JACKSONVILLE, Fla. – Wind shear is quantified as the variation of wind speed and direction from lower altitude winds and higher altitude winds. Typically defined by the difference in wind speed and direction from 5,000′ and 35,000′. In the tropics, wind shear greater than 30 mph typically weakens (and it can be sudden and dramatic) tropical systems. When we see these high values over the Main Development Region (MDR) for hurricanes and tropical storms, basically over the Central Atlantic Ocean to the Bahamas, tropical cyclone formation is suppressed, we saw this with Isaias. Isaias failed to intensify when near Florida ( a good thing ) as winds moving of the Florida coast were westerly, up to 50 mph.
Why does it weaken tropical systems?
Well, remember what the “eye” of a hurricane looks like? Like a giant rotating hole in the clouds, these clouds create this look because they are all in unison, spinning vertically stacked from just above the surface of the ocean all the way up to +60,000 feet. If however, there is a strong wind across the top (sometimes the lower part) of a hurricane, the hurricane tends to “tilt”. Wind shear greater than 30 mph tends to disrupt the vertical development of the storms near the “eye” so much, that dry air entrains (mixes) into the system and causes it to collapse.
Often times we see this on the satellite view as “pulsating” storms. There is a big eruption of storms within the tropical system that is very transitory (short lived). The impact on the satellite presentation can be very deceiving, as the tropical system can appear to be really developing, when in fact it is not. This is something we saw constantly with Isaias. Isaias constantly had wind shear up to 50 mph near it and only became a hurricane when it began to travel northeastward (towards the Carolina Coast) and moving with the upper-level winds.
Back to what’s happening now and what is expected to take place over the next week. On these pictures, the yellow areas represent wind shear (winds greater than 30 mph), the red areas represent winds greater than 45 mph. Anywhere in red would typically mean that should a hurricane move into that region of high wind shear, the life time of the hurricane would be very short.
On the other hand, areas of no yellow-orange-red are regions of light wind shear (bad news as storms can develop quickly).
Look at these two pictures closely, the yellow-orange-red really shrinks between right now and next week.
This has to be considered a bad sign, as tropical storm, more importantly, hurricane development will become more and more likely.