© Fernando Caracena, 2019
A recent headline on an AccuWeather article reads as follows: "
DC to feel almost as hot as Death Valley on Saturday"
The quoted article discusses the conditions expected at the surface, which will be miserably hot and sticky back East. DC will live up to the famous saying, "Capital punishment is having to spend the summer in the nation's capital! " Oddly, the mechanism left over from a cool wet spring is partially responsible for this flash heat wave. This mechanism is an overactive jet stream unusually far south for this time of year because of a blocking high pressure area over Alaska.
The above figure illustrates the situation as of the evening of July 18 2019, which corresponds to July 19, 0000 UTC (the old GMT and military Z, see article here). The figure depicts the temperature distribution as color filled areas (red for hot, and blue for cold) and height contours (in tens of meters) of the 500 mb pressure surface. This pressure surface, which is close to the mid depth of the atmospheric mass is often used by meteorologists in analyzing weather situations. This figure was generated by an objective analysis program that numerically analyzes numerous balloon soundings of the atmosphere, in this case, released at 0000 UTC. Note that the height contours in the figure have two interval spacings, 100 meters below 5800 meters, and ten meters above this level to contour the top of the ridge across the South. The course spacing for the lower parts of the surface was chosen to not obscure the pattern with a dense pattern of too many contours.
An advantage of depicting the height field of a pressure surface is that the local density of height contours indicates the strength of the wind there, which generally follows the direction of the contours. In the present situation, the height contours are compressed across the northwestern tier of states indicating a jet streak moving east. Initial conditions from global NWP (numerical weather prediction) models (not shown) indicate that this jet streak is part of a longer jet stream that is arriving from over the Pacific. The jet stream is situated over a sharp front, which separates unusually cold air to the north from unusually hot air to the south.
The situation painted by the figure is that there is a dynamic thermal separation maintained by the dynamics of a jet stream unusually south of its normal position, which is fighting against the normal expansion of a continental ridge northward at this time of year. What we have here is the famous kid's conundrum: "What happens when an irresistible force meets an immovable object?" A subsequent post will continue this discussion by addressing the question, "why is the hot temperature at 500 mb such a big deal for us humans down on the surface of this blue planet?" There is an important topic here that connects with the global climate.