Heatwaves have already shattered numerous temperature records across parts of the North American continent this summer. Soon after the historic heatwave baking the desert Southwest recently, a new one is expected to develop this week. It will be a very large and long-lasting one, thanks to a powerful heat dome that will dominate the United States for at least two weeks.
The record-breaking heatwaves that were ongoing over the Pacific Northwest and desert Southwest U.S. brought some records we could have never imagine they were possible to reach. Nearly 122 °F (50 °C) in Lytton, Canada is just one example. Or the recent one, 130 °F (54.4 °C) recorded in Death Valley – the highest temperature ever reliably recorded on Earth.
The temperatures this weekend are already pretty hot across the country, with the hottest day over the southern United States, as well as across parts of the Northern Plains and California. The highest temperatures are locally in the upper 90s to low 100s.
Actually, the chart below is pretty impressive, with basically every state of the Lower 48 having very high temperatures. So the heat dome is already taking its place over the US, gradually increasing temperatures underneath. Those will get much higher in the coming days.
While the upper 90s are about near normal to slightly above normal for the south, those are around 10-15 degrees above across the north and the northwest US.
The end of July often brings the hottest temperatures of the summer, and most of the high-temperature records are normally recorded in this few weeks period back and forth, up to mid-August. It will not be any different this year, the extensive and persistent heatwave is coming up over the next two weeks. Drought will also increase in some regions, leading to more wildfires on the West.
The heatwave might not be as extreme as the recent ones, but it will expand across much of the country. Literally from coast to coast, thanks to the stable weather pattern with hot air mass trapped for days, starting this week. It will not be dry heat as humidity will also be increasing, making daily Heat Index into triple digits to feel it even worse.
An upper High placed over the northwest parts of the CONUS this weekend will gradually expand east and south through the coming days, bringing potentially record-breaking temperatures to the Great Plains. Some states will experience temperatures above 100 °F.
Only the far northeast and southwest parts will see near-normal or slightly below, the rest of the country will be baking in locally severe heat.
And according to the mid-range weather trends, the heat dome with an extensive heatwave will sit over the United States for at least the next two weeks. Temperatures should stay mostly above-normal throughout this period, extending into the first week of August.
The heatwave this time will also struggle to push nighttime temperatures lower, thanks to higher humidity in the air. This is what makes this heatwave also an extreme one, besides being a long-lasting event.
The strengthening heatwave across the U.S. coming up through the remainder of July will be the fifth event over the country this summer. The weather pattern that leads to this heatwave is related to the so-called – heat dome. The use of it has become quite popular in recent weeks, thanks to the historical heatwave over the Pacific Northwest.
THE LID ON A PORT – THE HEAT DOME
A heat dome is a term that we use when an area of high-pressure parks over a large portion of the continent and stays there for days. It works like a lid on a pot, trapping hot air mass underneath. This feature is often blamed to be responsible for long-lasting and deadly heatwaves around the world.
Earlier this spring, the general long-range forecast by the National Oceanic and Atmospheric Administration (NOAA) hinted at a likelihood that a heat dome will develop this summer. This is basically because the La Nina global pattern was expected to end with the neutral ENSO phase developing into the summer months. During La Nina, the Pacific Ocean waters are cooler in the eastern Basin and warmer in the western Pacific Basin.
When such a large-scale circulation happens, the temperature difference creates winds that blow the tropical air mass from west to east and the weather pattern gets this air trapped in the jet stream which ends up across the western portions of North America. Through summer 2021, a neutral El Niño/La Niña Southern Oscillation (ENSO) is in effect.
When we talk about a heat dome, it is basically that trapping dome of heat beneath the extensive blocking (upper) High and surface high-pressure system. The heat dome normally results in a persistent stable air mass that begins to accumulate inside the lid and develops the heatwave, often a significant one.
During the heat dome events, the heatwave is normally an extended period of excessive heat that could last for several consecutive days/nights or even weeks. And the temperatures observed during a heatwave are typically well above normal.
If the heat dome is very strong, for example, similar to the record-breaking heat across the Pacific Northwest of Canada and the United States at the end of June this year, this large lid helps to sustain the heatwave at the surface.
Under the blocking High, a surface high-pressure system fully establishes. The subsiding air masses beneath it warms the lower atmosphere. As the parcels of warm air masses are also rising, this excessive heat at the surface also expands vertically into the higher atmosphere. Thus creating a dome of high-pressure area, trapping everything below.
Through the remainder of July, an impressive large upper-level ridge will develop across most of the United States, extending also into portions of southern Canada. A related heatwave at the surface will be significant for many areas.
The late June extreme heatwave that developed under the heat dome over the Pacific Northwest was a remarkable event as it was actually happening so early in the summer. Persistent and large heatwaves across the Western United States and Canada typically develop through mid-summer.
