Tuesday, January 19, 2016

Mid-Winter Fairbanks Update

The midpoint of meteorological winter season has just passed, so it's a good time to take a look at how the winter has turned out relative to normal so far.  To probably no one's surprise, it has been a warm one so far in Fairbanks, with the November 1 - January 15 mean temperature being the 7th highest on record (1930-present); but it's not as warm as last year, which was the 2nd warmest on record during the first half of winter.  Two years ago it turned very warm in the second half of January but there were several brief cold spells earlier in the winter (see charts below).

Temperature measurements from balloon soundings show that the warmth has become pronounced and persistent in the past few weeks, and in fact this morning's 850 mb temperature of -12°C was the first below-normal temperature at that level since the day after Christmas.  For 9 straight days around the turn of the year there was above-freezing air aloft in the Fairbanks sounding, which is almost unprecedented during December through February.  Only January 2014 had a longer spell of above-freezing air aloft in deep winter (10 straight days).

An interesting aspect of the vertical temperature profile so far this winter is that the warm anomaly has been quite shallow, with the average temperature anomaly dropping off quite rapidly with height (see the right panel in this year's chart above).  Since November 1 the mean temperature difference between 925mb and 700mb has been over 7.5°C, which is a record for the post-1991 period in which 925mb temperatures have been measured on every sounding.  The normal difference between these two levels is 4.8°C.

The rapid cooling with height is partly just a consequence of the fact that temperatures vary less in the middle and upper troposphere than they do at the surface and lower troposphere; so it is characteristic of a warm winter to have an enhanced vertical temperature gradient.  But there is more going on this winter, as the mean 500mb temperature has actually been below normal since November 1.  The reason for this is that 500mb heights have been lower than normal over the Bering Sea and western Alaska, and the trough axis (cold aloft) has been located not very far to the west.  However, at lower levels there has been strong warm advection from the south in response to low pressure in the Bering Sea.  This pattern of differential temperature advection (warm below, cold above) is what you get when there is a trough in close proximity to the west.

The maps below compare the normal height pattern to this winter's height pattern, for 500mb (top 2 maps) and 925mb (bottom 2 maps).
When it comes to precipitation and snowfall, it's been a tale of two winters so far in Fairbanks, with very snowy conditions in November but almost nothing since.  The 0.07" of liquid-equivalent precipitation and 1.7" of snowfall since December 1 is the 3rd lowest on record for this period, whereas November was the wettest since 1970.  The November precipitation was higher than the median for the first half of winter, so winter-to-date precipitation is still above normal; but this may not last much longer, as strong El Niño conditions are quite strongly linked to warm and dry conditions across interior Alaska in February and March.  The maps below show the temperature and precipitation patterns for late winter during the top 10 strongest El Niño episodes since 1950.

The chart below is an update of an earlier figure showing this winter's precipitation compared to the accumulated precipitation during 3 previous years with strong El Niño conditions in early winter.  In the 2 strongest El Niño's of the modern era, 1982-83 and 1997-98, Fairbanks precipitation saw a substantial deficit in the second half of winter.  It appears Fairbanks is already heading in the same direction this winter.

1 comment:

  1. In central Interior Alaska it's been windy aloft (http://aviationweather.gov/products/nws/winds/?area=alaska), with some mid-day mixing to the surface.

    Flying around the Tanana River valley this past few weeks I've noted a persistent flow from east to west in the upper valley to about Nenana, then a following SW vector towards McGrath supported by flow from the Yukon River to the north. The POES satellite infrared images can clearly show this when could cover is absent (http://pafg.arh.noaa.gov/poes.php).

    So what we get is a relative lack of a deep inversion and warmer temperatures at the surface. Soon the Sun's increasing angle and photoperiod will support this mild winter.