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A Surprisingly Tricky Monday Forecast

Living here on the coast, the ocean is always influencing our weather - and sometimes it throws some big curveballs to us meteorologists when we're making up our forecasts. One of these headaches are areas of low-pressure that form with little to no warning over the warm waters of a Gulf Stream. Seeing this happen can quickly turn a sunny day sour with clouds and showers streaming onshore. One of these areas of low pressure appeared offshore yesterday, and is making for a tricky forecast today.

Heading into the weekend, all indications were that we'd see high pressure dominate the Southeast and sunshine would prevail across the Carolinas. For the most part, that held true. But a slight weakness in the high pressure area offshore turned into a coastal trough Sunday afternoon - which did in fact send a few clouds into the Wilmington area. Now on Monday, the trough is too close for comfort and we are seeing some clouds coming onshore, and stand a very slight chance at seeing a shower or two on the coast.

Now for the record, none of the forecasting computer models had this trough developing in their predictions. Nor did the national weather service or other media outlets have it in their forecast, ourselves included. This system was largely a surprise, having formed in a high pressure regime. Why can this happen here at times? It's that pesky Atlantic Ocean.

There are a few reasons why the ocean makes for such interesting weather, but it mostly has to do with temperature differences. The coastline is the best example for this when we have differences between water temperatures and land temperatures. This is what drives our summertime weather when we see almost daily sea-breeze thunderstorms. But outside of land/water temperature differences, there are also water/water land differences. That's exactly what we see with the Gulf Stream.

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Looking at the map above of sea surface temperature off our coast, you can clearly see the Gulf Stream as shown by the warmer colors just offshore. Right now, the water there is a good 10-15°F warmer than it is here along the beaches. Since warm air rises through the atmosphere, this makes for a concentrated area of rising air.

 Once the air begins to rise in unison - it leaves a "gap" or a void at the surface, which puts less pressure on the water below. Simply put, the pressure is falling. Over time this creates an area of low pressure, which can sometimes organize into a storm. In this case, we have a long trough of low pressure with some clouds and a few showers associated with it.

Since we're still in the large scale high pressure regime with very little wind in the atmosphere to kick this trough out, it could very well sit just offshore for a couple days. While most of the weather associated with it will stay offshore, we have to keep a close eye to see what clouds or showers come inland to say hello. 

Just one of the things that can make forecasting a little tricky - and fun - around these parts. 

That's all for now. Thanks for reading.

- TB

By: Tim Buckley