Governor Rick Scott declared a state of emergency in Florida this morning in anticipation of Tropical Storm Erika’s arrival this weekend and early next week. The storm’s disorganized nature and erratic motion is making it a nightmare to forecast. Here’s what you need to know about Erika as it draws closer to the U.S.

A State of Emergency Sounds Worse Than It Is

Governors frequently declare states of emergency during natural disasters like hurricanes, tornadoes, floods, earthquakes, wildfires, or any number of other awful things that can cause significant amounts of damage and human suffering. A “state of emergency” makes it sound like the state is falling apart at the seams—we are talking about Florida, after all, so it’s doing that anyway—but this declaration is a formality that frees up resources and funds to deal with the disaster. It also clears the way for the Federal Emergency Management Agency (FEMA) to get involved in dealing with preparations and the aftermath, because the states often don’t have the equipment or funds to handle disasters on their own.

The Current Forecast

Above is the current forecast from the National Hurricane Center (NHC), such as it is. No offense to the good men and women at the NHC, of course, but this is a very hard forecast to nail. Don’t focus too much on the exact track right now—Tropical Storm Erika is probably not going to do what they expect it to do.

We demand perfection from an imperfect science. I know that it has to be frustrating as hell to live in Miami or Key West right now and look at this forecast knowing that the storm more than likely won’t do what the forecast is showing. This is an atypical storm that both humans and computers are having a hard time figuring out.

Florida is still firmly within the cone of uncertainty—not to mention a growing chance of tropical storm force winds, as shown above—so it’s necessary for anyone in or near the Sunshine State to keep a very close eye on the storm’s developments (or lack thereof) through the weekend. Your best course of action—forecast or not—is to prepare for the potential of a tropical cyclone that will produce gusty winds, very heavy rain that leads to flooding, and power outages. It’s better to be ready for a non-event than it is to be caught off-guard by a strong storm.

If the storm survives Hispaniola and ever actually makes that northwestward turn, the water is plenty warm for this thing to start strengthening, and it would eventually move toward the United States.

It’s Battling Dry Air and Wind Shear

For its entire life, the storm has battled the scourge of dry air and wind shear. Dry air was a significant problem when Erika first formed—as with Danny a few days before it—but Erika is a larger storm and proved surprisingly resilient to the lack of moisture. There’s still a good bit of dry air to the west of the storm, shown on the water vapor image above as warmer, brownish/orange colors. Dry air isn’t good for these systems. Tropical cyclones, by their very nature, need ample moisture to thrive.

Wind shear is playing the most significant role in Erika’s struggle these days. Unlike severe thunderstorms on land, which feed much of their energy from wind shear through the atmosphere, tropical cyclones need to be the only game in town. Any wind shear—strong winds in the middle or upper levels—will both knock the tops off the thunderstorms and blow existing thunderstorms away from the center of circulation. This is why we so often see lopsided storms in the Atlantic Ocean, and it’s why Erika’s center is almost always removed from the thunderstorm activity that makes the tropical storm a storm at all.

The Storm Keeps Faking Us Out

It’s been a while since we’ve had a storm that’s so messed up that it continuously moved off its predicted course. The storm hasn’t followed the National Hurricane Center’s official forecast since Thursday morning, with the center leaping west every time experts expected it to start moving northwest and pass through the Greater Antilles.

Why is it doing that? It’s weak and disorganized. The strength of the storm determines how the winds are able to steer a tropical cyclone. A stronger storm—one that has deeper thunderstorms surrounding its circulation—is able to tap into deeper wind currents higher up in the atmosphere. Weaker storms are shallow, so they’re driven by the steering currents closer to the surface.

Both the experts and the computer models have based Erika’s forecasts on the storm’s ability to strengthen and maintain its organization long enough to tap the steering currents in the mid- and upper-levels of the atmosphere. Those winds would take the storm northwest through the Bahamas and eventually toward the United States.

Instead, since the storm is such a mess, it keeps heading west in the lower-level winds blowing from the east. Tropical Storm Erika is like a boat adrift in the doldrums with a broken sail. They keep thinking the sail is going to fly skyward and take the thing northwest, but it’s just not happening. Not yet, anyway.

This tropical storm is a classic example of why the cone of uncertainty is so important.

Cone of Uncertainty

It’s been a long time since a storm has threatened this part of the United States, and many people don’t know what the cone of uncertainty is. It’s not their fault—we just don’t do a very good job of explaining it.

The cone of uncertainty is the margin of error in the forecast track of a tropical cyclone. The cones vary in color—the cone appears translucent white on my maps, but The Weather Channel’s cones are red—but they all mean the same thing: weather forecasts are imperfect, and while meteorologists are pretty good at figuring out the general direction in which a cyclone will go, it’s hard to predict the exact track a cyclone will take in a couple of days.

Forecasters are better at predicting the track of a cyclone one or two days out than they are predicting its track four or five days in advance. As such, the error in their forecasts (and therefore the uncertainty in the forecast) grows with time. These track forecasts have gotten pretty good in recent years, but they still have lots of room for improvement.

The above map shows the forecast for Hurricane Ike as it swirled in the middle of the Gulf of Mexico back in September 2008. The cone of uncertainty was very large for that storm—they had a pretty good idea that the storm would make landfall near Houston, but given their previous forecast track errors, they could be wrong and the storm could have wound up going anywhere from Corpus Christi to Lake Charles, Louisiana, or strayed even farther than that.

