Storms Could Ride Northwest Flow Into Rowan County This Afternoon

Rowan County will spend much of Tuesday dealing with another round of August heat, but attention will increasingly turn northwest later this afternoon as the potential develops for thunderstorms to move toward the area.

The Storm Prediction Center has placed Rowan County in a Marginal Risk, Level 1 of 5, for severe thunderstorms today. The primary concern will be damaging wind gusts.

Rowan County is also within the SPC's 5% damaging wind probability area. That represents a 5% chance of damaging thunderstorm winds or wind gusts of 58 mph or greater occurring within 25 miles of any given point in the outlined area.

Most of the Day Should Be Rain-Free

Today’s forecast calls for increasing clouds and a high near 95 degrees. Heat index values could reach around 102 degrees during the afternoon.

The chance for showers and thunderstorms is 30%, mainly after 5 p.m.

That means most locations will spend much of the day without rain. However, the relatively low rain chance should not be confused with the potential intensity of any thunderstorms that do develop or reach Rowan County.

Only a portion of the area may experience a storm, but an isolated storm could still become strong or severe.

Watching What Develops to the Northwest

The more interesting part of today's forecast involves what happens upstream of Rowan County.

A northwest flow pattern in the atmosphere is allowing disturbances and thunderstorm complexes to travel from the Ohio Valley toward the Mid-Atlantic and Carolinas.

One round of storms moving through West Virginia and Virginia early Tuesday is expected to weaken and should have little effect on Rowan County.

The bigger question involves additional thunderstorms that could develop behind that initial activity.

Some forecast guidance has suggested storms could organize along an outflow boundary left behind by earlier thunderstorms and then move southeast toward western North Carolina and the Piedmont.

There remains considerable uncertainty over exactly how that scenario evolves. Forecast models have not been particularly consistent about the timing, organization or eventual track of the thunderstorms.

That is why this is something to watch rather than a guarantee of severe weather in Rowan County.

The Atmosphere Could Support Strong Storms

If thunderstorms do approach Rowan County, they could encounter a very unstable atmosphere.

Forecast guidance indicates substantial instability could develop across portions of the North Carolina foothills and northwestern Piedmont this afternoon. That could help thunderstorms survive their trip toward the Piedmont instead of quickly weakening after crossing the mountains.

The combination of afternoon heating and instability means any storm that becomes established could produce strong wind gusts.

Damaging winds are the primary severe-weather concern, although thunderstorms could also produce frequent lightning and locally heavy rainfall.

Slow-moving or repeated heavy downpours could also result in isolated areas of excessive rainfall.

What the 5% Wind Risk Means

A 5% damaging wind probability can sound insignificant at first glance, but it is important to understand what the number represents.

It does not mean storms have only a 5% chance of producing damaging winds.

Instead, it means there is a 5% probability of damaging thunderstorm winds occurring within 25 miles of a given location inside the outlined risk area.

That probability is high enough compared with what would normally be expected on an August day for the SPC to highlight the area for an organized severe-weather threat.

Timing for Rowan County

The main window to watch appears to be late Tuesday afternoon into Tuesday evening, with the local forecast placing the best chance for thunderstorms after 5 p.m.

Storm chances remain at 30% through the early evening before decreasing after approximately 8 p.m.

The exact timing will depend heavily on whether thunderstorms organize to our northwest and how well they maintain themselves while moving southeast.

Not Everyone Will See a Storm

This does not currently look like a situation where widespread thunderstorms are guaranteed across Rowan County.

Some locations could receive a strong thunderstorm while others receive little or no rain at all.

That scattered nature is typical of summertime convection, but today’s environment means any storm that does develop deserves attention.

What I'll Be Watching

There are several things I'll be monitoring through the day:

• Thunderstorm development northwest of Rowan County.
• Whether an outflow boundary moves southeast toward the Piedmont.
• How much instability develops after afternoon heating.
• Whether storms remain organized as they approach or cross the mountains.
• Any indications of storms developing a stronger damaging-wind threat.

The radar picture later this afternoon will ultimately tell us considerably more than it can early Tuesday morning.

For now, there is no guarantee Rowan County sees a strong or severe thunderstorm today. But with the county under a Marginal Risk and a 5% damaging wind probability, it will be worth keeping an eye on the northwest as the afternoon progresses.

Rowan County Weather will continue monitoring radar and the evolving storm environment throughout the afternoon and evening.