Transitional Fall Storm Giraffe: Snow And Tornadoes — Maybe More


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The sharp dip in the jet stream that is responsible for Giraffe. 8 p.m., Oct. 12 analysis courtesy of NOAA

A sharp dip in the jet stream is responsible for Giraffe. 8 p.m. (EDT), Sunday, Oct. 12 analysis courtesy of NOAA

Different seasons are characterized by different kinds of weather. Storminess in the winter derives from the strength of the jet stream and a contrast of temperatures; summer storms feed on moisture and instability. In the spring and fall, transitional storms with characteristics of both winter and summer can evolve.

Giraffe will munch on energy in the jet stream — a pronounced dip is intensifying over the plains and midwest — and absorb moisture from the Gulf of Mexico.

Heavy snow fell Sunday in the high Rockies; severe thunderstorms, heavy rain and the possibility of a tornado will spread across the south and midwest today and Tuesday; and giraffe could bid the US goodby with an early-season nor’easter on the Atlantic coast.

The Fall Jet Stream

The jet stream is a reflection of temperature contrasts. In the summer, with the sun high above the northern hemisphere, the contrast is weak. As fall proceeds, the angle of the sun decreases in the higher latitudes; temperature contrasts, and consequently the jet storm, become more conspicuous.

The trough now moving across the Rockies and into the eastern United States is very pronounced. On the forward side, moisture-laden air from the Gulf of Mexico is being pushed northward; behind the trough, cold Canadian air is plunging south. The result is a wintry blast in the mountains and the setup for severe thunderstorms in the plains, midwest, and south.

The Path Of Giraffe

Severe thunderstorm risk for the period 4 p.m. Monday through 8 a.m. Tuesday. Forecast courtesy of NOAA

Severe thunderstorm risk for the period 4 p.m. Monday through 8 a.m. Tuesday (all times EDT). Forecast courtesy of NOAA

As the jet stream trough moves slowly east, inclement weather will engulf much of the eastern half of the US. The greatest chance of tornadoes will occur in Texas and Oklahoma on Monday, but flooding rains and damaging winds associated with thunderstorms can be expected over a wide area.

The center of a deep trough is characterized by very cold air at jet stream level. This causes a steep lapse rate (decrease of temperature with height), so a lifted parcel of air can find itself warmer than its surroundings and keep rising. This unstable condition sets the stage for powerful up- and downdrafts. Strong thunderstorms are the result, and if there is rotation at mid-levels of the atmosphere, a tornado can form.

Giraffe Could Exit Ungracefully

When the jet stream trough reaches the Atlantic coast, the temperature contrast with the warm water will produce additional potential energy (Warm air next to cold is a higher potential energy state than cold air beneath warm.). A nor’easter, a low pressure center that forms on the coast, is a possibility.

Is This Weather Evidence Of Global Warming?

It is extremely difficult to link any single weather event with climate change. However, the circumstances are suspicious.

The warming of the Arctic has contributed to rising pressure over the north polar region. As a result, the jet stream has been pushed south of its normal position.

Furthermore, there is a tendency towards sharper swings in the jet stream, with more omega blocks (blocking high pressure) and cutoff lows. Giraffe is an example of both a pronounced trough which will cut off for a time, and the jet stream positioned unusually far to the south.

Immediate Threats From Giraffe

A powerful squall line moved through eastern Texas in the early morning hours, and will continue into Arkansas and Louisiana. Winds in the Dallas area gusted to hurricane force and downpours flooded some streets.

This is just the start of Giraffe’s rampage. Residents of the midwest, Mississippi Valley, Deep South, and Atlantic coast should keep apprised of the latest National Weather Service watches and warnings for the next several days.

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