Severe thunderstorm event next Monday-Tuesday? History, setup suggest chance

Storm Prediction Center long-lead outlooks for next week. Thursday's discussion focused on next Tuesday (7/15), while Friday's focused on Monday (7/14) with note that Tuesday presents a threat but it is muddled. (SPC adapted by CWG)

Storm Prediction Center long-lead outlooks for next week. Thursday’s discussion focused on next Tuesday (7/15), while Friday’s focused on Monday (7/14), with note that Tuesday presents a threat, but that the setup is muddled. (SPC adapted by CWG)

On Thursday, the NWS Storm Prediction Center issued a rare “day 6″ outlook for severe weather next Tuesday across the region. Today, that outlook disappeared and a long-lead outlook for Monday appeared.

For this severe weather threat, you can largely blame the big swirl coming out of Canada that may or may not be the polar vortex.  Two things in particular stand out about its impingement on the region next week:

1) Unseasonably strong jet stream winds rounding its base and overspreading the northeast U.S..

2) A pattern that partially resembles other significant storm events in the region.

The strong winds aloft

"Bulk shear" on the GFS model for Tuesday evening shows high wind speeds through the column. In general, 20+ kts is supportive of pulse severe, with 30+ kts supportive of higher end severe including supercells. (Twisterdata.com)

“Bulk shear” on the GFS model for Tuesday evening shows high wind speeds through the column. In general, 20+ kts is supportive of pulse severe, with 30+ kts supportive of higher-end severe, including supercells. (twisterdata.com)

Severe storms thrive on strong winds aloft. These winds allow the storm to maintain a balance that prevents the updraft from eating up rain-cooled air, which results in a weakening of the storm. In many cases, the more upper-level wind, the better. As advertised, early next week – especially Tuesday — will have more than enough.

CWG’s severe weather expert Jeff Halverson confirms there is intriguing potential.

He notes, “The highly amplified jet stream pattern next week portends not only cold air aloft, but also strong upper-atmospheric dynamics, including unseasonably strong wind shear, upper-level spin, and a vigorous cold front.”

Similarities to historically well-known events

Re-analysis of two notable tornado events in the area with similar patterns to that advertised for early next week. The first produced the College Park F3 as well as a violent tornado to the west of the city. The bottom was the June 2012 tornado outbreak, one of the larger in the local area on record. (NARR via Penn State e-wall adapted by CWG)

Re-analysis of two notable tornado events in the area. These events had similar conditions to the possible event next week.  The first of these events (top) produced the College Park F3 as well as a violent tornado west of the city. The second (bottom) was the June 2012 tornado outbreak — one of the larger in the local area on record. (NARR via Penn State e-wall adapted by CWG)

History can be a guide, sometimes.

Before the June 1, 2012 tornado outbreak and damaging wind event, we pointed out that the meteorological setup appeared similar to that of September 24, 2001. The 2001 event produced a local outbreak, including an F3 and F4 tornado. During the 2012 event, Maryland saw its second-most tornadoes in one day.

While the June 2012 results were not the same, both days featured high-end storm activity, either in numbers or ferocity.

Mid-level trough (at 500mb) comparison between the 0z 7/11 European model and the 6z 7/11 GFS model. (Weatherbell.com adapted by CWG)

Mid-level trough (at 500mb) comparison between the 0z 7/11 European model and the 6z 7/11 GFS model. (Weatherbell.com adapted by CWG)

As you can see in the above model forecasts, both of these historical events as well as next week’s forecast feature a ridge in the west and a strong trough of low pressure in the Great Lakes region.

The mid-level trough in the Great Lakes is a particular mainstay of mid-Atlantic severe weather events. Examples of that include: Sep. 9, 2003 (re-analysis & reports), May 8, 1984 (re-analysis & reports), and Nov. 1, 1994 (re-analysis & reports), among many others.

While noteworthy, a pattern doesn’t guarantee much this far in advance. There are also cases where this pattern produces a run-of-the-mill storm event, like late June 2008 (re-analysis & reports).  So, there’s no slam dunk we’ll be running for cover.

The time is “right,” but details are foggy

Other ingredients, like low pressure forming to our west, causing surface winds to “back” to the southeast, are often needed to maximize such potential. Some others  include timing of storm initiation and available instability. Sometimes those factors are difficult to pin down until the day of the event.

Monthly tornado averages across the U.S. as per NCDC figured. (Author)

Monthly tornado averages across the Mid-Atlantic, as per National Climatic Data Center figures. (Author)

In her series of severe weather climatology posts, Kathryn Prociv pointed out that July is the peak month for tornadoes in the region. They often come in small batches rather than big events. Still, it’s worth mentioning here that we’re coming up on the 20th anniversary of the biggest tornado day in Maryland history, July 27, 1994.

July is also peak for severe wind (58 mph+) locally, though it is generally in the same ballpark as June. Wind damage is the chief threat in most of our severe storm situations, and this multi-day setup perhaps has the best potential to become a widespread wind event, given the strong winds aloft.

Severe wind reports 1950-2011. (Kathryn Prociv)

July severe wind reports, 1950-2011. (Kathryn Prociv)

Despite a favorable pattern, and currently models signaling a sizable events next week, storm setups can shift or fail all-together. We’ve already seen some of these shifts in recent days, and Monday is showing increased storm risk as it draws near. (Caveat: On Monday, that critical factor of strong wind aloft is currently marginal.)

Even in an idealized scenario for severe weather, with storm ingredients all coming together, numerous storms early in the day could cut down on heating potential. The location of the closed 500mb (mid-level) vortex is also in question, and changes there might help focus the greatest threat on a region other than ours.

But as Halverson told me in our discussion, “[modeled] factors may lead to a severe thunderstorm outbreak across the Mid Atlantic and Northeast.”  So, while it’s too soon to get overly worked up about severe weather early next week, it’s also worth an early heads up.

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