Tuesday, June 14, 2016

Heat and humidity: there's plenty more to come

Slow-moving afternoon thunderstorms were the focus over the past few days, even as heat and humidity started to build across the region. Until today, daily high temperatures for 3-4 days had been in the mid 90s with increasing humidity since the weekend. (Average high temperatures this time of year are near 90°.) Afternoon thunderstorms helped to cool things off, if not reduce humidity levels, for the past couple of days. Today, morning cloud cover and scattered showers that started (and ended) earlier than the past couple of days managed to keep highs in the upper 80s.

However, as we head into the next few days, an upper level ridge of high pressure that we wrote about in a weekend blog post will get stronger over the region, putting a damper on rain chances and allowing temperatures to climb once again. The bad news is that the humidity, as measured by the dewpoint, that rose the past couple of days will remain in place. That means we'll be dealing with dangerous heat index readings the next few days as the warmest and muggiest air of this early summer arrives.

A massive ridge of high pressure at the upper levels (the "hill" depicted in shades of brown) builds across the central U.S. as we head through the week. The European model valid on Friday morning is shown above. Graphic courtesy Pivotal Weather.
Only slight chances of rain are expected each day through the end of the week as high temperatures climb into the mid to upper 90s. With dewpoints in the mid 70s, heat index values will rise above the danger level of 105°, getting as high as 110° on Thursday in some areas. In addition, the high dewpoints will mean very warm temperatures overnight, with little relief as air conditioners continue to hum all night.

The high-res North American Model (NAM) depicts dewpoints for Wednesday afternon in the mid to upper 70s throughout the Mid-South. These values make the air feel uncomfortably humid and limit a person's ability to cool through evaporation of sweat due to high water content in the air. Graphic courtesy WxBell.

The high-resolution North American Model (NAM) depicts heat indices for Wednesday at 3pm in the 105° range (pinks) for much of the Mid-South, thanks to temperatures in the mid 90s and high dewpoints as shown above. Graphic courtesy WxBell.

By early Friday, a "backdoor" cold front will slip through the metro, bringing another small chance of thunderstorms. (It is termed a "backdoor" front since it will actually move through the area from northeast to southwest versus the more typical west or northwest to southeast. And like your back door neighbor, it probably won't knock before just coming in.) The best news from the passage of this front will be a reduction in humidity levels heading into the weekend. At this point, it doesn't appear that it will have much effect on temperatures (maybe 3-5° cooler this weekend), so expect highs in the 90s to continue, though heat indices will be back down below the danger level.

The National Weather Service surface map for Friday morning shows the backdoor cold front arriving from the northeast with drier, though not much cooler, air behind it.

The front will also be followed by a re-strengthening of high pressure at all levels of the atmosphere, meaning a period of dry weather ensues. This pattern looks to maintain itself well into next week with medium-range models from the Americans and Europeans indicating a very hot and dry week as we officially start summer on Monday. With only one day so far this month below average in the temperature department, it's very conceivable that we go through most of the month of June with above average daily temperatures. Hopefully this is not a foreshadowing of the rest of the summer! (And if you think it's hot here, take the next flight to Phoenix where the ALL-TIME heat record of 120° will be threatened early next week. [But it's a dry heat, right??])

The European model shows even stronger high pressure aloft by the middle of next week (compare to first graphic above), which encompasses all but the northeastern U.S. Temperatures in the Mid-South will be well into the 90s each day next week with a possibility of threatening the century mark, Graphic courtesy Pivotal Weather.
As you all know, heat exposure, particularly over a prolonged period when there is little relief at night due to high humidity, can have detrimental effects on the body and the effects compound as a heat wave lengthens. See the safety tips below as a reminder of how to deal with the heat and be sure to check on those who may need help finding relief, including our older generations, the very young, and our pets!

Erik Proseus
MWN Meteorologist

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