By Dr. Jeff Masters From Weather Underground

The traditional busiest month of the Atlantic hurricane season, September, is now over, and we are on the home stretch. Just three weeks remain of the peak danger portion of the season. September 2014 ended up with just two named storms forming–Dolly and Edouard. Since the active hurricane period we are in began in 1995, only one season has seen fewer named storms form in September–1997, with Category 3 Hurricane Erika being the only September storm. Between 1995 – 2014, an average of 4.3 named storms formed in September. With only five named storms so far in 2014, this is the quietest Atlantic hurricane season since 1986, when we also had just five named storms by the beginning of October. In terms of Accumulated Cyclone Energy (ACE), activity in the Atlantic up until October 1 has been only about 43% of the 1981 – 2010 average.

Figure 1. Tracks of Atlantic named storms in 2014. Note how all of this year’s hurricanes (tracks in red) have occurred well north of the tropics, north of 24°N latitude–a testament to how hostile for development conditions have been in the tropics, due to dry, sinking air. Image credit: National Hurricane Center.

Forecast for the remainder of hurricane season

Looking at climatology, since 1995, we have seen an average of 3.6 named storms form in the Atlantic after October 1. Two of those years–2006 and 2002–saw no storms form after October 1. The most post-October 1 storms was eleven, which occurred in 2005–no surprise there! The latest 2-week forecast from the GFS and European models show a continuation of the basic atmospheric pattern we’ve seen over the tropical Atlantic this season, with plenty of dry, sinking air. These conditions should lead to lower than average activity into mid-October, which is when historically, Atlantic hurricane activity begins to drop sharply. I expect we’ll see at least one more named storm in the Atlantic this year, with two a more likely number. It’s unlikely we’ll get three or more post-October 1 named storms.

During October, the focus of Atlantic tropical cyclone genesis shifts to the Western Caribbean, Gulf of Mexico, and the waters between the Bahamas and Bermuda. The Lesser Antilles typically see very few tropical cyclones after October 1, and I expect their hurricane season is over. Sea Surface Temperatures over the Caribbean are currently 0.2°C above average, and 0.4°C above average in the Gulf of Mexico.

Figure 2. Atlantic hurricane activity begins to fall off sharply around mid-October.

Figure 3. Vertical instability over the Caribbean in 2014. The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Normal instability is the black line, and this year’s instability levels are in blue. The atmosphere has been dominated by high pressure and dry, sinking air since June, which has made it difficult for tropical storms to develop, and no tropical depressions or tropical storms have been able to form in the Caribbean this year. Image credit: NOAA/CIRA.

Quiet in the Atlantic

A tropical wave predicted to come off the coast of Africa on Saturday is forecast by the UKMET and GFS models to develop by Monday in the vicinity of the Cape Verde Islands. An upper-level trough of low pressure over the Eastern Atlantic will bring high wind shear to this region early next week, though, making developing difficult. Another major invasion of dry air from the Sahara is currently in progress over the Tropical Atlantic, which will make it difficult for any tropical storms to make the crossing from Africa to the Lesser Antilles intact.

Figure 4. Dust from the Sahara can be seen streaming eastwards across the tropical Atlantic in this September 30, 2014 composite image from the VIIRS instrument on the Suomi spacecraft. Image credit: NOAA Visualization Lab

Eastern Pacific tropical disturbance 90E a heavy rainfall threat

In the Eastern Pacific, an elongated area of disturbed weather (Invest 90E) was located a few hundred miles south of the Pacific coast of Mexico on Wednesday morning, and was headed west-northwest near 10 mph. This disturbance has good support from all three of our top tropical cyclone genesis models to develop this week. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 90E 2-day and 5-day odds of development of 70% and 90%, respectively. 90E is a threat to bring heavy rains to the Pacific coast of Mexico throughout the week. So far, though, 90E’s heavy rains have remained offshore, as seen on satellite loops. Tropical Depression Rachel dissipated a few hundred miles west of Baja, Mexico on Tuesday.

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