By Bob Henson From Weather Underground

The next few days will be quite active for tropical weather watchers, as we have not one but two systems in the Atlantic that each show potential for developing into a tropical depression—an unusual twin prospect for mid-June.

Our top three models for predicting tropical cyclone genesis—the GFS, European and UKMET models—continued with their 0ZWednesday runs to advertise the potential for a tropical depression to spin up in the Western Caribbean or Southern Gulf of Mexico early next week. A large low pressure system, commonly referred to as a “gyre” of low pressure (see the diagnostics from SUNY Albany’s Phil Papin), will develop near Mexico’s Yucatan Peninsula this weekend, spreading heavy rains across the Western Caribbean and portions of Central America and Southern Mexico. The interaction between this gyre and a tropical wave expected to move into the region this weekend may be enough to trigger formation of a tropical depression by early next week.

While the skill of the models to predict tropical cyclone formation so far in advance has not been established, the fact that all three of the models have been consistently calling for development over multiple runs is noteworthy, and we should be watching the waters surrounding the Yucatan Peninsula early next week for potential development. (Note, however, that this morning’s 6Z run of the GFS model backed off on its forecast for a tropical depression to form.) In their 8 am EDT Wednesday Tropical Weather Outlook, the National Hurricane Center (NHC) gave this system 2-day and 5-day odds of tropical cyclone development of 0% and 20%, respectively.

An early area of concern in the deep tropical Atlantic

In a rarity for June, we have the possibility of a tropical cyclone forming over the next week in the Atlantic’s Main Development Region—the swath between the Antilles and Africa—where Cape Verde storms are more likely to develop from July through September. A broad tropical wave emerged on the Intertropical Convergence Zone west of the Sierra Leone coast on Tuesday and has continued to expand as it moves west at a very low latitude.

In its tropical weather discussion issued on Wednesday morning, NOAA placed the center of this wave at 6°N, 23°W, with a central pressure of 1011 millibars. This is too close to the equator for the system to leverage Earth’s spin to help itself get spinning, which will make any development slow to occur. Otherwise, conditions are reasonably favorable for this system to organize as it moves west, at least for the next day or two. Wind shear is relatively light (5 – 10 knots) along the path of the complex until it reaches about 40°W on Thursday, when it should encounter progressively stronger shear. The wave is traveling through a very moist environment at lower levels, as it is positioned well south of the dry Saharan Air Layer (see Figure 4 below) and should remain so. Sea surface temperaturesalong the track of the wave are running near 27°-28°C, which is about 0.5°C above average and more than ample for supporting a tropical cyclone.

IMAGES:

Probabilities of a tropical or subtropical cyclone of at least depression strength passing over various parts of the Gulf of Mexico and tropical Atlantic over the next two weeks, as predicted by the ECMWF model ensemble runs from 00Z Wednesday, June 14, 2017. Image credit: Michael Ventrice, The Weather Company.

Figure 1. Predicted total precipitation between 8 pm EDT Tuesday, June 13, and 8 am EDT Tuesday, June 20, from the 0Z June 14 run of the GFS model. Regardless of development of the predicted low pressure gyre over the Yucatan Peninsula into a tropical depression, the large size of the low and the abundant moisture that will feed it from both the tropical Atlantic and tropical Pacific (including the remnants of Tropical Storm Calvin), will generate very heavy rains that will cause flooding and landslide concerns in portions of Central America and Southern Mexico beginning this weekend; rainfall amounts of over ten inches (orange colors) are predicted in Guatemala and Southern Mexico. Note, also, the rainfall from the tropical wave predicted to arrive in the southern Lesser Antilles Islands and northern coast of South America early next week. Image credit: Levi Cowan, tropicaltidbits.com.

Figure 2.  Anomalies (departures from the seasonal average, in degrees C) in sea surface temperature for the week ending June 10, 2017. Ocean temperatures are generally 0.5°C – 1.0°C (0.9 – 1.8°F) above average from the Southern Gulf of Mexico to the coast of Africa. Image credit: NHC.

With ocean temperatures in the Western Caribbean and Southern Gulf of Mexico about about 1°C (1.8°F) above average, at 29.5°C (85°F), along with wind shear that is predicted to be moderate, conditions will be ripe for development–if the area of low pressure manages to center itself over the water. There is plenty of land in the region to potentially interfere with development, though, and the wind shear forecast is uncertain this far into the future.

The eventual track any potential storm is most likely to be to the west-northwest or northwest, with the primary threats to land being along the Gulf of Mexico coasts of Mexico and Texas by the middle of next week. The European model depicts a steering ridge of high pressure farther to the north, allowing for a storm track farther to the north towards Texas, while the GFS model has a more southerly position of the steering ridge, resulting in a greater threat to the coast of Mexico south of the Texas border. Keep in mind, though, that the models do not have much skill doing track forecasts so far into the future, and some of the ensemble members of the European model are showing potential tracks into Louisiana and the Florida Panhandle.

Figure 3. Meteosat-9 infrared image of a strong tropical wave located west of Africa just south of 10°N between 20°W and 30°W at 1200Z (8:00 am EDT) Wednesday, June 14, 2017. Image credit: NOAA/NESDIS.

Figure 4.  Dry air associated with the Saharan Air Layer (orange colors) should stay mainly to the north of the tropical wave tracking west across the deep tropical Atlantic. Image credit: University of Wisconsin-CIMSS and NOAA/HRD.

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