There are a number of regions across the United States that see an exorbitant amount of tornadoes in a given year. None more so than what’s classified as Tornado Alley by the National Climatic Data Center. But on a smaller scale there also exists Dixie Alley in the south, Hoosier Alley in the upper Ohio Valley, and out east the Carolina Alley. While every state in the contiguous U.S. has recorded at least one tornado since 1950, these regions often host the greatest threat in any given year.

Tornado Alley
Roughly 17 million people live in what is widely considered Tornado Alley, an area of approximately 500,000 square miles which spans portions of eight states across the high plains of the United States. Each spring weather enthusiasts, scientists, and extreme weather fanatics descend on the region for severe weather season as nature unearths some of its most powerful, least understood, and beautiful weather phenomena: tornadoes.

Tornado Alley

Tornado Alley

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Today we resume regular updates of the Tornado Threat Forecast every Monday and Thursday! As we have done for the past two years, the regular updates will be twice per week until things start getting quiet again in the later parts of summer. There will be a break from these forecasts during our storm chase expedition (or “chasecation”) in the back half of May, at which time daily updates of how our chasecation is progressing will be posted instead.

1-3 Day
20140421_tornado_forecast1-3
Monday

Northeastern Texas, southeastern Oklahoma southwestern Arkansas — TORNADO RANGE: 0-2 — CONFIDENCE: High
Expected Tornado Hotspot: None
Pros: Moderate instability, decent/good directional shear
Cons: Weak speed shear, winds not that backed at the surface, no strong upper-level dynamics, mediocre mid-level lapse rates

Tuesday

No tornadoes expected.

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Lone Tree, Iowa tornado on April 13. (YouTube)

Lone Tree, Iowa tornado on April 13. (YouTube)

Looking back at the week of April 13-19.

The middle of April has historically been a rough period when it comes to big tornado events in the United States. Just three years ago, a major outbreak sequence brought North Carolina its biggest tornado day on record.

Not this year, at least not yet.

Other than April 13 which dropped a smattering of disparate tornadoes, it was a fairly quiet week across the country, with only a brief additional bout of excitement brought on by a hefty squall line.

The lack of activity gave some time to look at the tornado drought, and our prospect of ending it any time soon…

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Looking back at the week of April 6-12.

Tornado watches, warnings and reports the week of April 6-12, 2014.

Tornado watches, warnings and reports the week of April 6-12, 2014.

April can be volatile, but lately it seems we talk more about lack of tornadoes and/or tornado droughts than anything else.

The week was dominated by a system that rolled through the south to start the period — Sunday into Monday, April 6 and 7.

As the week closed, some storminess resumed across the Plains, but nothing to write home about at this time of year.

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Tornadoes with a track length of 25 miles or greater ranked by strength grouping. (Ian Livingston/SPC Data) *select for larger*

Tornadoes with a track of 25 miles or greater sorted by strength grouping. (Ian Livingston/SPC Data) *select for larger*

Long-track tornadoes are unusual in the tornado world.

Using a moderately restrictive 25 mile or longer path as the definition, they’re about 2.5 percent of all twisters. In a more stringent 100 mile or longer path length, we’re looking at about 0.1 percent of the full modern record dating back to 1950.

Here, we’ll use 25 miles as a base, with additional looks at 50 miles or longer and 100 miles or longer tracks. In today’s lingo, you may hear the term referring to even shorter tracks, but we begin to wash the idea down heading much lower than 25 miles.

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(SPC)

(SPC)

If you’re a regular follower of our blog or of U.S. weather in general, then it’s no secret that this tornado season has been even quieter through early April than 2013 was.  But back on March 31, the 2014 tornado season actually broke a record for the latest in the year with no F/EF3+.

March 30 (local time) was previously the date of the latest F/EF3+ tornado on record in the U.S. since 1950, and it happened in 2002.  As of publish, there has not yet been a single EF3+ rated tornado in the United States during the 2014 calendar year. Caveat: There was a fairly large tornado in eastern North Carolina during the afternoon of April 7 that has yet to be rated.