The cone of uncertainty is much smaller these days—a testament to improved forecast models and greater expertise among the meteorologists at the agency—but things don’t always go as planned.

Historically, the center of a tropical cyclone will stay within the cone of uncertainty 66% of the time. Five days out, the average track error is 240 miles on either side of the forecast point. This means, on average, the National Hurricane Center’s forecast position for the center of the tropical cyclone five days out is within 240 miles of its actual location 66% of the time.

Not bad! However, Tropical Storm Erika is the 34%.

The above map shows six forecast points for Tropical Storm Erika between Tuesday morning and this morning. Each X represents where the center of the storm was located at the time of the advisory, and each cone moving away from the center represents the National Hurricane Center’s cone of uncertainty surrounding their forecast track at the time.

Things started to fall apart on Wednesday night, when Erika started moving much farther west than forecast. It was originally going to brush the Leeward Islands and move north of the Greater Antilles. Then it crossed the Leeward Islands and was forecast to graze Puerto Rico. Then it was forecast to hit Puerto Rico, but instead it skirted south of the island territory. Then it was forecast to cross the eastern Dominican Republic, so obviously it went south of the Dominican Republic, where it sits and spins as of the writing of this post.

Pretty rough. The center of storms historically stay in that cone two-thirds of the time, but there are cases—like Tropical Storm Erika—where that just doesn’t happen.

Prepare Now

Even if it turns out that Erika falls apart or veers far away from Florida and the rest of the southeastern United States, you need to prepare now for this and any tropical cyclone that could threaten in the future. We’re still climbing toward the climatological peak of hurricane season—which is September 12—and storms are possible all the way through the end of the season on November 30.

If you live in an evacuation zone, get your plans ready just in case you’re told to evacuate and you decide to heed the warnings and orders of officials. Failing to evacuate when you’re told to do so can land you in a whole bunch of bad situations—remember that emergency crews will not come to your aid if the storm or flooding is so severe that it threatens their own safety. You’ll be put in a queue to come rescue/recover when the storm is over.

Like most people, though, if you’re reading this in an area vulnerable to hurricanes, you’re not in an evacuation zone. You might be far enough away from bodies of water that you don’t have to worry too much about freshwater flooding or storm surge at your home. That’s fine. During or after any storm, those affected are still vulnerable to power and utility outages for an extended period of time, not to mention the potential for vehicle damage, road closures, and businesses closed due to damage or power/utility outages.

Here’s a non-exhaustive list of what you need to ride out any tropical cyclone and survive the aftermath:

  • Enough non-perishable food to last you and your family at least three days. It’s a good idea to have extras just in case.
  • Five gallons of water per person, which the CDC says should last you three to five days.
  • Fill up your bathtub(s) and sinks with water. If the water goes out, you’ll need these reserves to flush the toilet. (Kind of important!)
  • First aid supplies—enough to treat and bandage minor injuries, take care of stomach issues, and minor pain medication like ibuprofen or acetaminophen.
  • Flashlights and radios are important, as are enough batteries to power both for extended use. Hand-crank flashlights and hand-crank radios are especially useful (and they give you a great workout).
  • Personal hygiene is really important here—if the water goes out and you can’t take care of certain things in this department, it could make you or your loved ones very sick at a very bad time to get very sick. Depending on your needs, you should have hand sanitizer, baby wipes, toilet paper, napkins, paper towels, tampons and pads, dental care, deodorant, soap, and anything else you normally use to keep yourself feeling fresh ‘n’ clean.
  • Make sure you have enough of your prescription medication to last you several days after the storm. In the worst case scenario, it might be a week or longer before you can get a refill.
  • Cold hard cash. You won’t be able to swipe your card if the power, internet, or telephones go down, so you’ll need cash to buy things.
  • Make sure you have enough gas in your car to last you a while. Also, make sure your car battery is healthy and it’s not about to die. It would suck to lose power at home, only to lose power in your car shortly thereafter.
  • Gather your important personal documents—stuff like social security cards, birth/death/marriage certificates, insurance papers, deeds, mortgage papers, and computer backups—in one, water- and fire-resistant spot. Put them in container after container if you can. Make sure that the worst flood couldn’t touch them and you can grab them at a moment’s notice.

We’ll have to wait and see what Erika does in the coming hours and days before we start to really worry about what could happen to Florida (or any other state) if and when it draws closer to the mainland. Based on its erratic movement and disorganized structure, there’s a very real chance that the storm dissipates or moves away from land and this turns out to be a non-issue for the contiguous United States.

Pay close attention to forecasts from the National Hurricane Center this weekend. They issue full forecasts every six hours—5:00 AM, 11:00 AM, 5:00 PM, and 11:00 PM EDT—with position and intensity updates every three hours in between as long as watches and warnings are in effect.

And, as always, I’ll have updates for you here on The Vane all weekend as the situation evolves.

[Images: NASA, author, NOAA | Corrected to fix an embarrassing error. | Update: The forecast track map was updated to reflect the 5:00 PM advisory from the National Hurricane Center. ]

Email: dennis.mersereau@gawker.com | Twitter: @wxdam

If you enjoy The Vane, then you’ll love my upcoming book, The Extreme Weather Survival Manual, which comes out on October 6 and is now available for pre-order on Amazon.