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Speaking of Hurricanes - 07.18.05
Doc Weather explains some of the terms used by forecasters to describe the forces that support or weaken hurricanes.
On websites devoted to hurricane prediction technical language is often used that, if it were understood could help non specialists to understand the tracking problems that complicate the prediction of hurricanes. The following excerpt is from the description of the tracking coordinates of Hurricane Emily. This storm at the time of the forecast was making a transit moving west out of the Caribbean Sea and into the Gulf of Mexico.
Fig.1
"Emily remains on the south side of an east-west low/mid level ridge extending from the Atlantic across the northern Gulf Coast to Texas. Large scale models forecast the ridge to generally persist with a weakness perhaps developing in 36-48 hr over the western Gulf coast as a mid-latitude shortwave trough passes to the north." This ridge is depicted in figure 1. Forecasters call a ridge with such a large area a steering ridge. High- pressure over the eastern Atlantic was linked to the Bermuda High and extended into the North American continent across the Gulf Coast. The blocking pattern in the ridge was steering the storm horizontally across the western Caribbean Sea and not allowing it to make any turns to the north. The previous storm Hurricane Dennis found a weakness in the steering ridge and was able to turn into the Mississippi delta. The weakness was from a mid-latitude trough that moved across the continent, as Dennis was moving slowly northward towards the Ohio Valley.
For Emily, the ridge pattern can be seen in figure 1as a continuous high-pressure area centered off of the east coast of Florida. Another upper level high- pressure area is centered over the Great Basin and extends eastward into northern Texas. These ridges are the steering ridges for this storm. The mid-latitude short wave trough formation mentioned in the bulletin can be seen moving southward over western Canada. With Emily making a move south of Cuba the thinking was that it might be possible for the trough to dig south into the Midwest between the two steering ridges (arrow) and help to steer the storm into the Midwest following in the footsteps of Dennis the week earlier. This was not to be.
Fig.2
This is a chart for the day after the bulletin. We can see that the high-pressure off of the east coast of Florida has grown stronger and moved west. This has forced the trough coming out of the northwest to stop digging south and move to the east, taking a track across the northern tier of states. This effectively shut the door to the hurricane forcing a horizontal track to the west for a landfall in the western Gulf of Mexico. With the closing of this door the storm weakened as it crossed the water between Cuba and the Yucatan peninsula. The bulletin goes on to say that "it is unclear whether the weakening id due to an internal cycle or external forcing. While cirrus outflow has diminished over the southern semicircle there are no obvious signs of shear. Emily continues to generate cold convective tops so there is potential for re-intensification before landfall in the Yucatan."
Fig.3
This image shows a large thunderhead from the circulation around hurricane Dennis. In the image we see the cloud as a wall forming out of a lower mass. At the top of the wall the thunderhead is breaking up into a flurry of cirrocumulus formations that are spreading out across the sky from the top of the cloud. These cirrus formations are the result of a tremendous outflow of warm air from the cloud that is turning into ice at the upper levels. This is cirrus outflow as a result of the strong cooling effects of the convection at the top of the cloud. (cold convective tops). When the thunderheads that surround a hurricane produce such cold convective tops, the storm is considered to be well organized. If a wind were to enter into the cloud and blow aside the currents of rising warm air so that they couldn't rise to the upper layers then that side wind would shear off the top of the cloud and prevent cirrus outflow. Often such shear currents are present in the form of strong easterly currents in the tropics. Lacking these shear currents in the prevailing easterlies the storms can build up to great heights and achieve outflow at the upper levels. The outflow shows up in satellite photos as masses of cauliflower looking clouds swirling around the central eye of a hurricane. The bulletin suggests that since there is still cirrus outflow from Emily that it is still likely that the storm will build strength when it moves across warmer water either before it crosses the Yucatan or afterwards.