A World of Weather
Fundamentals of Meteorology

1. Introduction

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When defensive football coaches study game films of an upcoming opponent, they look for tendencies in the formations and plays used by the opposing team’s offense. By recognizing such patterns, defensive coaches can design strategies to predict the play the opposition will run on any given down.

In similar fashion, weather forecasters look for patterns on weather maps in order to predict the tendencies of potentially offensive weather. Such pattern recognition can play an instrumental role in short-term forecasts (one to three days ahead), medium-range forecasts (four to ten days) or long-range outlooks (months to seasons).

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This pattern of steering winds around the Bermuda high and a trough over the Midwest favors a tropical cyclone moving northward up the Atlantic Coast.

For an example of pattern recognition, suppose it’s mid-September, the heart of Atlantic hurricane season. If upper-level winds resembled the pattern shown in the figure above, forecasters at the Tropical Prediction Center in Coral Gables, FL would watch for the possibility of a tropical system running a northward “fly pattern” up the East Coast. Indeed, with a lethargic trough of low pressure drifting over the Midwest and the Bermuda high lined up in its typical formation, high-altitude steering winds (which, like football pass patterns, are marked by arrows in the figure) would tend to direct any tropical system over the Caribbean or the tropical Atlantic right up the Eastern Seaboard. In August and September 2004, the prevailing pattern did not invite tropical cyclones up the East Coast, but steered Hurricanes Charley, Frances, Ivan and Jeanne more westward on a collision course with Florida (check out the 500-mb pattern during the period, September 1-4, when Hurricane Frances approached the Sunshine State).

Forecasters at the Storm Prediction Center (SPC) in Norman, OK, apply the method of pattern recognition to predict outbreaks of severe thunderstorms. For example, they don’t hesitate to issue severe thunderstorm watches when they see a pattern like the one shown in the figure below.

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A classic set-up for an outbreak of severe weather during the spring. The hatched area represents the area at greatest risk.

When composing seasonal outlooks, meteorologists look at (among other factors) teleconnections from either El Niño or La Niña, if one of these is present or imminent in the tropical Pacific Ocean.  Recall that a teleconnection is a correlation between an atmospheric or oceanic anomaly in one part of the world and an anomaly somewhere else. Both El Niño and La Niña involve significant departures from average ocean temperatures. These anomalies incite anomalous exchanges of heat, moisture, and momentum between the ocean and the atmosphere that can have far-reaching effects on seasonal weather patterns in other parts of the globe.

There are many other examples of pattern recognitions that meteorologists use to make short-term, medium-range, and long-range forecasts. We will further explore the forecasting method of pattern recognition later in the chapter. First, we need to lay some groundwork. Just as some coaching staffs rely on computers to quantify the tendencies of opposing teams, forecasters rely on computer simulations of the atmosphere for guidance in molding their forecasting strategy.