Harvey was 1000 year storm

[jfre81]

Originally Posted by LizzySWW

really?

Doesn't really teach how probability works, but that should give people some pause when they think of picking up and moving here.

[Matadora]

Why do ENSO forecasts use probabilities?

So what does historical probability tell us?

An 80% chance is a pretty high probability, which means if the climate were a betting game, we’d be wise to bet on El Niño. But what if the models predicted that the odds of El Niño this winter were 50%? Should we shrug that off?

This is where it helps to have a second kind of information: the climatological probability—the average odds of El Niño, La Niña, and neutral conditions based on how frequently they’ve occurred over a long historical record.

When people envision random chance (“we don't know”), they may think of a two-sided coin flip, with each side having a 50% chance of occurring. However, in the case of ENSO forecasting there are three possible outcomes: El Niño, neutral, and La Niña. Imagine a coin with a very thick edge, so that the chances of getting heads, tails, or having the coin land on its edge are all 33.3%. For any random coin toss—any given season we might consider—intuition might tell us there is a 1 in 3 chance of it being an El Niño season.

While a 1 in 3 chance of El Niño, neutral, or La Niña is a reasonable starting point for a “first guess” as to what we might expect, observations from a recent 30-year period tell a somewhat different story. Those baseline, or climatological probabilities, are shown on the chart as dashed lines.

As you can see in the figure, the climatological probabilities are not flat, horizontal lines at 33.3% all year round. Rather, they vary noticeably by season. Looking at the lines on the figure, we see that during NDJ (November-December-January), the climatological probability for El Niño or La Niña is higher than at any other time of the year, and exceeds 35% for each, leaving less than a 30% likelihood for neutral ENSO. By contrast, during the late spring and summer the reverse is true, as the typical chance of neutral conditions exceeds 50%.

Any bar that is greater than the dashed line of the same color (showing the climatological probability) indicates a heightened chance of that ENSO category occurring relative to the historical average. For example, in the case of the second season being forecast in the figure (JJA), the probability of El Niño is 69% (red bar), but the climatological probability is only 26% (red line), meaning that the chance of it occurring is considerably higher than average—more than double.

The difference between the forecast probability and the climatological probability, called the probability anomaly (69% minus 26%, or 43% in this example), can be important in decision-making, such as in the case when El Niño is associated with drought and/or increased forest fire danger. When a consequence is negative, the probability anomaly may be as important as the actual probability in terms of preparedness for adverse impacts.

2018 March Quick Look

“Two La Niñas are never alike, and they don’t happen in a vacuum,” says Tom Di Liberto at the NOAA Climate Prediction Center in Washington DC. He says other factors such as the Arctic Oscillation, a southward shift in arctic air masses, could also impact on Atlantic hurricanes.

The Climate Prediction Center does consider ENSO forecasts when it issues its outlooks, says Di Liberto. But when it comes to hurricanes, he says it’s better to wait and look at what the ocean conditions will be during the summer than in winter.

[okcthunder1945]

Climate change + poor land use policies. Houston needs several more Addick and Barker sized reservoirs and dams to the west and northwest to hold some of the water before it reaches population. Better land use policies can minimize damage.