August Outlook: Cooler Than July
By Erica Grow on August 1, 2012, 11:22am
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As we turn the calendar from July to August, it's worth noting that the month ended much cooler than it began. In Hartford, the high temp on July 31st was 77 degrees; compare that with a 92-degree high temp on July 1st! While the Storm Team 8 Day forecast certainly has summerlike heat in it, the long-range weather pattern isn't looking as hot and dry as July.
The picture above is showing the longwave pattern in the upper atmosphere from July 30th. Specifically, we're looking at the 500mb "steering layer" which we can use to find the large-scale features that impact our weather trends. As you can see, there is a trough in this pattern over the entire eastern United States. Basically every location east of the Mississippi river is in this trough pattern, which brings us cooler and wetter weather. Not at all like the picture below:
By contrast, the overall pattern over New England in early July was zonal, which means the flow was nearly horizontal. A zonal flow brings dry air to our area, because the winds are moving from west to east. With the high sun angle of mid-summer, a westerly flow will not only be dry, but very warm. This is one of the reasons we had such a dry start to July, and we also had some record-breaking heat to boot. This zonal/slight ridging pattern persisted until late in the month, when the cooler and wetter pattern began establishing itself.
The large-scale (synoptic) wave pattern in the atmosphere is governed by huge movements called Rossby waves (named for one of the pioneers of contemporary meteorology). Rossby waves are on an atmospheric scale of about 10,000km-- much larger than a thunderstorm or even a hurricane. Have you ever noticed that big things tend to move more slowly than small things? This is true with the atmosphere as well. Rossby waves can take weeks to completely shift or break down. I believe that the pattern we are currently experiencing will stick around for at least the first half of August. Of course, small-scale changes will lead to brief warmups, but for the most part, I believe our temperatures will be close to normal or slightly below for at least the next 2 weeks. We will also continue to see increased rainfall, and the month of August should help to erode or completely eliminate our current precipitation deficit. It should be a good month for farmers and gardeners here in Connecticut, because you won't be fighting drought-like conditions in order to keep a strong harvest!
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