Question:
Why would Tropical Storm Patty form if sea surface temperatures are in the 70s? I thought a minimum of 80 degrees was required for formation.
Answer:
Anytime you want to know more about a current (or recent) tropical storm, I recommend checking out this link: https://www.nhc.noaa.
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The 9am discussion on November 2 mentions that Patty had non-tropical origins, starting as a low pressure system over cooler water. They also say:
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“The low has become detached from fronts and has a shallow warm-core structure, though it remains within a cooler airmass behind a cold front over the eastern Atlantic. Despite SSTs around 21 deg C, instability aloft has allowed the system to sustain some moderate convection that wraps most of the way around its center in geostationary and passive microwave images.”
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The forecasters mention that sea surface temperatures (SSTs) are low for a tropical storm, but instability allows for enough lift to create condensation and clouds, plus the latent heating that fuels tropical storms. Instability comes from the difference in temperatures between lower and upper levels of the atmosphere – the cooler the temperatures aloft are compared to the surface, the greater the lift.
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“Little change in strength is expected today, and Patty is forecast to be a fairly short-lived subtropical cyclone. This is because westerly shear is forecast to increase over the system during the next couple of days, which will likely make it difficult for the system to sustain convection near and around its center.”
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Here the forecasters mention why they believe that Patty won’t become a major hurricane. Fascinating stuff! I learn more from these subject experts every time I read a discussion.
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3 am discussion on November 4Â mentions:
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“Patty has continued producing deep convection since the time of the previous advisory, although the convective structure has recently degraded slightly on the latest infrared images. The infrared satellite images and an 03/2138 UTC ASCAT pass depict a more compact cyclone with a confined radius of maximum winds than earlier in the system’s life. Patty is also no longer co-located with the upper-level low that was earlier responsible for its hybrid characteristics. Based on these observations, Patty has made the transition into a tropical storm. The earlier ASCAT pass showed tropical storm force winds as high as 39 kt in the southern semicircle. The initial intensity is therefore held at 40 kt.”
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I’m guessing that because Patty became smaller (conservation of angular momentum: the ice skater spins faster when she brings her arms in, reducing the radius) and moved away from the cold core low pressure system, it can now take advantage of temperature differences between the sea and higher in the atmosphere. Also, if there’s less wind shear (which I would expect as Patty gets further from the upper-level low), the tropical storm can continue to create convection and stay together as a storm.Â
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- Tropical Storm Patty initially formed from an upper-level low or cold-core system, which can bring colder temperatures in the upper atmosphere. The contrast between the cooler upper-level temperatures and the relatively warm sea surface can create enough instability for convection (thunderstorm activity) to persist and organize into a tropical system.
- This process is more common in subtropical storms, which can transition to fully tropical systems if they acquire enough tropical characteristics, like a warm core, organized convection, and a more symmetric structure.
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For more explanation on what’s going on, keep reading the discussions – these forecasters specialize in tropical weather and have the most recent knowledge of the subject.
