Here come da ridge!
[ECMWF model forecast; image source: wright-weather.com]
For extremity of summertime ridges of high pressure aloft over the United States, meteorologists look at "500 millibar heights" -- in non-technical terms, that represents how high the pressure is a few miles above the Earth's surface -- and in particular how close those heights get to 600 decameters (19,685 feet). That's the benchmark, as it's about as high as those "heights" ever get.
And they're gonna get pretty close to that in a few days. [July 17 update: The 500 mb height reached 600 dm this evening over Omaha. Preliminary data suggests that's a record for that location and the farthest north 600 dm has been reached in the central states.]
Translation of all of that into what it means for people: The extreme heat of 2011, which has already been remarkable in parts of the country during the early part of the season including before summer officially even started, is about to expand as we enter into the next phase of the pattern.
[July 18 addendum: Wichita's # of 100+ degree days up through this point in the season and the average temp since June 1 have exceeded that of any year of the 1930s, and each is just shy of the highest, which occurred during the extreme heat wave of 1980.]
In the southern states, particularly the southern Plains, afternoon high temperatures have consistently been particularly extreme, assisted by how dry the soil is. Rather than some of the sun's energy going into evaporating soil moisture, it gets efficiently converted into quickly-rising temperatures each day.
And in turn, the soil dries out even more, worsening the drought.
Immediately adjacent, it's been the opposite, with exceptionally wet conditions including record flooding.
As the uber ridge expands and the heat surges north during the coming days, the atmosphere will have to work harder there than farther south for each degree of the afternoon high temperature, but any limitation in that department will be made up for in the heat index, a measure which is an attempt at quantifying the combination of heat and humidity.
Soil and crop moisture evaporating will boost the dewpoint, which translates to how humid the air feels. Dewpoints and heat indices are expected to rise to exceptionally high levels as far north as parts of the Dakotas and Minnesota this weekend into early next week.
What's more, overnight low temperatures will be quite high along with oppressive humidity.
Please be careful and take precautions! Heat (including the effects of humidity) nowadays is typically the #1 weather-related killer in the U.S. (other than vehicular accidents due to wet, snowy, or icy roads), with an estimated 1,500 each year dying on average.
This season is shaping up to be a memorable hot summer along with those such as the ones in 1930, 1934, 1936, 1954, 1980, and 1988.
We'll have to see when all is said and done, looking back from the vantage point of when we get to September and October, exactly how 2011 ends up stacking up.
There are various ways of comparing the heat, including the persistence, expanse and extremity of it.
In regard to the latter, many state high temperature records were set in the 1930s during the peak of the Dust Bowl, especially in 1936.
A significant contributor to that was the expanse of the drought.
You might be familiar with Drought Monitor maps that appear on The Weather Channel and weather.com. The Drought Monitor is a great initiative and set of products, however a significant limitation for historical perspective is that the maps and data that we have become so accustomed to and reliant upon exist only back to 2000.
The Palmer Drought Severity Index (PDSI), one of the inputs to the Drought Monitor product, is available back more than a century.
It focuses on long-term water levels rather than short-term moisture (though is at least partly also reflective of the latter).
The map plots below represent, as best as I can tell, a reasonable apples-to-apples comparison between the current drought and that during the peak of the Dust Bowl, in particular 1936, when 14 states set record high temperatures that still stand. (There were other factors that made the Dust Bowl what it was; here I'm referring specifically to meteorological and hydrological ones.)
These are maps for June since that's the latest month for which one is available for in 2011, and there haven't been any huge changes during the first couple weeks of July.
You can see the extraordinary dichotomy of extreme wet/dry that exists in such close juxtaposition to each other this spring & summer in the U.S., as well as the much greater expanse of drought in the mid-1930s. That helped boost temps in many states to values that have not been exceeded since.
Although the current drought is not as expansive, by this measure the driest categories are actually more prevalent than in June 1936. [July 17 addendum: A 9-month lack of precipitation in Midland shattered the previous record.]
The combination of expanse and severity stands out more in 1934. In both of those 1930s Junes, the focus was farther north than in 2011.
[PDSI images source: NOAA/ESRL Physical Sciences Division.]
What happened in the 1930s and other decades reinforces that there have always been extremes in weather, and there is always natural variability at play. What's changing now is the nature of those extremes, and also what's important is the context.
This time, the extreme drought, heat, and wildfires are occurring along with U.S. extremes this year in rainfall, snowfall, flooding, and tornadoes, and many other stunning temperature and precipitation extremes elsewhere in the world in recent years as well as, as I posted on my TWC Facebook "fan" page, record-shattering 500 millibar heights in high latitudes. And all of this is happening while there's an alarming drop in the amount of Arctic sea ice.
The nature and context of the extremes is the difference between the 1930s and now.
[Source: Polar Science Center; click on image for full-sized version.]
