In yet another shining example of good information falling into uninformed hands, a rumor is spreading through Facebook that a tropical storm could hit southern California in the next couple of days. Thankfully (or not, given the crushing drought), there is a 99.5% chance that it will not happen.

The rumor in question started with a routine weather model image published by Wunderground a couple of days ago. The product in question is called a "spaghetti model plot." Forecasters have quite a few weather models at their fingertips, and a good number of them can be used to track tropical cyclones. When one looks at a weather model to track the path of the center of a tropical cyclone, it can be hard to eyeball subtle differences between the forecast tracks produced by the different models.

Spaghetti model plots show users the forecast track for the center of a tropical system produced by many different weather models. The end result looks like a handful of spaghetti thrown on a map, hence the name. Spaghetti models show how much consensus (or lack thereof) exists between the models. All of the lines close together shows a strong consensus in the models that a tropical cyclone will move in that direction, and lines that are fanned out in all different directions show low consensus. A great example of low consensus is from 2010's Tropical Storm Lisa, a spaghetti model forecast for which is posted above.

For the most part, the causal user of weather information sees spaghetti models in three places:

  1. Checking Wunderground
  2. Watching The Weather Channel
  3. Following weather geeks on social media

I still haven't been able to find "gossip zero," so to speak, but when the rumor began sometime on Sunday or Monday, the image people kept posting was an ensemble spaghetti plot chart from Wunderground, much like this one (which is current as of this post):

You may be familiar with the "big" weather models such as the GFS (American global model), ECMWF (European model), and the NAM (North American Model), but few people outside of meteorologists and hardcore weather enthusiasts have heard of "ensemble" models. An ensemble weather model is basically a smaller, slightly different run of one of the bigger models.

An ensemble model changes the starting parameters fed into the model to arrive at a different result. Think about it like this: you're on your way to work and there's a big car accident on the interstate. You decide stop for gas and that decision puts you square in the traffic jam. If you'd left home ten minutes earlier, you'd have missed the crash and gotten to work on time. If you'd left four minutes earlier, the man texting his wife while combing his mustache might have hit you instead of someone else.

By slightly changing the starting condition (what time you left home), the results could be dramatically different. That's basically what an ensemble weather model does; it changes the starting conditions to see how much that change affects the end result. In the ensemble model that people are talking about, there were two out of ~20 ensemble runs that showed something hitting California. It didn't show what — a hurricane, a depression, remnants, sharknado — but it showed that the starting conditions fed into two of the ensemble runs wound up sending the system into California. The rest join every other reliable model, as well as the National Hurricane Center's official forecast, in sending it out to sea.

It's overwhelmingly likely that Lowell will continue out to sea and no landmass anywhere will even see a cloud from the storm.

As a general rule, the water is just too dang cold off the West Coast to sustain a tropical system. In the unlikely event that a tropical system or its remnants are forecast to hold together long enough to have any major impact on the state, you can bet that more people than your best friend's 15-year-old cousin would be talking about it.

Only two tropical systems in recorded history have hit California: a tropical storm made landfall near Long Beach in 1939, and a hurricane came very close to making landfall in San Diego (but ultimately missed by a hair) back in 1858. The state can and does see the remnants of dissipated tropical cyclones, though, and those are still dangerous as they can bring heavy rainfall.

[Images: author / FSU / Wunderground]