanks.  This ties the record for most 50-plus days in March; the record was previously set in 1998.  For the month as a whole, it was the warmest March since 2005 with a +3.6 °F departure from normal, but it was nowhere near a record owing to the cold spell during the Iditarod.

The late winter warmth has taken a few inches off the snowpack in Fairbanks (now at 14 inches), but Keystone Ridge retains a healthy 25 inches, and the Little Chena Ridge SNOTEL reports 5.1" of snow water equivalent, which is very close to the 1981-2010 median for the date.  So while there is legitimate concern about early snowpack disappearance and wildfire danger in other southern portions of the state, the snowpack is relatively healthy in the vicinity of Fairbanks, especially in the higher elevations.

The chart below shows the history of thawing degree days (daily mean temperature above 32 °F) during March.  The long-term median is only 1, so this year the TDDs are above normal, but the situation is not particularly unusual.

Does the recent warmth mean that breakup will be earlier than usual this year?  Perhaps, but it's a small effect, because thawing is barely getting under way.  In 17 previous years with at least 8 TDDs in March, the Tanana River breakup date at Nenana averaged 2-3 days earlier than usual.  The eventual breakup date is affected much more strongly by weather conditions in April, and especially right around breakup, of course.

On another note, a reader recently asked about the rate of spring warm-up in Fairbanks versus other locations, and specifically why Fairbanks warms up so quickly.  Reader Gary provided helpful answers in comments and I agree with his suggestions.  However, it's interesting to note that on average the rate of spring warming is actually no greater in Fairbanks than in locations higher up the Tanana River valley, such as Tok and Northway.  In fact, because the higher elevation locations such as Northway are colder in winter, one could argue that they see more a more rapid warm-up in late winter.  Of course, in late spring the warming is curtailed at higher elevations because summer temperatures are cooler than in low-lying Fairbanks.  These differences are evident in the chart below.

The other town mentioned by the reader was Galena, which is certainly slower to warm up in March and April, and the main reason is the relative proximity to the west coast.  The northern Bering Sea is largely ice-covered in spring and provides a source of cool low-level air that delays the seasonal warm-up for locations in western Alaska; a relative abundance of cloud cover closer to the coast may also play a role.


  1. I bet Galena's proximity to the large Yukon River at times has a cooling effect. It's certainly a greater surface area than the Tanana River near Fairbanks, either when frozen or during periods of open water, and tends to thaw at a later date in the Spring.

    Until they warm, large bodies of water (lakes and rivers) tend to cool the air depending on the direction of flow (ask a boater where to cool off on a hot day).


  2. Good point, Gary. The location for which the NCDC normals are provided is only a few hundred feet from the river.

    1. Still, to be safe place another bet on more cloud cover in Galena versus Fairbanks. I've not spent much time in that area so can't speak from personal experience as to its climate.

      I lived in Tok in 1974 and worked for some time summers in the Northway area. As I recall it snowed in June and August that year, and flying/driving from there to Fairbanks usually resulted in warmer temps, but no hard data.

      The coldest summer locale I've encountered in Interior Alaska is the Denali Highway between Paxon and Denali Park. That includes the Brooks Range and North Slope exclusive of the coastal margins. Lots of downslope flow of cold air along that highway from nearby mountains and glaciers.