Finishing Off July Pleasantly Mild But Below Average - KMSP-TV

Finishing Off July Pleasantly Mild But Below Average

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As the month of July winds down and we head into August, there is increasing confidence that this will be a cooler than average end to the month. Climatology tells us that the last week of July is the hottest week of the year on average across much of the country, but it may not follow the norm this year. 2 different cold fronts are expected to move through the eastern half of the country this upcoming week bringing in unusually cool air straight from northern Canada. It's all associated with a strong trough that will dive into areas east of the Rockies. Even though it hasn't felt like we have seen all that warm of a July so far, that's just not the case. As of Saturday, we sit 4.5 degrees above average for the month. If temperatures fall low enough to drop our monthly average temperature to sub average, this would be the first cooler than normal July since 2009.

A trough can be seen by watching the orientation of the jet stream. The jet stream is the river of air at the top levels of the atmosphere that separates cooler air to the north of it from warmer air to the south of it. Notice in the picture below depicting the location of next weekend's jet stream, that the jet dives southward into the eastern U.S. This would be our trough.

This trough is what brings in our cold front at all levels of the atmosphere including right here on the surface. But let's take a peek at a level above the surface… right around 5000 feet. We look at this level (called 850mb in meteorological terms) because air this high up in the atmosphere is typically unaffected by surface heating from the sun. Therefore, we are able to determine just how warm we can get on the ground due to the physical laws of potential lapse rates (a natural occurring phenomenon where the air cools as it rises and warms as it sinks… this ultimately allows the temperatures to warm no more than 10°C per KM of altitude (18°F per two thirds of a mile or about 3500 feet). So, take a look at the 850mb temperatures for Tuesday afternoon across the Midwest and Great Lakes.

They look very mild with plenty of greens and yellows which typically indicate 70's or close to 80 at the surface depending on the time of year. Also notice the wind direction as the barbs are pointing out of the north meaning that we have northerly winds (AKA we are behind our cold front) which is ushering in our cooler air. Now look at Saturday.

Noticeably colder with greens and even a speck of blue showing up… so this next cold front could potentially be cooler than the first dropping highs 10 or 15 degrees below average. But remember, it is still a week away and many things can change before then. But, what makes us more confident in this cool down then others is just how many resources are showing a cooling trend. The next two images are produced by the Climate Prediction Center (CPC) which houses many experts in the field of long range forecasting. Even they are seeing signs of cooling across the eastern U.S. This first graphic is the probability of experiencing above, below, or near normal temperatures for the 6-10 day forecast period (Friday through next Monday).

This shows that there is an increased probability, especially in the Great Lakes of getting sub normal temperatures over this time frame. Average high for the Metro is 84… so highs will likely be below that. However, one thing to remember about these forecasts is that it's for a combination of multiple days. So there is the possibility that the first couple days could be above average followed by a few below average days which rounds out to a below average timeframe when combining all of the temperatures. In this case though, there is very little evidence to suggest that this will be the outcome.

The second graphic shows the probability of experiencing above, below, or near normal temperatures for the 8-14 day forecast period which takes us into the first week of August.

Temperatures still have a heightened probability of being below average which would keep any heat waves at bay through the first week of August. Good for your power bill, but maybe not so good if you are looking for some heat.

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