National Weather Service Weather Forecast Office

Got hot chocolate or Champurrado?

It’s a question that many Rio Grande Valley residents ask several times each winter, when temperatures plunge and relatively rare cold weather settles in. Typically, the region sees a day or two of chilly weather each winter followed by a quick return to more "normal" conditions...daytime temperatures around 70°F or so. These cool days are often accompanied by a brisk north wind, a steely gray sky, and occasional drizzle or light rain. Immediately after the calendar turned to 2013, brisk, chilly, and occasionally wet weather arrived in the Rio Grande Valley, and would continue through at least 4 or 5 days. While many welcomed the prolonged chill after a record warm 2012, others were ready for recovery back to the balmy temperatures that make Valley winters pleasant when compared with most of the Lower 48 states.

Given that some folks were asking "When is it going to warm up again?", we took a preliminary look at winters (defined climatologically as December through February, with November included) since December, 1999, to find out how often the Valley experiences prolonged cool/damp spells. We defined a ’spell’ as three consecutive days with temperatures below 60 degrees during the majority of the daylight hours. We emphasize daylight since, in a number of cases, the calendar day high temperature occurred between midnight and daybreak before the front arrived and temperatures plunged. The following table describes the cases for Brownsville/South Padre Airport, since November 1999.

*Daytime temperatures on Dec. 15, 2000 touched 60°F
†Precipitation that fell after front in cooler air.
‡Temperature after 12 PM (noon) on first day of period was below 60°F.

The Pattern
Each meteorological event has a unique atmospheric footprint. Each footprint can vary around a common theme, and some can be strikingly similar. The footprints that produce prolonged brisk, chilly, and damp weather for the Rio Grande Valley vary around a theme of what meteorologists term split flow. Split flow is simply two or more jet streams traversing North America, each bringing different elements to the weather mix. For the Valley, the two streams include a subtropical/tropical jet, which provides moisture that may originate over the tropical Pacific Ocean thousands of feet above the ground. This jet also helps lift lower level moisture from areas such as the southwestern Gulf of Moisture overtop of denser, colder air near the surface. The dense, colder air is provided by the second stream, known as the polar jet. When the polar jet "buckles", it can flow from north to south and bring air from frigid locations in Canada, Alaska, and even Siberia into the United States. Dense, cold air can sink toward the surface and spread, or ’ooze", deep into the United States and all the way to northern Mexico.

Examples of remarkably similar split flow patterns that caused prolonged periods of dreary chill are shown at the bottom of this article. The surface map (below) shows the less frequent, but predictable, result of this atmospheric footprint: High pressure with origins at high latitudes, a coastal trough of low pressure in the western Gulf of Mexico formed by land(colder)/sea(warmer) temperature contrasts, and enhanced lift provided by upper level disturbances moving east from the trough. As shown in the table above, every time this pattern sets up, warm, humid air above the surface overruns the shallow, colder air and produces clouds at periods of light rain or drizzle. Precipitation type is determined by the strength of the cold air and amount of lift; precipitation amount is determined by the strength of the trough, source region of the moisture through the entire atmosphere, and persistence and production of moisture in the source region. For example, heavier rainfall, as in December, 2009, may be helped by El Niño conditions which favor improved moisture production in the eastern tropical Pacific Ocean.