06.30.10

Hurricane Alex

Posted in Tropical Weather, Weather News at 1:36 pm by Rebekah

Last night, Alex became the first hurricane of the 2010 Atlantic hurricane season and the first June Atlantic hurricane in 15 years.

Alex is currently a Category 1 hurricane with maximum sustained winds of 85 mph and a minimum central pressure of 962 mb.  Hurricane Alex is expected to make landfall late tonight as possibly a weak Category 2 (sustained winds of at least 96 mph) in far northeastern Mexico, just south of Brownsville at the Texas border.

Alex is a large tropical cyclone; hurricane-force winds (sustained of at least 74 mph) extend outward 60 miles from the core, while tropical storm-force winds (sustained of at least 39 mph) extend outward 200 miles from the core.

Threats from Alex include:

• Rainfall – 6 to 12 inches expected over northeastern Mexico and southern Texas, with isolated amounts of 20 inches
• Flash floods and mudslides – as a result of the heavy rain, especially in mountainous terrain
• Wind – tropical storm-force winds are already affecting the coast, and hurricane-force winds are expected to reach the coast within the hurricane warning area tonight
• Storm surge – 3 to 5 feet along the coast, north of where the core makes landfall
• Tornadoes – expected in the outer rainbands in southern Texas today and tonight

The right-front quadrant of a Northern Hemisphere tropical cyclone is always the most dangerous part to be, so even though Brownsville will not receive a direct hit from the eye, the rain, storm surge, winds, and tornadoes could be worse than in Mexico.

In the Northern Hemisphere, low pressure systems, including tropical cyclones, rotate counter-clockwise.  Thus on the right side of a cyclone, with respect to its motion, winds will be stronger than on the left side, as the rotational speed combines with the forward speed (on the left side, the winds are weakest, as the direction of rotation is opposite from the direction of motion).

These stronger winds on the right side of a tropical cyclone additionally result in more water being pushed towards land, thus the storm surge will be the worst on the right front side (as it makes landfall).

A tornado watch was issued by the Storm Prediction Center this morning for southern Texas, and so far three tornadoes have already been reported around Brownsville, as the outer rainbands of Alex are already coming on shore.  The outer rainbands are made up of multiple severe thunderstorms, so supercells are often found in this area if the wind shear is great enough (friction as the cyclone comes ashore contributes to greater wind shear).

Tropical cyclone tornadoes do not tend to be as strong as tornadoes of the Great Plains, largely because shear is not as high.  Lightning, also, is not very prolific in tropical cyclones, partly because there is not as much ice above a warm-core system (ice is needed to create an electric field).

For the latest information on Hurricane Alex, check out the National Hurricane Center and the Brownsville National Weather Service!

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06.29.10

World Wide Weather #8: Brownsville, Texas

Posted in Tropical Weather, Weather News at 5:19 pm by Rebekah

This week’s post in the global weather and climate series features Brownsville, Texas.

The southernmost city in Texas, Brownsville is located on the US/Mexico border (marked here by the Rio Grande), just a few miles from the Gulf of Mexico.  Brownsville’s history stretches back to 1845, when construction on a fort was begun at the Mexican border.  During the Civil War, Brownsville was used as a smuggling point for Confederate goods to Mexico and cotton to Europe.  On Christmas Day 2004, Brownsville had its first measurable snow in 109 years, with 1.5 inches (3.8 cm) on the city’s first White Christmas.  Today, Brownsville remains an important shipping point.  Brownsville is home to 139,722 people (337,227 in the metro).

A few more facts about Brownsville (from Wikipedia):

• Time zone: Central Standard Time (UTC-6) or Central Daylight Time (UTC-5)
• Elevation: 33 ft above mean sea level
• Climate zone: Subtropical
• Average high temperature: 82 °F (28 °C)
• Average low temperature: 65 °F (18 °C)
• Record high temperature: 106 °F (41 °C)
• Record low temperature: 12 °F (-11 °C)
• Average annual precipitation: 28 inches (700 mm)

Current weather:  As you may or may not be aware, Tropical Storm Alex, the first named storm of the Atlantic hurricane season, is soon expected to become a hurricane and make landfall Wednesday night as a Category 1 hurricane, somewhere from Brownsville to the northeast Mexican coast.

The National Weather Service issued a hurricane local statement this morning for the Brownsville area: “THE PRIMARY IMPACT FOR ALL OF DEEP SOUTH TEXAS WILL BE INLAND FLOODING FROM 12 TO 18 HOURS OF TORRENTIAL RAINS…WITH SIGNIFICANT WIND DAMAGE IMPACTS MORE LIKELY TOWARD THE COAST”.

They emphasize that people should not focus on the forecast track, as the storm can make landfall anywhere within the National Hurricane Center’s forecast cone, and even if the storm makes landfall elsewhere, it can still be wide enough to affect many surrounding people.

For more information on Brownsville, here’s a link to Wikipedia.

For weather maps and information on current and forecast Brownsville weather, see the Brownsville National Weather Service site.

Next Tuesday I plan to take a look at the climate and weather in another part of the globe.  As always, if you have any comments or suggestions for future cities, please leave a comment on this post!

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03.20.10

Tropical Cyclone Ului Makes Landfall in Australia

Posted in Non-US Weather, Tropical Weather, Weather News at 12:37 pm by Rebekah

Severe Tropical Cyclone Ului made landfall in Queensland today (about 1:30am Sunday, local time), on the northeast coast of Australia.  With a central pressure of 964 mb, the cyclone brought sustained winds of over 90 km/hr (56 mph) and wind gusts of 205 km/hr (128 mph) to the coast.  According to the Australia Bureau of Meteorology, this is a Category 3 tropical cyclone.  According to the National Hurricane Center in the US, this would only be a tropical storm.  Why the large discrepancy in ratings?

Tropical cyclones have different names in different ocean basins; in the North Atlantic, a strong tropical cyclone is called a hurricane, while in the Northwest Pacific, a strong tropical cyclone is called a typhoon.  Tropical cyclones also have different ratings in different ocean basins, each based on a set of criteria developed by the agency that monitors the storms in their particular region.

The table below (click to enlarge) shows worldwide ratings for tropical cyclones by ocean basin and agency (courtesy of Australia Severe Weather; they reportedly got it from Wikipedia).

You may notice that the NHC ratings look a little different from what you may be used to.  Note that these classifications are based on the 10-minute average wind speed in knots (1 knot = 1.15 mph), while the NHC’s Saffir-Simpson Scale is based on a 1-minute average wind speed…thus the wind speeds in this table have been “downgraded” to fit the 10-minute average that most everyone else in the world uses.

For more information on Australia’s particular rating system, see the Bureau of Meteorology page about tropical cyclones.  For information on current tropical cyclones around Australia, see this Bureau of Meteorology page on tropical cyclones.  For information on current tropical cyclones in the Pacific and Indian Ocean, see the Joint Typhoon Warning Center.

Severe Tropical Cyclone Ului at 1530UTC (2:30am local time in Queenland); infrared satellite image courtesy of the University of Wisconson-Madison.

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