BREAKING: Isaac Is Declared A Category 1 Hurricane But Size And Slow Movement Poses Serious Threat

Courtesy of NOAA

Isaac Has Had A Whole Host Of Problems To Contend With

Well Isaac has just been declared a 75 mph, category 1 hurricane despite the look of one over the past 12 hours with an eye and eyewall feature with substantial convection circling this inner core. The issue has been that although winds aloft (at flight level) have been supporting hurricane-force conditions with winds during the last recon mission recorded at 102 mph, winds of only tropical storm force were recorded nearer the surface and so that is the NHC’s reason for not declaring this storm a hurricane.

That forces me to ask the question, does that mean there is a much weaker low level circulation than in the mid and upper levels? Perhaps, and that could be due to dry air partially eroding the surface low. Remember, these systems system draw air in and upwards within the centre and it you have a layer of air air flowing in, that chokes of the rapid upward motion needed to support regenerative ‘hot towers’. Hurricanes have vertically stacked surface, mid and upper level low pressure systems which make up the warm core, the engine. My suspiction is that part of the lower core has been missing or weakened by that persistent feed of dry air wrapping itself around the circulation. You’ll have noticed that generally at any given time, part of the storm had a lack of convection, even cloud, that was the dry air.

Bundling A Huge Circulation Takes Time

As well as persistent dry air to contend with, the shear size of this system went against the likihood of strengthening any time soon. The vast energy field can take a long time to draw inwards and consolidation takes a lot of time. The fact that Isaac was drawing in dry air and trying to bundle, this meant double the effort for this storm strengthening.

There is a constant cycle of air being sucked inwards, constantly forming rings of convective towers around the core. The deeper the convection within the core, the more pressures fall within the centre and the faster the air gets drawn in from all directions, known as inflow. The more this process evolves, the faster the rate in which air travels in, up and out from the top of the storm, this is known as outflow. as pressures lower, so the rate of air spinning around the centre increases and eventually this solid wall of thunderstorms will break away from each other forming a clear slot known as the eye.

This process has now been completed allowing Isaac to become a hurricane.

What Can A Big Storm Do That A Small One Can’t

In Isaac’s case, just like was Katrina, Rita and Ike, the storm was very large and so too was the windfield. Hurricane put tremendous force down on ocean surfaces and as they travel towards a given area, they push down on the water, generating a small wave. This small wave may raise the sea level in a region downwind anywhere from 1 foot to 20 feet. However, while not all hurricane provide a massive wall of water as they come ashore, the larger the hurricane, the larger this surge often gets. With Charley in 2004, the storm was very small so it didn’t grab a vast area of ocean and help build a wave. As for Katrina, Rita and Ike, not only where these powerful storm out at sea, which helps builds heavy seas and that wave but the size of the circulation means impact across a larger area.

Katrina was a perfect example of a doomsday scanario of size and intensity. The day prior to Katrina making landfall, the storm supported winds of 175 mph, this combined with vast size meant a vast wall of water was coming with Katrina onshore along the Gulf Coast. In fact a storm surge along the Mississippi coast was estimated at 30 feet and this was down to intensity out in the Gulf and vast size.

As for Isaac, while not a strong system in terms of wind. The size does mean a 6-12 foot wall of water coming ashore later tonight into the very areas impacted 7 years ago tomorrow by Katrina.

48-72 Hour Downpour Could Dump Over 24 inches of Rain

The other major issue with Isaac other than potential storm surge flooding is flooding caused by excessive rains. The crawl of Isaac onshore will be painstaking with it taking perhaps a full 12 hours to go from the Mouth of the Mississippi up to New Orleans. With this slow movement and vast size of the storm, this could bring catastrophic flooding in itself, never mind wind or surge. The city of New Orleans can’t worry solely on surge but the 10-20 inch rains which will fall. These torrential, persistent, 48 hour rains will keep those pumps working hard that’s for sure.

Inland areas could see significant flooding from this.

Courtesy of NOAA

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