Why aren’t South America and the west coast of Africa hit by hurricanes?

Posted on June 10, 2024 by WeatherGuys Editor

Tropical storms hitting South America have distinct disadvantages compared with the North American situation.

The energy to drive tropical systems is derived from warm ocean waters. For tropical storms to form, the ocean temperature needs to exceed 79.7 Fahrenheit, which is common in the North Atlantic and Caribbean oceans between June and November. Reaching these temperatures is rare in the South Atlantic Ocean, even during their summer. Hurricanes do not form within 5 degrees of the equator and that includes a large geographic region of South America. The small Coriolis forces there inhibit circulation development.

Tropical Cyclone tracks from 1985 – 2005. Credit: NOAA

Tropical storms that develop in the eastern Atlantic will be carried away from the African continent by the winds that steer them. The east coast of Africa does get hit by typhoons that form over the Indian Ocean or Bay of Bengal.

Although the moisture-laden remnants of a tropical storms can result in flooding in California, the West Coast rarely sees landfall of a storm. The only known tropical storm to make an official landfall in California occurred on Sept. 24, 1939. Hurricanes have difficulty forming off the U.S. west coast due to cold water, cold currents and unfavorable winds. Storms that might approach southern California from western Mexico would be steered away by the prevailing upper-level winds.

The waters off the west coast are colder than the threshold for tropical storm formation. The California Current brings cold water north to south along the coast. Also, the prevailing northerly winds in the summer and fall push waters away from the coastlines. This results in upwelling that brings cooler ocean water from below to the surface. This upwelling process also occurs off the western coastline of South America.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Tropical

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Why is NWS predicting an active hurricane season?

Posted on June 3, 2024 by WeatherGuys Editor

The National Weather Service is expecting the 2024 Atlantic hurricane season to be an extremely active one. The forecast predicts 17 to 25 total named storms. Such storms have sustained winds of 38 mph or higher. Additionally, four to seven of those storms may fall into the major hurricane category, with sustained winds of 111 mph or higher.

An average year sees 14 named storms during hurricane season, which runs from June 1 to November 30.

One factor contributing to an above-average hurricane season is the near-record warm ocean temperatures in the Atlantic Ocean. Warm ocean temperatures increase evaporation, which provides energy for a storm to develop and intensify. The ocean waters must be above 26.5 C, or 79.7 F, for hurricanes to form and sustain. Large regions of the tropical East Pacific and the tropical Atlantic surpass this threshold.

Surface water temperatures measured by satellites on May 30th revealed conditions more typical of August in the Gulf of Mexico and western Atlantic Ocean.
Credit: NOAA CIMSS

The forecast is also driven by the atmospheric circulation pattern transitioning from El Niño to La Niña conditions. El Niño and La Niña are the two extremes of a recurring climate pattern that can affect weather around the world. On average, La Niña years are above-average hurricane years. La Niña creates conditions for less vertical wind shear, which refers to the change in wind speed and direction between roughly 5,000 and 35,000 feet above the surface. Hurricanes will not form when the upper-level winds are too strong. Strong upper-level winds inhibit the storms’ structure by displacing the warm temperatures above the eye and limiting the vertical ascent of air parcels.

Typhoons are tropical storms that form over the Pacific Ocean. While La Niña tends to increase hurricanes in the Atlantic, those conditions tend to decrease the number of typhoons in the eastern and central Pacific Ocean basins.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Severe Weather, Tropical

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Why is Memorial Day weekend weather highly variable?

Posted on May 28, 2024 by WeatherGuys Editor

Memorial Day weekend weather can be absolutely glorious in the state of Wisconsin. But it can be rainy and cold as well. Perhaps no other major holiday suffers such a Jekyll and Hyde split, and there are really good scientific reasons that underlie this duality.

By the end of May, the Northern Hemisphere is just about completely over the prior winter and the cold air that characterized it is almost completely left to very high latitudes, where the longer days act quickly to erode what is left even near the North Pole. The process of “shedding” the cold air from winter sometimes involves the excursion southward of regional cold air vortices in the mid-troposphere, which meteorologists refer to as “cut off” low pressure systems.

