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COMMENTARY

Cooler-than-average September to follow this warm, rainy August

On this Labor Day weekend, let’s first look at how we’ve done in August and during this summer.

The monthly mean temperature has been just a bit above average, at about plus 0.7 degrees. There were no 90-degree-mean days, with 13 days’ highs exceeding 80.

There has been just one 90-degree day this summer, which is below our average of three.

Rainfall has been about 0.45 inches above average for the month.

Incidentally, rainfall from June 1 through Aug. 30 has been 10.05 inches, which totals out at exactly average, a statistical rarity. So, after the wet spring, our yearly surplus is down to a modest 3.3 inches of liquid.

July was 2.9 degrees above average, with rainfall well below average, and June was minus 1.1 degrees, with rainfall plus 0.94 inches.

September will be off to a cooler-than-average start. The average high at the start of the month is 77, but by the end of the month, the average dips to 65. A tendency toward below-average temperatures obviously becomes more noticeable late in the month (insert “duh!” here). Even during cooler periods, there will be exceptions.

After Labor Day, Tuesday looks to be the warmest day this first week of the month:

We will be back to below-average temperatures by mid- and late week into next weekend, but nothing drastically cool. The culprit will be a dip or a trough in the northern jet stream entering the Great Lakes.

Note the forecast position of Hurricane Dorian. For more information on this dangerous storm, see my current article, which will include periodic updates in the comments section.

In extended-range model ensembles, there is a tendency for warming showing up in the following week into at least mid-month. There is good agreement in the Canadian ensemble for mid-month as well, so current indications do not point to the cool trough later this week persistently sticking around. The Climate Prediction Center has the highest probabilities for below-average temperatures in the 6-10 day period in our backyard. In the 8-14 day period, there is a somewhat lesser probability for below-average temps.

However, I do believe the cooler 8-14 day probability is weighted by the first few days in this period, and some more moderation will return by Sept. 10 and beyond. As for CPC, their experimental weeks 3-4 outlook is more optimistic for above average temperatures.

CPC also produces precipitation probabilities in this extended range — in this case, they are predicting below-average rainfall over most of the country in weeks 3-4 — but I tend not to place much credence in extended-range precipitation outlooks in most circumstances. Temperature trends are usually more predictable further out in time, while precipitation forecasts much beyond the seven-day period have shown little statistical skill. Uncertainty always increases with time, and this “spaghetti plot” in the American GFS ensemble illustrates the wide spread in upper-air solutions 15 days out in time. This kind of spread makes the prediction of the movement and position of individual storm systems two weeks out in time an unlikely venture.

Somehow, I doubt the Farmers Almanac is much into analyzing spaghetti plots. Although they may be adept at throwing spaghetti up on a wall, just for the heck of it.

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