A mid-Atlantic tropical wave worth watching; more on this year's steering currents

A tropical wave in the mid-Atlantic, near 9N 38W, has grown more organized since yesterday. This system has been labeled "96L" by the NHC. The wave has a small closed circulation, as seen on both visible satellite loops and this morning's 4:28am EDT QuikSCAT pass. Winds from QuikSCAT were as high as 35 mph. Wind shear is about 10 knots, and is forecast to fluctuate between 10 and 20 knots in the region over the next two days. This is low enough wind shear to allow some slow development. Sea surface temperatures are 27-28 C, which is above the 26 C minimum temperature tropical storms typically need to form. There is one cluster of strong thunderstorms near the center of circulation, but dry air to the north appears to be limiting the thunderstorm activity. The Saharan Air Layer (SAL) lies just 50-100 miles north of the storm's center of circulation, as seen in water vapor satellite loops. The GFS model does indicate a tropical depression might form here, but does not have a very good handle on it, since it is showing far too slow of a forward speed. Our other three reliable models, the NOGAPS, UKMET, and ECMWF, do not develop the system. Climatologically, formation of a tropical depression in this region of the Atlantic this time of year is quite rare. Given this fact, plus the presence of so much dry air near a relatively small circulation, I am not expecting this to become a tropical depression. Movement of 96L will be just north of due west over the next few days at 15 mph, as seen in the model forecast plots (Figure 1).


Figure 1. Computer model forecast tracks for 96L.

More on steering currents for this hurricane season
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.

Jeff Masters