If such a cut off low parks itself over the Great Lakes states, the weather is often persistently showery and chilly. This is because the air at about 3 miles above the ground is colder than normal in such a circumstance and, coupled with the relatively warm surface temperatures of late May, the atmosphere is made less stable and the threat of showery weather increases substantially. These cut off lows can persist for several days in a row. It turns out that the seasonal maximum for such cut off lows over our area runs from mid-May to mid-June.

Consequently, Memorial Day weekend can be plagued by the presence of one. Our recent holiday was influenced by the proximity of such a disturbance that continues sliding southeastward toward the Great Lakes states.

Unsettled weather and copious clouds over the Great Lakes on Memorial Day 2024.
Credit: NOAA CIMSS

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Seasons

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Has this year’s tornado activity been abnormal?

Posted on May 20, 2024 by WeatherGuys Editor

A remarkable number of tornadoes have occurred across the United States this year.

Preliminary reports from the Storm Prediction Center tally 784 tornadoes across the United States through May 13, the second-largest number on record. That number is far higher than the historical average to that date of 549.

Storm Prediction Center (SPC) 2024 tornado tally as of May 21st.

Remarkably, however, this year’s total pales in comparison with the 1,314 tornadoes that had visited our country by the same date in 2011. That year’s total of 2,240 exceeds the next highest total (1,677 in 2019) by 33.6%. Other seasons that crossed the 700 tornadoes threshold by mid-May were 2023, 2022 and 2017. Of those three, 2017 ended with the largest total (1,522, third all time).

It is hard to say whether this rapid start to the season will mean that 2024 will contend for a top spot on this harrowing list. It is also very difficult to say whether the background climate change has played a role in increasing the number of tornadoes the country experiences.

Recent work by Victor Gensini at Northern Illinois University and Harold Brooks at the National Severe Storms Laboratory has found that Tornado Alley has shifted eastward during the past 40 years. Once again, the role of background climate change in this shift is an open question.

Regardless of the cause, such trends are important for understanding the potential for changes in tornado exposure. These storms, and the associated severe thunderstorms that deliver them, are responsible for an average of $5.4 billion in damages each year across the United States — with seasons exceeding $10 billion in damages becoming more common.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Severe Weather

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Do tornadoes occur over all parts of Earth?

Posted on May 14, 2024 by WeatherGuys Editor

The only continent that has not recorded a tornado is Antarctica. Otherwise, tornadoes can happen in just about any location.

A tornado spins west of Hawley, Texas.
(Credit: R. Erdrich, Abilene Reporter-News)

North America, and specifically the United States, accounts for more than 75% of the world’s reported tornadoes. Other tornado-prone areas around the globe include western Canada, Argentina, the western Mediterranean, Japan, Bangladesh, Australia, New Zealand and South Africa.

Tornadoes form in regions of the atmosphere in which warm moist air is surmounted by warm dry air, large vertical wind shears exist, and weather systems such as fronts or thunderstorm complexes can force air upward. The U.S. provides these three ingredients in abundance. The distribution of tornado reports reveals a “tornado alley” of highest frequency in the central Great Plains. In this region, warm moist low-level air from the Gulf of Mexico is often capped by very dry air blown out over the Plains at mid-levels from the Mexican Plateau. This type of stratification sets the stage for the explosive thunderstorms that can spawn tornadoes. Another area of frequent tornado occurrence is over the Southeast.

Although tornadoes can occur at any time of day or night, favorable tornadic conditions benefit from solar heating. Therefore, the most likely times for tornadoes are late afternoon or early evening. In fact, more than half of all U.S. tornadoes occur between 3 and 7 p.m.

The total number of tornadoes in the U.S. each year has stayed relatively consistent over the past several decades. Recent research has indicated that there are now fewer days with tornadoes each year, but there are more tornadoes on the days when they do occur, indicating that they tend to now happen in more concentrated bursts over fewer days of the year.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Severe Weather

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