High pressure usually means dry weather. But there are two types of high pressure areas, and the distinction is important. We'll break down the differences and use that knowledge to track the heat as it pulsates in different parts of the country over the next few days.
Surface high pressure is usually associated with cooler temperatures as clockwise winds drag air from north to south. The second type, upper-level high pressure, is an entirely different ball game. The presence of warm, dry air in the mid-and-upper atmosphere causes the pressure to rise, which helps deflect any incoming weather disturbances.
To track where the weather will be hottest across the country over the next few days, let's use the Global Forecast System computer model (the GFS) to look for upper-level high pressure ridges. Specifically, we'll examine the 700 millibar layer, which is about 10,000 feet aloft.
There are two distinct areas of high pressure that show up today, which I've highlighted with an "H" in the first image. The dominant ridge stretches from the Mid-South to the Mid-Atlantic. Nashville will be in the middle 90s today, but Washington, D.C. will sizzle in the upper 90s. Even places like Boston and New York will feel the sizzle of the middle 90s this afternoon. For the record, the second, less hot upper ridge can be found centered over West Texas. That ridge will gain strength over the next few days.
By Saturday afternoon, the Mid-Atlantic ridge has been pushed east into the Atlantic Ocean by a trough of low pressure that moved from Hudson Bay in Canada toward the Northeast. After a few days of mid-to-upper 90s, by the weekend the Eastern Seaboard gets a break and returns to the 80s. Without an upper ridge right on top of us, Tennessee's heat will abate, albeit ever-so-slightly. We'll still be around 90° in Nashville on Saturday. What you'll notice in the second image is the strengthening of the ridge in the Central Plains. Instead of the 80s, this part of the country will be well into the 90s on Saturday, and approach 100° on Sunday.
We may see some heat relief by Tuesday of next week in the Mid-South, but it's not set in stone. The third image shows a very amplified jet stream pattern, by summer standards. An amplified pattern has lots of north-and-south oriented movement, not just the straight west-to-east flow we often see in the warm months. I've highlighted the return of an upper-level-low to the Northeast (the third image is valid Tuesday of next week). As that trough spins from Hudson Bay to New England, it gives the hot ridge of high pressure in the Plains a push back to the west.
If the GFS model verifies, it will allow temperatures in Tennessee to drop dramatically, from the 90s Monday into the lower 80s Tuesday. We'll see, however. The European model, which often proves reliable in the mid-to-long range, doesn't project that the Northeastern trough will be strong enough to completely push the ridge away from Tennessee. If that's the case, our heat wave may continue through next week.
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