There's a decent chance this afternoon's high will reach 90 degrees for only the fourth time this summer, so we took this rare opportunity to update our 90-degree day graph. On average we would already have ten days above 90 degrees by now, but every few years, the last being 2009, the extreme heat disappears. If today's high reaches 90 we'll tie 2009 and will have doubled 2004's frequency. One thing we won't do is have at least ten 90-degree days this month, something that has happened in July for the last four years.
Temperature usually decreases as you go up in the atmosphere. That's true this morning; for the first couple of miles up the temperature reverses course and the air starts getting warmer. That temperature inversion is the key to this afternoon's forecast because it is preventing clouds from forming. The lack of clouds lets the sun heat the humid air near the surface. That heat is expected to build up to the point where it bursts through the inversion with clouds, showers and thunderstorms following soon thereafter. Showers are possible by mid-afternoon and becoming more likely by early evening. Today's high will be really, really close to 90.
A cold front will pass through the city around sunrise tomorrow, but the rain may linger well into the morning, especially east of the city. Thursday's high will be in the lower 80s. Once high pressure moves in it promises mild and dry days for Friday and Saturday, but rain is possible on Sunday as we get a textbook example of an upper-level disturbance moving across the country and pulling the jet stream into a trough over the East Coast.