Arlene moves into Mexico as a tropical storm; Watching for Bret
Tropical Storm Arlene moved ashore this morning a little before 4AM CDT as a strengthening tropical storm with maximum sustained winds of 65 mph and a minimum central pressure of 993 mb. In Tampico, Mexico, winds in the order of 20-30 mph have been reported, with gusts higher than 35 mph. These winds are expected to get stronger as the day goes by and the city gets stuck under the right quadrant of Arlene's eyewall. At least 1-3 inches have already fallen in the city, and another 2-4" can be expected before Arlene moves into inland Mexico. This is likely to cause problems traveling, and mudslides/landslides will be a problem as well. Further north, in Brownsville, Texas, a little under an inch and a half of rain has fallen this morning as a result of Arlene's outer bands, and its possible that Brownsville could receive another 1-2" more, which is no undoubtedly helping the drought situation in that area. In addition to the heavy rains, wind gusts up to tropical depression strength have been reported within the heaviest rain bands. An earlier microwave pass of Arlene reveals that had the tropical system been over water another 6-12 hours, we would have probably seen our first hurricane. Because of the relatively flat terrain of the coast of Mexico, added to the cyclonic curvature of the Bay of Campeche, Arlene was able to really start to ramp up when it approached landfall earlier this morning. This same situation happened to Hurricane Alex of last year, which intensified from a strong Category 1 to a strong Category 2 hurricane between 2 NHC advisories.
Figure 1. Tropical Storm Arlene shortly after landfall south of Tampico, Mexico.
The intensity and track forecast for Arlene is not complicated, as the system should move westward into the mountainous terrain of inland Mexico. Since Arlene doesn't have an energy source anymore, it should rapidly weaken today, and likely dissipate before tonight is over. Some of the computer models are hinting at an area of low pressure developing south of the Baja of California in the Pacific later over the weekend, but it isn't a threat to develop at this time, with cooler Sea Surface temperatures.
Watching for Bret
Now that Arlene is out of the way, it is time to start watching for our second named storm, Bret. Typically, we do not see our first named storm until July 9th, and we do not receive our second named storm until August 1. So, we are ahead of schedule, and may stay ahead of schedule as I will type about later on in this post. Overall, the Atlantic is hostile, with 20-50 knots ruling most of the basin. There is a pattern of troughs and ridges across the Caribbean basin at this time. To sum it up, it is a pattern of high wind shear and low wind shear. As tropical waves pass through this area in the next couple of weeks, some will get caught under the troughs and likely not develop, but some could get caught under a ridge, just like Arlene did, which will provide a low shear environment, and allow for possible intensification. Sea Surface temperatures and Ocean Heat content are high enough to support a tropical cyclone, with much of the Caribbean running 2-4 °C above the 26 °C isotherm. In addition, several of the models are predicting a moderate to strong MJO signal by the end of the first week of July. This allows for a confluent area aloft, and enhances thunderstorm development, making it easier for a tropical cyclone to form. Considering that Arlene formed under a weak upward pulse, it will definitely need to be watched closely. None of the reliable computer models, such as the ECMWF and GFS, develop anything over the next week or so, but there have been hints by the majority of the models that there are going to be a couple of cold-core lows that form off the East Coast. While this isn't necessarily a threat, the lows will need to be watched in case one of them separates from a frontal boundary and becomes warm-core.
Figure 2. Current Wind shear across the Atlantic basin.
I plan on taking a week or two off from posting, at least until the Atlantic basin starts to shift into high gear, since that is the basin I primarily cover. So, my next blog post will be between July 7-14.