If that tornado is not rated EF3+, then the streak will likely continue until at least Monday, April 14, when the next significant trough ejection is modeled to occur across the central U.S. If the NC twister does attain the rating in question, we still have set the record by over a week.

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Looking back at the week of March 30-April 5.

Tornado activity the week of March 30 - April 5, 2014.

Tornado activity the week of March 30 – April 5, 2014.

It was a wild week of weather, with even a predominantly cold storm producing some action. Two significant systems rolled across the country. The first was the cold one, with blizzard conditions spread across the north central U.S.

The second, while also featuring snow in the northern Plains and Midwest, promised to be the first legitimate tornado event of the spring. While it did produce, it may be said it did so on the lower end of potential.

Still, a number of tornadoes occurred on multiple days, with a focus on the Ozarks and mid-South, but also targeting the broader Dallas-Fort Worth metroplex.

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Tornadoes will threaten the South today and tomorrow as a high shear, low instability environment develops over the region. The environment for tornadoes will be fairly conditional, but with the strong shear, some upside risks are in play for areas that might get greater daytime heating, but the chances of that happening are low.

Just a reminder that for 2014, we have changed up the forecast format a bit, and the details can be found in our forecast from last week.

20140406_update
Sunday

Western and central Gulf — TORNADO RANGE: 2-7 — CONFIDENCE: Normal
Expected Tornado Hotspot: Southern Louisiana, southern Mississippi
Pros: Strong speed shear, good directional shear, good upper-level forcing, height falls moving in late
Cons: Low instability, okay/poor lapse rates, widespread cloud cover minimizing any chances for pockets of greater instability, greatest potential appears to be in the overnight hours

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During a period of just 18 hours between April 3rd and 4th, 1974, the so-called Super Outbreak dropped 147 tornadoes across 13 states. This was the largest and most violent tornado outbreak of the 20th century, and likely only matched in scale by the outbreak of 2011. In all, there were 30 F4 and F5 tornadoes recorded during this period, and over half of the tornadoes recorded were classified as F2 or higher.

Related: Looking back and recreating the maps | A Numerical Comparison of 1974 and 2011

Forty years later this outbreak continues to be an important case study for severe weather. Just this week the Storm Prediction Center released a simulation of what today’s storm-scale models may have forecast for this event. While warning lead-times have more than doubled since the 1970s, forecasting where and when tornadoes will form remains a critical challenge still today.

spc-1974-storm-scale

Storm-scale simulation of April 3-4, 1974 Super Outbreak

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It’s 2014, and we have a couple of changes to the format of the Tornado Threat Forecasts. There may be some additional tweaks as we get settled into the 2014 tornado season. Firstly, the Tornado Potential, which was just a gauge of how many tornadoes to expect, will be replaced with an actual tornado range, since that’s just basically what the Tornado Potential was, anyway. Forecasting an actual range once you get above several tornadoes can be pretty difficult, so the Forecast Confidence and the text discussion will balance how likely I think that the actual tornado count will fit within the given range. A wider range may also hint at the amount of uncertainty in the forecast.

Sometimes there will be a large risk area, but most tornadoes will be concentrated in a certain part of it. To help give more detail to the forecast, we have added the Expected Tornado Hotspot for where tornadoes will be most likely and/or most numerous.

(Note: This is not an April Fool’s joke… I blame the bad timing of the more active weather pattern.)

20140401_update
Tuesday

North-central Texas, southern Oklahoma — TORNADO RANGE: 0-3 — CONFIDENCE: Normal
Expected Tornado Hotspot: None
Pros: Moderate instability, strong directional shear, good speed shear
Cons: Strong capping inversion leading to questionable storm coverage

California — TORNADO RANGE: 0-2 — CONFIDENCE: High
Expected Tornado Hotspot: None
Pros: Good upper-level forcing, strong directional shear
Cons: Low instability, weak speed shear in the low to mid levels

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