January 2015: A Tale of Two Seasons

One can use the phrase "A Tale of Two Seasons" to describe nearly any month in Oklahoma.  This phrase can especially be applicable during the winter months, and ironically, we wrote a post last year about January 2014 being "A Tale of Two Seasons."  But January 2015 was special in its own way.  For the first fourteen days of the month, perhaps you thought that this winter would be brutally cold and that spring could not get here quick enough.  After all, those fourteen days reported average temperatures which were generally well below average.  But then a pattern change occurred, and suddenly the average temperatures were above normal.  This lasted through the end of the month, including a stretch of three record-setting high temperature days at Oklahoma City during the last week in January.  This blog post will explain the temperature anomalies in January and why they occurred.

The graph to the right, courtesy of the National Weather Service in Norman, shows the daily average temperatures (highs and lows) in Oklahoma City in January and their departure from average.  Again, it may be stunning to see the abrupt pattern shift.  For the first fourteen days on the month, the average temperature was consistently below normal.  For the last seventeen days of the month, the average temperature was consistently above normal.  In the end, the warm temperatures overpowered the cold temperatures, and January for Oklahoma City finished with an average overall temperature of 40.4 degrees.  That was 1.2 degrees above average.  In Tulsa, the average overall temperature for January was 38.4 degrees.  That was 0.7 degrees above average.

Do you want to see more cool graphs?  Let's take a look at the ones here, showing the daily temperature data for each day of the month for Oklahoma City and Tulsa.  The blue bars denote the observed temperature range, with the top of the bar denoting the daily high temperature and bottom of the bar denoting the daily low temperature.  The brown "envelope" denotes the normal temperature range, while the record maximums and minimums are denoted by the red and blue lines, respectively.  Graphs like these are useful for showing the range in the daily high and low temperatures and their deviation from normal.  For example, in Oklahoma City on January 26, the morning low dropped down to 28 degrees (which was 1 degree below the normal low of 29 degrees), but the afternoon high soared to a record-shattering 77 degrees (which was 26 degrees above the normal high)!  Contrast this with just a few days later on January 31, when the high temperature was 45 degrees, and the low was 41 degrees.  This particular small temperature range to close out the month was caused by extensive cloud cover and rain showers which occurred all day and did not allow for surface heating during the day or cooling at night.

Spring fever?  Across a majority of the state, from January 26 through January 28, we were treated to unusually warm temperatures in the upper-70s and in a few cases, the low-to-mid-80s!  While cloud cover on Wednesday likely spoiled Oklahoma City's chances of reaching the 80-degree mark, Tulsa reached exactly 80 degrees on Wednesday afternoon, marking only the third time in recorded history (since 1905) that Tulsa has hit 80 degrees in January.  The last time was in 1911.  Note that highs this warm are more reminiscent of early-to-mid May, or late-September to early-October, not late-January.  All of this, of course, occurred at a time when the bulk of attention to weather was pointed towards the Northeast, where a major winter storm unfolded.  While this may sound ironic, a connection can be made with the unusually warm temperatures across the southern and central Plains and the winter blast across the Northeast.  We'll discuss this below, but for now, take a moment to look at the graphic above, which displays how impressive Oklahoma City's record-setting days were.  That last date for the old record is not a typo.  The old record high for January 28 was indeed in 1893, only three years after records began for Oklahoma City.  Pretty impressive to break a 122 year-old record, right?

So what caused the sharp pattern change?  Simply put, an upper-level trough was generally present over much of the country (especially the eastern and central United States) during the first half of the month, while an upper-level ridge was present over the central and western United States during the second half of the month.  During this time of the year, troughs fuel arctic air from Canada and allow cold air to surge southward.  The arctic air can be accompanied by strong north winds, especially after a cold front.  Ridges, which are more prominent in the summer and are responsible for the frequent triple-digit days, allow for warmer air at the surface.  From January 26 through 28, a strong ridge was present over the western and central United States, while a strong trough was present over the eastern United States.  The ridge led to unseasonably warm, record-setting temperatures over a majority of the central and southern Plains and parts of the west, and the trough, along with a prominent surface low over the Atlantic Ocean just off the East Coast, led to the major winter storm over the Northeast during the same time frame.

On January 26 and 27 in Oklahoma City, the temperature warmed rather rapidly, and perhaps shockingly, during the day from such cool morning temperatures.  As we made reference to earlier, the temperature spread on January 26 was an impressive 49 degrees.  A typical day may see a temperature spread between 20 and 30 degrees.  Giving an explanation for why the temperature warmed so rapidly means we need to turn to an atmospheric sounding.  We have shown a couple of these in previous blog posts to describe winter precipitation, more specifically the type of winter precipitation that may be falling (freezing rain, sleet, or snow).  We now use the sounding to explain a dramatic rise in temperatures during the day.  The first sounding to the right (you can click on the sounding to enlarge it) is from January 26 at 12 Z (6 AM CST).  All soundings pictured were launched from Norman, OK at the National Weather Center.  Notice how the red line (denoting temperature) was very nearly flat from the surface to approximately 970 mb.  This is called a temperature inversion, where the temperature increases with height in the atmosphere.  Since the line was nearly flat, the temperature increased rapidly over a small change in height.  Thus, while the surface temperature may have been cool, the temperature just above the surface was actually rather warm.  Since there was little to no cloud cover and relatively calm winds at the surface on the morning of the 26th, strong vertical mixing occurred, and the warm air (along with the stronger winds) from just above the surface mixed down to the surface, while the cooler air and lighter winds from the surface mixed up to 950 mb or so.  Mixing occurs best on days with no cloud cover and light surface winds.  A similar situation occurred on January 27 and 28, with January 27 probably demonstrating one of the most impressive inversions out of the three days.  Since the inversions were so shallow, it would have been possible to stand in the parking lot of the National Weather Center and observe one air temperature, then go to the roof of the National Weather Center and observe one that was at least 10 degrees warmer!

A recent cold front has dropped temperatures back to below-average levels.  Today's high in Oklahoma City was a chilly 39 degrees, which is 13 degrees below the average high for this time and 38 degrees below the daily high a week ago.  In Tulsa, it was even colder with a high of 33 degrees.  That's 17 degrees below the average high and also 38 degrees colder than the daily high a week ago.  We'll be on sort of a roller coaster ride of temperatures through the end of the week, but by the weekend and into the future, temperatures look to remain above average according to the Climate Prediction Center.  We'll have to see how February plays out.

AC

Sources: Google Images (story photo), NWS Norman (climate data and graphs), NWS Tulsa (climate data and graphs), SPC (soundings)