The steering current pattern for June featured a pattern much like we saw in 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. The position of the Bermuda-Azores High (Figure 6) was pretty close to average. Its strength was only 1 mb below average, driving slightly slower trade winds than average across the tropical Atlantic. I expect this pattern to continue for the first half of July, and the troughs should be frequent enough and strong enough to recurve any tropical storms or hurricanes that penetrate north of the Caribbean Sea. Steering current patterns are not predictable more than about two weeks in advance, and there is no telling if we are in for a repeat of the favorable 2006 steering current pattern that recurved every storm out to sea. It is encouraging to note that in 2006 the steering current pattern locked into place in late May and stayed that way for almost the entirety of the hurricane season. The atmosphere often stays locked in to a particular steering pattern for an entire summer, and it would not be a surprise if that occurred again this year. If this pattern holds, expect a below-average chance of hurricane landfalls along the U.S. Gulf Coast, and normal to above normal chances along the U.S. East Coast.
Complete update
Just wanted to add another section on this years steering currents written by jeff masters.
Steering currents for 2007 part2
Yesterday, I posted my bi-monthly hurricane outlook, for the first half of July. Since it was getting a bit long, I presented only a short steering current analysis. More follows here. There are several ways to look at steering currents. I presented the position of the surface high pressure system known as the Bermuda High (or Bermuda-Azores High). Another way is to study how close to the surface a pressure of 500 millibars (mb) is found. When there is low pressure aloft, due to a trough of low pressure, the height at which a pressure of 500 millibars is found moves closer to the surface. If one plots up the "500 mb height anomaly"--the difference of where a pressure of 500 mb is found above the surface, compared to the average height from a climatology of the past 30 years--one gets a good measure of where above or below average storminess occurred. Higher than average 500 mb heights imply less storminess and possible drought conditions. The 500 mb height anomaly plot for June 2007 (Figure 2) shows higher than average heights across the southwestern U.S., where drought and high temperatures were observed in June. Lower than average 500 mb heights imply an above normal preponderance of troughs of low pressure and thus storminess. This was the case over Texas and Oklahoma in June. If these troughs are over the Atlantic, they act to recurve hurricanes out to sea at the longitude they are at. This only occurs if a hurricane penetrates far enough north to "see" the southernmost part of the trough of low pressure. Typically, this happens northward of about 20 degrees north latitude. Figure 2 shows lower than average 500 mb heights occurred over most of the Atlantic, meaning there were many more troughs of low pressure than usual. Had any hurricanes occurred over the Atlantic north of about 20 degrees north latitude, they would have gotten caught up in one of the troughs and recurved out to sea. The latest 2-week forecast from the GFS model shows a continuation of this above average frequency and intensity of troughs of low pressure over the Atlantic--much like we saw in 2006. Thus, we can expect any tropical cyclones that penetrate north of about 20 degrees north latitude to get recurved. This will very likely be the case for 96L, if it ever becomes a tropical storm.
Figure 2. Difference in height (in decameters, or tens of meters) from average of the 500 millibar height above the surface for June 2007. Image credit: NOAA/ESRL.
