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Severe weather ramps up in the U.S.


Severe weather season is about to begin in the U.S.
Severe weather season is about to begin in the U.S.

Dayna Vettese, Meteorologist

April 17, 2013 — It’s that time of year: Severe weather season in Tornado Alley is starting to ramp up, storm chasers are getting antsy, forecasters are brushing up on their severe weather forecasting techniques and the public is preparing for bad weather.

There is a risk for lightning in parts of the U.S. on Wednesday
There is a risk for lightning in parts of the U.S. on Wednesday

Tornadoes and other forms of “summer severe” weather occur all year round in the United States, but this time of year through to the beginning of June is when Tornado Alley and Dixie Alley really start to get going.

Tornado Alley and Dixie Alley are nicknames given to certain regions of the United States that are prone to severe weather outbreaks -- much like the outbreak that U.S. forecasters are calling for this Wednesday.

The Storm Prediction Center (SPC) in the United States is responsible for outlining severe weather risks and identified a "Moderate Risk" for severe weather in its Convective Outlook.

The levels of risk include, from least to greatest: Slight Risk, Moderate Risk and High Risk. These risks include the risk for any severe weather including wind, rain, hail and tornadoes.

The SPC issued a Moderate Risk three days in advance of Wednesday, which is very rare, especially in April.

A statistic from the SPC states that, including this moderate risk, only five have been issued three days in advance during the month of April in the history of the agency.

This includes the 2011 outbreaks between April 25 and 28, which included 358 tornadoes and over 300 fatalities. This outbreak did eventually include an upgrade to High Risk.

On average, between 1991-2010, roughly 155 tornadoes occur in the United States in April with 28 occurring in Texas, 13 in Iowa, 12 in Kansas and 11 in Oklahoma.

In May, those numbers almost double for Tornado Alley.

So what will Wednesday hold?

Wednesday's severe weather risk in the U.S.
Wednesday's severe weather risk in the U.S.

A large trough will dig in and swing across the central and southern plains of the United States. With the proper ingredients coming together, considerable convection should develop in the outlined risk areas.

These ingredients include warm, moist air from the Gulf of Mexico and cold air surging south from us here in Canada. 

There is enough shear (or, spin in the atmosphere) predicted to be within the risk area that organized, rotating thunderstorms could form posing a risk for heavy rain, lightning, hail, strong winds and possibly tornadoes.

The SPC also includes its Day 2 and Day 3 outlook probabilities. The percentages mean the probability of severe weather occurring within 40 km of a given point within the probability area.

There is a step up from this, however. If an area is hatched, that means there is a 10% or greater probability of significant severe weather within 40 km of a given point within the risk area.

What does this mean for Canadians?

This same trough and associated low pressure system will eventually make its way into Canada, via Ontario.

For northern Ontarians this could spell out another significant snowfall event. 

For southern Ontarians this could bring some very warm temperatures but also a lot of rain and the potential for thunderstorms.

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