A prolonged period that began with clouds on December 9th, persisted with damp chill from the 10th through 12th, and ended with plenty of fog through the 14th, will be followed by – you guessed it – another round of England–like weather for the bulk of the week before Christmas. About the only difference between the Lower Rio Grande Valley and London will be the length of day, though the lack of appreciable sunshine through at least the 17th will make it appear that night will last longer than it really does. Similar to a winter’s day on the British coast, Deep South Texas will have all the ingredients: brisk winds, rain and drizzle, and temperatures right back into the 40s for most areas beginning during the evening of Tuesday the 15th and continuing right into Thursday morning the 17th. Added to the London–like brew will be locally heavy rainfall (contours, above) on already saturated soils, especially on the 16th. While these rains will not come in buckets, event totals may add up to more than 3 inches in some locations toward the coast. Cities and towns receiving close to 3 inches of rain may see nuisance flooding in poor drainage locations.
The week ahead (through December 19th) will feature a return to a more vigorous subtropical jet stream, a classic signature of a moderate to strong El Niño across the southern U.S. and northern Mexico (below). Embedded within the jet is a deepening disturbance up near 18,000 feet, which will ease across southwest and south central Texas similar to a prior system on November 30 and December 1. This system, like its earlier sibling, will induce a surface cyclone, which may become a full blown gale, just east of the Lower Texas and Tamaulipas coast on the 15th, which will gradually move north and northeast through the 17th. Even after the system departs, residual moisture trapped below a developing inversion should continue clouds and spotty light rain or drizzle into Thursday the 17th, which may well keep temperatures struggling to reach the 50s as the length of day continues to shorten. Cool temperatures will continue right into the upcoming weekend.
Forecast surface pressure field (light blue dashed lines), and 500 mb pressure heights (solid yellow lines), for Wednesday, December 16th 2009.