Above: A haboob approaches Phoenix on Tuesday evening.
Above: Another image of the haboob approaching Phoenix on July 5, 2011. Image credit Chris Voyles.
Above: A 1930s era image of a dust storm approaching a Texas Town.
Above: An image of a haboob enveloping Al Asad Airbase, Iraq in April 2005.
We've not experienced any haboobs lately in Florida, but it remains very dry. South Florida is experiencing an exceptional drought according to the National Weather Service's Climate Prediction Center. Some relief is forecast this week as an inverted trough or tropical wave moves slowly across Florida. We are hopeful for copious amounts of rain. I have never seen it this dry for this long.
Above: The current National Hurricane Center forecast map graphic showing the wave traversing Florida.
If we get many of these troughs this summer we might return to some semblance of the previous normal. Currently many lakes in east Central Florida are dry or nearly so.
WHAT IS AN INVERTED TROUGH OR TROPICAL WAVE?
In the mid-latitudes of the Northern Hemisphere a trough is usually seen as a southerly bulge in the height contours. The lowest heights are generally located to the north of the trough. In an inverted trough situation, the height contours bulge to the north. This is more common in the tropical regions where regions of low pressure ride south of a mid-latitude high pressure but can happen in the mid-latitudes when low pressure is south of high pressure. An inverted trough bulges to the north. At first it may look like a ridge, but on further inspection it is a trough. Both a trough and an inverted trough have a cyclonic (counterclockwise) flow pattern. A trough will tend to have more westerly winds associated with it while an inverted trough will tend to have more easterly winds associated with it. If an inverted trough is actually a ridge then the winds will be flowing with the height contours in the opposite direction (anti-cyclonic direction). The direction of windflow through the feature is how a ridge is discerned from an inverted trough.
Tropical waves will show up as inverted troughs because they are generally south of mid-latitude high pressure and have an easterly wind associated with them. Below are an example of a trough and an inverted trough.
PHOENIX'S HABOOB (DUST STORM) EXPLAINED
On Tuesday, July 5, 2011 Phoenix experienced a haboob. Haboob is the Sudanese name for duststorm or sandstorm with strong winds that carry small particles of soil or sand into the air. These events are particularly severe in areas experiencing drought, like the American Southwest.
Below: The Phoenix storm marches across the desert.
Phoenix's sandstorm dropped visibilities to near zero and coated surfaces with a gritty later of dust and sand. The phenomena occurs when the outflow from a thunderstorm kicks up desert dust. The Phoenix storm was due to a large complex of thunderstorms known as a mesoscale convective system (MCS) that developed to the east of Phoenix. As the outflow from the MCS hit the ground, large quantities of sand and dust became suspended in the air by 50 - 60 mph winds. The amount of dust was much greater than is usual for one of these storms, due to the large size of the thunderstorm complex, and the extreme drought conditions the region has been experiencing. As the haboob hit Phoenix, winds gusted to 53 mph at Sky Harbor International Airport, and the airport was forced to shut down for 45 minutes due to visibilities that fell as low as 1/8 mile. The airport received only 0.04" of rain from the storm, but large regions of Southern Arizona got 1 - 2 inches of rain overnight due to the monsoon thunderstorms.