Rainfall departures from average, nationwide, November 7 to December 6, 2009 (click to enlarge) Rainfall departures from average, nationwide, November 7 to December 6, 2009 (click to enlarge)
El Niño Makes Presence Felt across RGV
November Value nearing "Strong" is Followed by Cool, Damp Weather

As November turned to December, the force of a late autumn and early winter El Niño began impacting the weather across the southern third of the United States. November's El Niño Southern Oscillation (ENSO) index of 1.7°C moved the three–month running mean (September to November) into the Moderate range – now 1.2°C for the three month period. The atmospheric pattern that one expects during such a situation, not factoring in other teleconnections, was underway. The pattern features a pronounced low latitude, or subtropical, jet stream well above the earth’s surface, leading to a more southerly track of low pressure systems which produce more rainfall and more frequent cold fronts across locations south of 30°N latitude, including the Rio Grande Valley.

After alternating warm and cool spells through November, and rainfall generally below average, a vigorous upper level disturbance moved from Baja California east through southwest and south Texas to close the month and open December. This system dropped heavy mountain snows from New Mexico through the Guadalupe Pass, and eventually lifted deep tropical moisture behind a cold front across the Rio Grande Valley on December 1, dumping an inch of welcome rain across the Rio Grande Plains and 1 to 3 inches across the Rio Grande Valley. The rains were accompanied by gusty northwest winds and temperatures which held in the 40s and 50s, the coolest readings of the season. Colder air and more light rains would follow on the 4th and 5th, as a broader upper disturbance pushed modified arctic air into Texas. Rare accumulating snow fell as far south as areas just north of Corpus Christi, with 4 inches measured between Baytown and Victoria. Across the RGV, blustery north winds in gusting over 30 mph would combined with temperatures in the upper 30s to lower 40s to make it feel like the upper 20s to lower 30s; the frosty night which followed was the coldest in the region since December 2004.

A cracked door of warmth by December 7th and 8th would be slammed shut once again on the 10th and 11th by another round of chilly, cloudy, damp weather. The general outlook for the rest of December is for more cool than warmth; while total rainfall through the holiday season remains uncertain, each front will likely bring more clouds, wind, and chill.

The maps above are beginning to match expected conditions for the October–December, and November–January composites. For the time period beginning at the end of the first week of November and ending during the first week of December, observed data at the three primary stations of Brownsville, Harlingen, and McAllen showed temperature departures of -2.4°F, -2.1°F, and -1.2°F, respectively, and rainfall departures of +1.78 inches, +1.42 inches, and +1.53 inches, respectively. For the first ten days of December, temperatures have been 5 to 7°F below average across Deep South Texas. For a full menu of El Niño composites, click here.

How will the remainder of the 2009/2010 winter fare? Predictions continue to show El Niño remaining strongly moderate through mid winter, and if past history and the current pattern are any guide, residents of Deep South Texas can expect continued cool and frequently damp conditions through December and likely into January, as well. Still, there are many factors at play in how the actual weather evolves through an El Niño episode; only time will tell before we can fully assess the late autumn and winter of 2009/2010. Stay tuned!

For more on the relationship of El Niño to observed weather in Deep South Texas, check out an earlier article on the topic.

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