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The Las Vegas summer that would not end

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Posted at 1:44 PM, Oct 12, 2015
and last updated 2015-10-12 16:44:51-04

This year it seems summer just won’t leave Las Vegas.  

The past 4 days have seen high temperatures, which are more appropriate to mid September than mid October, and in late September we saw highs which were normal for late August.  

Throughout the year Las Vegas has seen a slew of record warmest months and seasons – February, March, June and September were all the warmest ever recorded, along with this past winter, and Las Vegas is on track to set a new warmest yearly average temperature set just last year.  So what’s causing is?  Global warming? El Niño? Well, not entirely.

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Global warming certainly has a role to play here, but it can’t completely explain everything. Ultimately Las Vegas has been under a semi-permanent ridge of high pressure which affected the West coast last winter.  That ridge kept the skies generally clear and contributed to the California drought and with the clear skies kept the temperatures much above average. That happened again for March, but it doesn’t quite explain June or September.

June saw an average temperature about 0.5° above the record and almost 5° above normal besting the old record set just two years ago. Both significant margins in terms of monthly averages. Those high temperatures can generally be blamed on a persistent area of high pressure which set up over the desert southwest and wobbled between southern Utah to the Four Corners area. That’s a normal summer pattern, and is the normal driver of our monsoonal flow for thunderstorms in the summer but it set up very early likely due to the ‘ridiculously resilient ridge’ which affected our area over the winter and was unusually strong allowing for the intense early summer heat.

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There is another important ingredient that has significantly affected the warmer temperatures – the urban heat island effect and Las Vegas’ impressive expansion over the past 50 years. The heat island effect is a city’s tendency to trap heat and keep overnight temperatures warmer than the surrounding areas. The city of Las Vegas has dramatically expanded in the last 20 years exacerbating the heat island effect and therefore pushing overnight temperatures upwards. That can be seen in the number since we’ve seen about 10 record high temperatures but nearly 40 record warm low temperatures just this year.  

The building El Niño also has affected our area. It’s done so far by way of keeping generally warmer water (which warms the air) closer to the west coast. Warm ocean water north of the tropics generally creates a surface high pressure system, which creates warmer water due to the generally clear skies, which in turn strengthens the high, which warms the waters, etc.  A portion of that stagnant high pressure system stuck around over Las Vegas for most of the month of September contributing to our record warm month just last month.

So why the especially hot October? Basically it’s a mixture of all those things. The persistent El Niño influenced high has kept skies clear and enhanced a massive northward bend in the jet stream that has really allowed the warm air to remain in the valley and just bake.  It is only a matter of time before the cooler air rolls into the valley with more bends in the jet stream. Until then, enjoy the late pool weather as roll along on the way to potentially our 5th record warmest month of the year and, if things stay like this, a new record warmest year.