Posted by: annemartinfletcher | August 25, 2011

How to Analyze Hurricane Computer Models

How to Analyze Hurricane Computer Models

This is the second in a series of posts on things the military taught me that all citizens should know. Thanks to the U.S. taxpayers, I earned a BS and an MS in Operations Research in the military. Operations Researchers construct and analyze mathematical models to solve real world problems. This education helps me to understand the Hurricane Computer Model results that are distributed by the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS), and used by TV weather forecasters. As I sit here in my barrier island home, street level 5’ MSL and house level 9’ MSL, occasionally lashed by the first, mild rain showers spun off from Hurricane Irene, I want to share how I make my decision to stay or evacuate.

First of all, the graphic most frequently shown on TV, the Tracking Map from Weather Underground or the 5 Day Forecast Cone for Hurricane Irene from NWS, is virtually worthless for deciding whether to evacuate. Unless, of course, you live on a barrier island in NC, in which case you are already packing.

This is a terrible graphic to use on TV because most viewers assume that the cone shows the entire width of the storm, and where that storm is going to go. FALSE! The EYE of the storm has a 68% chance of being somewhere inside the cone. The statisticians among you know that 68% equates to one standard deviation.  There is a 32% probability that the eye will NOT stay within this cone. The cone helps viewers estimate if they will be hit with the more dangerous quadrant of the storm, the right side. To estimate the actual width of the storm’s effects, assume the eye is on the closest part of the cone, and then extend it towards you by the width of the storm. Fortunately, NOAA/NWS does this for us with some more meaningful maps.

For all of the NOAA hurricane graphics, be sure that you select the timeframe most pertinent to your location. For me and Hurricane Irene, this Thursday evening I am looking at the 24 to 48 hours forecast. New Jersey should be looking at the 108 to 120 hour (5 day) forecast. Check back frequently, because the models are updated with increasingly accurate predictions as the storm approaches your location.

Next, I need to decide whether to go through the hassle of putting up my house’s storm shutters. My personal preference is to install the shutters if the winds are predicted to reach 50 Knots. Imagine a truck traveling at 58 mph and sending a pebble against your car’s windscreen — that is how fast debris will be flying in a 50 knot wind. Plus, my windows will be covered if the storm strengthens or turns towards my location.  Luckily, NOAA/NWS provides a graphic showing the probability of being hit by 50 Knot winds.

For Irene, there is less than a 5% probability of my location experiencing steady winds of 50 Knots. For Wilmington, NC, however, there is a 70% chance that they will experience 50 Knot winds.

After deciding not to install my hurricane shutters, I still need to decide whether to evacuate. I am a hurricane magnet. I was in Miami going through simulator training during the approach and aftermath of Hurricane Andrew. Why my superior officer made these poor decisions is a story for a different blog post. Hurricane Hugo severely damaged my house 30 miles inland from Charleston. Hurricane Erin strengthened during the night and suddenly swerved north to swipe a beach condo I was in at Panama City Beach. I have enough experience with these storms to know that if you are high enough to avoid the storm surge and protected enough to survive a spawned tornado, then the actual storm is not the worst part. The worst part is when the storm is over but the roads are blocked by trees and downed electrical wires, and you have to live for three to five days (or more) without water, sewer system, air conditioning, and electricity. Of course the weather will be muggy and buggy. This long story is my way of saying that I evacuate my island home “even” for a category I hurricane.

Don’t wait for evacuation orders. If you do, you are likely to be stuck in long lines of traffic and all the inland hotels will be full.  Oh, joy, creeping along a highway at 5 mph with several thousands of your closest friends. Hope you have gas, water and snacks. Make your decision to evacuate even before the hurricane warning is announced for your location.

How can you tell if some part of the hurricane, other than the eye, has a probability of hitting your location, before the hurricane warnings go out? Go to the NOAA website and look at the Hurricane Force Wind Speed Probability. This is the graphic that I wish the TV weather broadcasters would use. My location has less than a 5% chance of experiencing hurricane force winds, but Wilmington, NC, has a 30 to 40% chance of getting hurricane force winds. Unlike Hurricane warnings, this graphic gives you an idea of what to expect five days out. Just keep updating it.

My last consideration about whether to evacuate is based on tide levels. It is very pleasant to live on an island—not so much to have my house BE the island. With the right combination of storm surge, high tide, and heavy rain, the water will be lapping at my eight and a half foot foundation. NOAA/NWS has a new, experimental model that graphically predicts storm surge. My location has about a 35% probability of seeing a two foot surge. That won’t effect me — unless it is combined with the high tide. I check out the local NOAA tide charts and compare high tide times with that Forecast Cone model, telling me when, approximately, Irene will be near our location. Lucky me, it will be closer to low tide than high tide. For this storm, I will wait in place.

Update: this is the Saturday storm surge graphic for NYC and NJ. At the top of the graphic, I selected the probability of the area receiving a 4 foot or higher storm surge within the model’s time frame. It shows that much of NYC has over a 50% chance of experiencing a four foot or higher storm surge, while the Brunswick area has an 80% chance or more of seeing over a 4 foot storm surge. Localized flooding can be higher, depending on rain, tide, and the canyon effect of buildings or sea walls. To update this graphic or adjust the scale to see your area, please go to this link.

I hope this analysis helps you folks farther north make informed, and EARLY, decisions, using the NOAA/NWS weather models, about whether to evacuate. Of course, if the local authorities tell you to leave, I recommend you follow their advice. Good luck, everybody!

Resources:

National Hurricane Center.

http://www.nhc.noaa.gov/refresh/graphics_at4+shtml/211943.shtml?50wind096?large#contents 

http://www.nhc.noaa.gov/refresh/graphics_at4+shtml/211943.shtml?hwind120?large#contents

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Responses

  1. Update: I added a direct link to the model that depicts the probabilities of various size storm surges.

  2. I followed your information with great interest. Thank you. Aaron from NJ

  3. Aaron, you are more than welcome. Hope you stay dry!

  4. Anne, very nice and informative.

  5. Anne, excellent post! I used a very similar process for here in Jacksonville, Florida. Interestingly, I was in Panama City, Florida just as you were for Erin (I evacuated, my wife was TDY to Keesler). Here in Jacksonville I don’t have to worry so much about the storm surge (the city is 20 miles inland) but are more concerned about the winds. Not only will high winds topple the ancient oaks around here, but they close the city bridges at 40 mph. Since I live on the south side of the city, I have to evacuate early to beat the winds and bridge closings (very long drive to evacuate without crossing a bridge). Fortunately, Irene passed us by with only a few rain showers. I got to see it from the air, as I flew by on a leg from Orlando to Charlotte just as the hurricane passed Jax.

  6. Daniel,
    Welcome to my blog! You made the wiser choice that night in Panama City. Are you dual military? (don’t answer if you are not comfortable with everyone seeing the answer.) Or perhaps USAir?

  7. Wow…and my dream as a child was to become a weather man.
    This stuff looks so complicating!
    Thank you, Anne for a very interesting post.

    • Thank you, Mark. I took a look at your site and liked it so much that I added a link to in my sidebar under “Women in Military or Aviation.” I called the link a “Primer on Becoming a Pilot.” I hope my readers will check out your website.

      • Wow. Thank you very much, Anne. I am very flattered and grateful.
        It is great to meet another person so passionate about aviation, and it is reflected in the quality of your posts.

        Looking forward to being an avid reader of your writing,
        Mark

  8. […] how I make my decisions, you can read a previous post on how to interpret computer models at this link. Additionally, I looked at the “Spaghetti Models” which are simulations based on […]


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