The western United States is famous for its calm, predictable weather, so it's noteworthy when extreme weather happens. Parts of California and Oregon have been sweltering under heat exceeding 100 degrees in some spots, and it's making people ask "what gives?"

The answer lies in the jet stream. When meteorologists talk about upper-air patterns, they use the terms "troughs" and "ridges." A trough is an elongated area of lower pressure that often brings stormy weather and generally cooler temperatures, and a ridge is an elongated area of high pressure that often brings calm, stable weather.

The jet stream for the first half of this week has been incredibly wavy. The above image shows the jet stream this evening as predicted by the North American Model. Large ridges dominate the west and east coasts, while a deep trough dips down through the center of North America.

As ridges consist of high pressure, the air inside of a ridge is sinking. Sinking air is conducive to clear skies and generally calm winds, allowing the atmosphere to warm up, and considerably so at times.

Temperatures are following the textbooks pretty closely. The east coast is experiencing fairly warm temperatures, with much of the eastern United State experiencing 90+ degree temperatures before a cold front sweeps through, and the west coast is baking in 90-100+ degree weather for one more day.

The trough in the center of the country is ushering in cooler air from northern Canada, bringing northerly winds from the Arctic Circle straight down to the southern Gulf of Mexico. The sharp cold front is crashing into the warm, unstable and triggering heavy thunderstorm activity along and east of the Mississippi River, which could lead to some flash flooding over the next couple of days.

The ridge will start to break overnight and during the day tomorrow, allowing high temperatures to begin to moderate across much of the west coast starting on Thursday. Tomorrow's high in San Francisco should only reach the low 80s, with temperatures returning to average (upper 60s) by Friday.

[Images via NWS and TwisterData]