Boris a Dangerous Rainfall Threat for Mexico and Guatemala

Torrential rains are lashing Southeast Mexico and Southern Guatemala as Tropical Storm Boris lumbers northwards at about 5 mph, with landfall expected to occur Wednesday in Southeast Mexico. Even though Boris has top winds of just 40 mph, and will, at worst, be a weak tropical storm at landfall, it is an extremely dangerous rainfall threat to the region, as the storm's slow motion is expected keep heavy rains over the region the entire week. The NHC forecast is for 10 - 20" of rain with isolated amounts of 30 inches (750 mm) or more in the Mexican states of Oaxaca and Chiapas. Rains of this magnitude are capable of causing widespread flooding and heavy loss of life. Tropical Storm Agatha hit this region at the end of May 2010 as a weak tropical storm with 45 mph winds, and dumped up to 22.27" of rain. The resulting catastrophic flash floods and landslides killed 190 and caused $1.1 billion in damage, mostly in Guatemala. Heavy rains from the precursors of Boris triggered a landslide in Guatemala over the weekend, killing five people. In neighboring areas of Mexico, several landslides closed mountain roads on Monday, and evacuations began Monday evening from coastal and low-lying areas next to rivers that are prone to flooding.


FIgure 1. Latest satellite image of Boris.

Satellite images show that Boris is poorly organized, but has several clumps of heavy thunderstorms. Mexican radar showed the heaviest rains were offshore this morning, but these rains will move inland today as the storm heads north at 5 mph.


Figure 2. Satellite rainfall estimates for the 24-hour period ending Tuesday morning, June 3, 2014 over Central America. Rainfall amounts in excess of 200 mm (7.87") were estimated along the Guatemala coast and near the Belize/Mexico border. Image credit: http://climaya.com.

Will Boris emerge into the Gulf of Mexico late this week?
If Boris continues due north along its current path at its current speed for the remainder of the week, the storm will cross the narrowest part of Mexico and potentially emerge over the Southern Gulf of Mexico's Bay of Campeche late this week. Once over the warm waters of the Gulf, the remnants of Boris will have the potential to regenerate into a tropical depression. The 06Z Tuesday run of the SHIPS model, which uses the GFS model to diagnose wind shear, is predicting that wind shear in the Bay of Campeche will be moderate, 10 - 15 knots, on Friday and Saturday. However, a band of high wind shear associated with strong upper-level winds from the subtropical jet stream is predicted to lie over the Central Gulf of Mexico, and these winds may interfere with development. Sea surface temperatures (SSTs) in the Southern Gulf of Mexico are about 28°, which is about 0.5° above average. These warm waters do not extend to great depth, and the total heat energy available to intensify a potential storm is rather low. SSTs cool quickly as one goes to the north, are a marginal 26°C in the Central Gulf of Mexico. The GFS model is bullish on developing a tropical depression in the Gulf of Mexico late this week. But according to WSI's tropical weather expert Michael Ventrice, who has guest blogged on El Niño in my blog, the GFS likely has insufficient resolution to handle a large tropical low pressure system forecast to set up over Central American late this week. These large low pressure systems often have "spokes" of extra spin that rotate around the main low, and these "spokes" are often erroneously developed into tropical depressions by the GFS model. The European model is much less gung-ho about developing a tropical system in the Gulf of Mexico late this week, and NHC is currently giving no chance that such an event will happen by Sunday. I put the odds at 10%.


Figure 3. Sea surface temperatures in the Gulf of Mexico were a very warm 28°C in the southmost Bay of Campeche on June 1, 2014, but diminished quickly to 26°C in the Central Gulf. Image credit: NOAA/AOML.

Jeff Masters