I’ve created some combined hovmöller diagrams for those that don’t want to have to toggle the waves on and off. My goal with these is to emphasize readability, so the contour intervals have been tweaked and WIG waves have been removed.
Back in the late spring there were signs of La Niña. Some networks stopped just short of forecasting a La Niña event, while others were fairly bullish about the impending event. However, over the past couple of weeks, there have been many articles claiming that La Niña isn’t actually going to happen. Even the National Weather Service’s Climate Prediction Center has adjusted its forecast to ENSO neutral, which is a fancy way of saying nothing. Now that summer is (almost!) over, it’s worth looking at what happened with La Niña, and why we should care…
Someone asked me the other day if heat dome is a real meteorological term. This was the first time that I’d heard it, but apparently I live under a rock (check the twitter hashtag #HeatDome). It seems to be another buzz term, the summer equivalent of polar vortex, which the popular media has been misusing for the past few years. However, unlike polar vortex, heat dome is not a word that I’ve ever heard meteorologists use. Google trends tells us that the term has existed for a few years now and spiked previously in 2011, which I presume was during another heat wave. My guess is that heat dome is a fancy way of saying high pressure system.
It’s clear from any surface analysis that high pressure has dominated the eastern half of the country for the past few days. Although there’s not much of a northerly flow over the East Coast (at least not from what I’ve looked at), subsidence and clear skies within the broad area of high pressure allow the temperature to rise as it pleases.
Below is a map showing temperature anomalies for Sunday, July 24, 2016 (also known as today). You’ll notice that most of the CONUS (continental United States) is painted red, indicative of warmer than average temperatures. You can see evidence of the ridging in the contours over the eastern CONUS. (This would be more apparent if I’d chosen a better contour interval.)
To put this into perspective, I downloaded temperature data for Reagan National Airport (DCA), just south of DC, from the National Centers for Environmental Information. The data plotted in the histogram are maximum temperatures for all 31 days in every July from 1981 through 2015, which is 5 years longer than the time period that the National Weather Service uses for climate statistics. The maximum temperature reported at DCA for July 23, 2014 was 98 degrees. There are only 79 days out of 1085 (7.3%) that have temperatures of at least 98 degrees. Another way to put this is that about 93% of all July days in DC have high temperatures cooler than 98.
Clearly, this isn’t a common occurrence. But, what if we rephrased the question: how many July days in DC are warmer than 95 degrees? Can you really tell the difference between 95 and 98? It turns out that roughly 17% of all July days have high temperatures of at least 95 degrees in DC and about half of all July days have maximum temperatures of at least 89 degrees. 24 of the 79 days (30%) with high temperatures greater than 98 degrees have happened since 2010, but only 20% of the days with high temperatures greater than 90 degrees have occurred since 2010. The average temperature in July from 2010-2015 is 2.33 degrees higher than the average temperature in July from 1981-2009.
So what do all these numbers actually mean? Well, in a nutshell, DC has been warmer in the last half-decade than it was before. This is probably partially related to anthropogenic global warming. The numbers also tell us that while 98 degrees is an abnormally high temperature for DC, temperatures of at least 90 degrees are as about as common as getting heads on a coin flip.
I suppose after a post about all this heat, it would be nice to take a look to the future; when will we see abnormally cold temperatures? As shown in the map below, the CFS model has a hint of abnormally cold temperatures for the eastern CONUS about 2.5 weeks from now. I don’t want to get anyone’s hopes up though; you don’t need me to know to be cautious in believing 3 week forecasts.