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As we head into the second half of summer, a moderate El Nino pattern that began in March is showing signs of strengthening in the Pacific Ocean. And the official forecasts from several different meteorological agencies in the U.S., Australia, Japan and the United Kingdom indicate a high probability that El Nino will last several more months into early 2016. Not only that, but many climate models used by forecasters also indicate this El Nino may end up being one of the strongest on record. As a result, significant changes to weather patterns all over the world are expected.

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Why are tornadoes more frequent in the United States than anywhere else in the world? The main reason is the United States’ unique topography. Dry, cooler continental-based air from the mountains and Canada collide with moist, warm air from the Gulf of Mexico. This creates the perfect environment for tornadoes to form. The differing winds within these air masses cause vertical wind shear, a change in winds with height, which is critical for tornado formation. The area where all of these ingredients come together is often referred to as Tornado Alley, stretching across most of the Central United States. However, tornadoes occur in every state, including Hawaii and Alaska.

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The wind is howling, the flakes are falling, it’s a classic blizzard! But suddenly, you see a flash, and then boom! Thankfully, that was not your roof collapsing under the weight of the snow, but instead a rare phenomenon called thundersnow. Many people who experience this phenomenon are perplexed and skeptical that thunder and snow can happen simultaneously. Granted, it is indeed a rare phenomenon, but when certain meteorological parameters are present in winter weather, it can occur.

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The extreme temperature and moisture differences between the hot, dry Sahara desert and the cool, wet coast of the Gulf of Guinea in west-central Africa give rise to the African Easterly Jet (AEJ), a belt of easterly mid-level winds. The migration of the AEJ from south in the “dry” winter and spring months to north in the “wet” summer and fall months constitutes the West African Monsoon.

At the surface, the monsoon is basically a shoving match between the Harmattan, a scorching, often dust-choked layer of Saharan air, and the moist southwesterly flow off of the Atlantic ocean. The further south you go the weaker the influence of the Harmattan, and the longer the duration of the rainy season, ranging from 2-3 months at 16˚ N to nearly 8 months at 8˚ N. Areas south of 8˚ N even experience a relative “break” in the monsoonal rainfall in July and August as the AEJ and associated storm activity shift just to the north. This monsoonal fluctuation is the prime determining factor in the precipitation patterns between 9˚ and 20˚ north latitude that are so vital to agricultural concerns in West Africa. Even minor variations in the strength and orientation of the monsoon can spell dire consequences for subsistence farmers in the Sahel, the semi-arid region between the Sahara desert to the north and the grasslands to the south. Desertification due to over-farming and global climate change may contribute to famine during abnormally dry periods such as the one from 1972 to 1984 that killed more than 100,000 people. However, studies indicate that the Sahel is actually in a multi-decadal period of increased rainfall that will peak in 2020.

 

The AEJ can often be found stacked on top of the Inter-Tropical Convergence Zone (ITCZ), a narrow band of converging, storm-generating surface winds that circumnavigates the globe. Ripples in the AEJ known as African Easterly Waves (AEWs) traverse the continent every 3-5 days, interacting with the ITCZ to create giant clusters of thunderstorms. Some of these clusters survive the transition onto the open Atlantic Ocean, begin to spin due to the Coriolis force, and intensify into tropical cyclones, even hurricanes (see figure below). Generally speaking, the more rain that falls in West Africa, the more hurricanes that form over the Atlantic basin. About 60% of all Atlantic tropical cyclones (and 85% of all major hurricanes of Category 3 or higher) can be traced back to AEWs. In addition, studies suggest that virtually all tropical cyclones in the Eastern Pacific basin originate from AEWs.