Raven reads from the JetStream school lesson on surface maps. "Here is one town, one day, two fronts."
Rocket copies the numbers into a table as she reads. "Fifties in the morning, mid seventies by afternoon, forties by night?"
"What do you notice about the wind?" Raven asks.
"East, then south, then northwest," Rocket says. "It swings around like a clock."
Nova draws a tiny L on the screen with a red line to its east and a blue line to its southwest.
"Now move the whole picture over the town," she says, "and watch the fronts pass one by one."
Rocket slides the L across. "Warm front first, then the cold front. The table makes sense now."
The National Weather Service JetStream school describes a day when a warm front and then a cold front cross one location.
The table below puts that description in order. The temperatures and winds are the ones the lesson gives.
| Part of the day | Sky | Temperature | Wind | Which front |
|---|---|---|---|---|
| Early | Clouds lower and thicken, several hours of light to moderate rain | In the 50s | From the east | Warm front approaching |
| After the warm front passes | Rain ends, partly cloudy | Mid 70s | Gusty from the south | Between the fronts |
| A few hours later | A line of thunderstorms sweeps across | Just ahead of the cold front | ||
| After the cold front passes | Skies clear | Falls into the 40s | Shifts to the northwest | Behind the cold front |
Forecasters do not know exactly where every shower will land, so they give a probability of precipitation.
The probability of precipitation is the percent likelihood of a measurable amount of rain, or melted snow. It applies to any given point in the forecast area during the forecast period.
Measurable means 0.01 inch or more. A 40 percent chance means a 40 percent probability of at least 0.01 inch of rain at your spot.
| Percent | Forecast word |
|---|---|
| 10 percent | isolated or few |
| 20 percent | slight chance |
| 30, 40 or 50 percent | chance |
| 60 or 70 percent | likely |
| 80, 90 or 100 percent | occasional, or periods of |
Excellent reading. Tomorrow you connect fronts to the forecast you hear every evening.