HEATWAVE STRENGTHENS OVER THE NORTHWEST US, EXPAND ACROSS MUCH OF THE COUNTRY THIS WEEK
The weather models are in generally good agreement that the next week or two will see about 10 to 15 °F above-normal temperatures on average across much of the contiguous United States, even more than 20-25 °F in some parts. The heatwave will begin strengthening over the Pacific Northwest and the Rockies on Monday, then gradually expand east towards the Great Plains through mid-week days.
The attached chart below indicates the developing upper High with above-normal temperatures across the north-northwest parts of the U.S. on Monday, while temperatures are pushing higher also further south across the Deep South and the Southeast US.
The ongoing upper low, sitting over the desert Southwest, will provide high moisture for the monsoonal rains for a few more days. A rather typical setup for mid-summer, but quite an intense one this year.
A much above-average temperatures are likely to develop by Wednesday and Thursday, literally expanding from coast to coast. The hottest temperatures and the strongest anomaly will be over the Great Plains, with temperatures close to about 20 °F above normal for the typical late July period.
Millions across the country will be under the effect of severe heat, as the heat dome aloft will allow temperatures to climb much higher than usual every day.
Mid-week days will bring the highest temperatures across the Midwest and the majority of the central US as the heat dome slides southeast with time. The associated surface high-pressure system will contribute to mostly stable weather. Much higher temperatures will develop across the Northwest US as well on Wednesday.
Humidity will also be very high, giving the temperature feel (known as Heat Index) of more than 100 degrees F across the Great Plains all week. Under the heat dome, the normally significantly moist air mass advecting north from the Gulf of Mexico will bring the Heat Index into the triple digits. Those will be experienced from Texas, Oklahoma and Kansas to South Dakota as well.
Extreme, potentially record-challenging high temperatures will develop through the mid-week days across the northern Plains. Parts of Nebraska, South Dakota, and Minnesota could see the peak afternoon temperatures into the 110-115 °F range, possibly even a tad more in some places. This is *extreme* heat.
Many daily station records will be challenged or easily surpassed. And the high moisture will make it pretty exhausting with the Heat Index approaching 120F in places.
Nevertheless, it will be very hot across most of the central and northern Great Plains through mid-week. With temperatures from the upper 90s to low 100s from Wyoming to Montana, Dakotas, Colorado, Kansas, Iowa, and Wisconsin. With the worst being in-between as we see from the chart above.
The general weather pattern will lead to a temporal weakening of the massive heat dome aloft towards the weekend, but the heat underneath will remain severe. The trends are then hinting at a development of an Omega blocking pattern into next week that could push the temperatures much higher again and extend the heatwave into August.
OMEGA BLOCKING PATTERN DEVELOPS NEXT WEEK, EXTENDS THE HEATWAVE THROUGH EARLY AUGUST
The heat dome gets some changes towards the end of the week, thanks to some noticeable pattern deformations. A deep trough develops over eastern Canada and digs south into the eastern portions of the US. Bringing quite a significant refreshment for the Great Lakes and the northeast US.
Further west, upper-level ridging begins strengthening back north into western Canada, gradually developing an Omega blocking pattern. Such a pattern consists of the upper High in the middle, having two upper lows or troughs on each side. Looking at the chart below, the left image is showing these two lows – one over eastern Canada and another near the West Coast.
A very cold air mass is likely to spread across eastern Canada through early next week, with some lower temperatures also across the US. Those will come close to normal values, so at least some refreshment after a week of baking hot temperatures.
But a building Omega blocking normally means that stable weather develops, and so are the temperatures getting higher. So, with time, the heatwave is expected to intensify next week with the heat dome re-building across much of the United States.
Although these trends are basically mid-range, the ensemble forecast does support the extension of the sweltering heat well into early August. At least for some parts of the country.
EXTREME DROUGHT ACROSS THE WEST WILL WORSEN
The wildfire season across the western parts of the United States, including the Pacific Northwest region, is already at a much higher intensity than typically for mid-summer. The extensive and extreme heatwaves in the recent weeks brought the conditions favorable for wildfires earlier and more intense this year.
Exceptional drought is spread across the West, being the worst across California, Arizona, New Mexico, Utah, Nevada, Oregon, and Washington. Severe to extreme drought is also developed across Idaho, Montana, Wyoming, and across Dakotas to Minnesota.
Drought data are showing that it is already the worst Western drought so far this century. The U.S. Drought Monitor shows that nearly 70% of the Western U.S. is under extreme to exceptional drought conditions. And with these heatwave events, drought is only intensifying.
Thanks to the persistent massive heat dome, the upcoming heatwave will worsen the drought conditions, wildfire threats will increase. It could get especially dangerous when frontal systems return and thunderstorms with lightning help triggering more fires and dry winds increase.