By: Lead Forecaster Daniel Crawley
Today, September 10 is the climatological peak of the hurricane season in the Atlantic Basin. But if you have been following the weather over the past 1-2 weeks, its been very quiet for the peak of the season.
People have asked us…Why are the tropics quiet?
Well, there are several factors at play that is keeping the basin relatively calm. This blog post will hopefully break down some of those factors.
Hostile conditions currently throughout the Atlantic Basin
There are a lot of things going on right now that is a deterrent to tropical development throughout the basin. If you look at the water vapor we have marked a few of those things. First…notice the overall lack of water vapor in the Main Development Region (MDR). Dry air aloft can prevent tropical development. Last week was a great example of that as the former Invest 91L could never organize due to dry air surrounding the environment.
The second factor lies in a massive upper trough across the Eastern United States and a Tropospheric Upper Trough (TUTT). Those features are supplying some wind shear that prevents any storms from organizing.
If you look at the 500 mb pattern below, strong westerlies lie in the mid-latitudes and the lack of a prevailing Western Atlantic Ridge is keeping wind flow very light down into the MDR. Look at where we highlighted the area off the coast of Africa and westward through the Atlantic, the winds are variable in nature.
The pattern right now is just very hard to get convergence to occur in the lower levels of the atmosphere. You need that as the genesis of tropical development.
Atlantic Basin sits in unfavorable global teleconnection
So, on top of the baseline conditions in the Atlantic basin, we also are dealing with an unfavorable global teleconnection. Some of this we’re about to explain may be a bit much in terms of weather-geek terms but we’ll do our best to simplify it.
The Madden-Julian Oscillation (MJO) is a global teleconnection that signifies which regions near the equator experience enhanced upward motion (lift). The MJO entered phase 5 and 6 (Western Pacific Basin) when Erin departed last month and since then, the MJO has slowly wondered into the null phase. The expectation is for the MJO is to enter into Phase 8 and 1 over the next 1-2 weeks. By looking at the graphic below, 8, 1 and eventually 2 supports better overall atmospheric lift in the Eastern Pacific and Western Atlantic Basin.
An MJO in the 1 and 2 phase can work as a catalyst to promote tropical activity and overcome some of the current deterrents (dry air) that already exists.
Does computer model guidance indicate us going into favorable MJO?
The answer to that is…yes. However, we are still looking at a process that is a good 10-14 days away. If you look at the graphic below, the area in brown indicates where sinking air is present. The Western Caribbean, Gulf Coast and MDR region is definitely in a Phase 6 / Null phase of the MJO currently.
One week from now you can begin to see a little improvement throughout the Atlantic Basin with a hot-spot in the lower latitudes of the Pacific. This is a phase 8 look as the Pacific is prime from an uptick and working across Central America. Given the current baseline conditions, I think the Atlantic Basin will remain fairly quiet through mid next week.
By Day 12 (Monday Sept 22) that pulse of the MJO has transitioned across the Caribbean and into the MDR across the Atlantic, a Phase 1/2 look. One would believe that the current deterrents in the Atlantic should be negated at this point giving the entire basin to perk up in activity for the last 7-10 days of September.
Summary
For right now, expect continued quiet conditions overall in the tropics. Precipitation anomalies are quite low for week 1 as the Eastern US trough will prevent any significant development. The MDR may get more active in terms of moisture days 7-10 along with the Western Caribbean.
The same applies to the timeframe of Sept 18-25 as there are no obvious signs of a tropical threat to the US.
