Typical July Climatology Surrounding Hurricanes
We have all heard that this years Hurricane Season is suppose to be Normal to Above Normal in terms of events, yet it is July and it is so quiet.. why? Well we thought we’d take a minute or two to explain the current conditions and why this is usually the norm.
First off is the Atlantic Basin, in July we usually see a typical set up of a strong High Pressure and sequentially strong Trade Winds in the Main Development Region (MDR).
The relevant effect of this is usually extremely dry air and dust pushed off the Africa continent.
We have talked a bit about this dust prior in our Hurricanes: What is SAL post, but to recap :
Saharan Air Layer (SAL) – SAL is a dry air mass driven off of Saharan Desert and can sometimes be abundant with dust. This elevated layer of Saharan air and mineral dust occurs during the late spring through early fall over extensive portions of the North Atlantic Ocean between the Sahara Desert, the West Indies, and the United States. Dry air will definitely suppress thunderstorms and weaken tropical cyclones causing downdrafts to occur around the storm and in turn inhibit hurricane development.
Below is an image of SAL taken on July 15th, 2016 the yellow and red show the great amount of dry air / SAL currently in the MDR.
Typically in late summer we should see a shift and weakening of the High Pressure and subsequently a reduction in SAL, however; this is a significant detriment to the development of tropical cyclones and is watched through the entire season.
One of the other factors that usually are a good inhibitor is the presence of Wind Shear in the Gulf and the Caribbean Looking at current conditions though we have some areas of 20+ knots of wind shear its not overwhelming right now .
The one other item worth mentioning is the current positioning of the tropical waves and precipitable water, this is also typical in the early hurricane season. This is noticeable in the SAL image above but you can also see it here in the below satellite pic.
The MDR usually is between 10°N (Red Line) and 20°N. These storms are traveling across the Atlantic and right now feeding the activity in the pacific. This will change as we head into August as the Monsoonal Trough moves northward.
We basically have covered most of the items surrounding why July is usually a slow month for hurricanes but, we do have them historically. In fact we usually don’t look to the East Atlantic for these storms to develop, they usually occur closer to home. Here is a map outlying where storm development occurs by month:
Hurricane Outlooks for 2016
We discussed why July is so slow typically but what about the months going forward. First statement we would like to make is that it’s anybody’s guess. Yes we said ‘guess’ , there are so many competing factors when it comes to storm development, intensity, path, and timing that prediction of hurricanes is a very difficult and volatile especially when we are talking seasonal forecasting. Yet there are some fundamental meteorological and atmospheric conditions that can guide us in understanding what may occur during the 2016 Hurricane Season and we will try to cover them here.
ENSO and the Atlantic Multi-Decadal Oscillation (AMO)
El Nino / La Nina along with sea surface temperatures usually have a dominant influence on the Hurricane season.
Here is what we typically expect for relative to ENSO influences on the Hurricane Season :
As notated in the graphics El Nino / La Nina impacts are primarily caused by changes in the vertical wind shear, which refers to the change in wind speed and direction between roughly 5,000-35,000 ft. above the ground. Strong vertical wind shear can rip a developing hurricane apart, or even prevent it from forming.
Now we can take a look at July’s SST Anomalies (departure from normal) where you can see a huge difference year on year.
********** 2015 El Nino Year **********
************** 2016 CURRENT ***************
As you can see what a change year on year.. we are seeing a more La Nina type pattern emerging as we head into the Hurricane Season but we are currently in a Neutral ENSO for mid July. Here is the outlook prepared in June with respect to an emerging La Nina: (clicking below chart will bring you to the CPC ENSO Page)
The CPC / IRI forecast currently a 71% chance of La Nina
during the prime hurricane months of Aug. Sept. and Oct.
Atlantic Multi-Decadal Oscillation (AMO)
The AMO is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time. Since late 1990’s we have been in a warm phase. During warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases, at least twice as many per NOAA. The switch form Warm phase to Cold phase is hard to predict and NOAA has noted its uncertainty relative to the AMO for the 2016 Hurricane Season.
Current SST from OSPO NOAA
Looking at current SSTs does look as the Atlantic is primed for the Hurricane Season. Hurricanes usually like temps of 79°F (26°C) or greater that definitely doesn’t look like an issue here.
In synopsis a “La Nina does favor a stronger Hurricane Season by producing more conducive atmospheric conditions within the MDR, including 1) weaker vertical wind shear resulting primarily from weaker upper-level westerly winds, 2) decreased sinking motion, and 3) decreased stability. However, the overall hurricane season strength depends not only on La Niña, but also on the phase of the AMO. If La Niña develops, an above-normal season would be much more likely if the warm AMO signal also appears. A near-normal season would be most likely if La Niña develops but the warm AMO signal does not appear.”
NOAA 2016 Hurricane Outlook
So what we mentioned earlier about good ‘guess..’ it still applies here.. why.. because all this information just really just tells us about the chances of hurricane developing and how active the season will be. IT IS NOT A LAND FALLING FORECAST ! Realistically you live in the Mid Atlantic region ( our assumption ) and well historically we don’t have lot of land falling hurricanes but it doesn’t mean we shouldn’t be prepared. We do, and have had impacts from tropical systems. July is the slow month and should be used as preparation of a ‘What If..” scenario. Most of our region will shelter place but if you live along the Coast and or Delaware River know your evacuation routes have a plan developed that you an your family can execute and rely on in case of impacts. Visit www.ready.gov to create a plan.