Tuesday, August 29, 2023

Hurricane Franklin and Tropical Storm Idalia: August 28, Update A

I'll sum up some wrap-up thoughts from last week on the US west coast some other day. Today, it's once more unto the breach dear friends, and over to the Atlantic and Gulf of Mexico where the game's afoot. Are your supplies ready? Water? Candles? Ice Cream? Wine?

Tropical Storm Idalia

She is getting stronger as I'm sure everyone in Florida and the southeastern US are already aware... 

She's currently at 22.0N, 85.0W, heading N at a stately 8 mph. Her track is bringing her into Florida, but the center could be anywhere in that cone so don't focus on the center of it. We are still over 1.5 days away and that could shift slightly.

One of the big issues is going to be storm surge along that coast of Florida and the amount of surge depends on the track, but also on a number of other things.

<Science Alert!> Storm surge is water rising along the coastline as a storm approaches and makes landfall. How high the storm surge is depends on a number of factors: 

1. Location at Landfall. Storms are low pressure systems, so in the northern hemisphere they rotate in an anticlockwise direction.  This means generally water will be pushed onshore on the right side of the eye (eastern or southern side depending on if the coast runs E-W or N-S) side of the eye, and will be pushed off shore on the left side (western or northern side depending on coast orientation). In this case, if Idalia makes landfall just to the north of Tampa Bay (for example), she will push water up into the bay and storm surge will be higher. 

2. Intensity. Storms are low pressure systems, which means that there is low atmospheric pressure in the middle. But obviously we don't have a low pressure "hole" between the air and the ocean. Instead the water 'rises' up to fill the 'gap' which is created by the atmosphere's low pressure. So, the stronger the storm is, the lower the central pressure, the greater the 'gap' for the water to rise to 'fill' and therefore the higher the storm surge. 

3. Speed of the storm. If a storm is moving quickly, then there is simply less time for water to get pushed onshore before the storm has swung by. Storm surge is greater for slower storms. 

4. Angle of storm approach to land. Depending on if a storm is approaching land directly or at an angle can affect the amount of storm surge because of the direction of the winds. Storms that are heading directly onto shore result in greater storm surge than those that are approaching it from an angle - skirting along the coastline. 

5. Shape of coastline (bays etc). Water can get piled into bays and upstream in estuaries of course. 

6. The depth of the seafloor just off the coast is important. For areas with a shallow seafloor, more water piles up compared to areas where the seafloor is deep just off the coast. 
<End Science Alert!>

For those who need a reminder, I use NOAA's tidesandcurrents website. For those who haven't seen this before, I'll cover how to find out what you need to know in tomorrow's post. 

Idalia is currently a Tropical Storm with winds of 70mph, central pressure 983mb, which makes her a very strong Tropical Storm (TS range: 39 - 73mph)... actually, I think she is already a hurricane because her circulation (vorticity) is already present throughout the troposphere.  She just clipped Cuba but that wasn't enough to inhibit her development: 

She looks like she has a lot of convection - that's because she is moving over a part of the ocean which is very warm, with sea surface temperatures over 30 deg C. But not only is the surface hot, the upper 125 meters of ocean is warmer than 26 deg C - so anything she is churning up is definitley feeding her! Her blossoming like this in this part of the world is not unusual though, because she is going over the Loop Current System... yes, squeezing in another <Science Alert!>. :-)  

<Science and Forecasting Alert!> The Loop Current (System). The Loop Current is part of an ocean current system that flows from the Caribbean into the Gulf of Mexico through the Yucatan Strait, and then out of the Gulf of Mexico through the Straits of Florida, and up the east coast of the US. How far it extends into the Gulf of Mexico varies from year to year and month to month. As this current flows around Florida, it changes name to become the Florida Current. Then as it flows up the east coast, it changes name again and is known as the Gulf Stream. It leaves the US east coast around North Carolina and flows across the north Atlantic to the UK. Why is this current system important for tropical storms? This current system is well known because it has the deepest warm waters, and is very fast flowing - but the current also has offspring that can also have deep warm waters (all these offspring are called Eddy by the way). The deep warm waters mean that tropical storms that pass over any part of this current system or over any warm water Eddies have a jolly good (British understatement) chance of becoming stronger. 

So, how can you see where these areas are? One of the places I look is buried within the vast network of websites that NOAA has. In particular, the Office of Satellite and Product Operations has a site called Satellite Heat Content Suite (I know, such a catchy name - made for a song lyric I'm sure! ;-)). I use the North Atlantic page: 


If you click on this, you will see all sorts of maps. The two most useful for our purposes is the one on the top left - Sea Surface Temperature - which looks like this today: 

You can see that the waters where Idalia is are the darkest of dark reds - over 30 deg C. The other most useful map (more so than the sea surface temperatures actually) is the depth of the 26 deg C Isotherm - third one down in the right column and that looks like this today:

From this, we can see that Idalia has been passing over that area of yellow/green just north of the western tip of Cuba - this is where the upper 125m of water is warmer than 26 deg C. No wonder she picked up some convection. <End Science and Forecasting Alert>

The current forecast is for a major hurricane at landfall on Wednesday. A major hurricane is a cat 3 or higher storm (111mph winds or higher). At the moment, I don't see anything that will inhibit her from growing in the next few hours - the water is toasty, and there isn't much wind shear around. There is a bit of dry air to her northwest, over the Gulf, but I'm not sure it's close enough to combat that convection. However, the 26 deg C depth does get shallower as she moves northwards, so there is a possibility that will help to keep her growth in check - but to decrease her intensity requires strong wind shear as well, so that's what I will look for tomorrow. 

Hurricane Franklin

Just a quick note on Franklin, who is in the Atlantic at 29.4N, 71.0W, heading N at 9mph. He is already a major cat 4 hurricane with winds of 150mph, central pressure of 926 mb (Cat 4 range: 130-156mph), but is going to avoid Bermuda and stay away from the eastern seaboard as well: 

He's a good looking chap (what else would you expect with a storm that strong?), but not a big fella... 

Bermuda will get some rain, a bit of a breeze, maybe not a golfing day on Wednesday, but hopefully nothing too bad. 

For those in Florida - get ready and listen to your emergency managers - they know the area the best. If you are in an area that floods easily (yes, I know we are talking about Flat Florida - hard not to get some flooding!), then please evacuate if you are told to. 

Until tomorrow!

Toodle pip,




These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and local weather service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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