Hottest Day of the Year Map

Here is an interesting map showing the average day with the warmest temperatures around the U.S.   Amazing differences, with some places having their warmest temperatures in June (southern Arizona and New Mexico), while others having their typical high temperatures in September (coastal CA).  

A three month range!

Why do different locations have such radically different dates of warm temperatures?   Let's think about it.

On a superficial level, one might suppose that the hottest day of the year would be the day when the sun is strongest and most overhead:  the summer solstice (June 21st).

But thinking about it a bit more carefully, it is apparent that warming occurs as long as the amount coming in, i.e., the solar radiation, exceeds what is going out (infrared radiation to space).  And that might be  later than the maximum solar radiation.

Consider the daily (diurnal) temperature variation--the warmest temperatures are not at solar noon (roughly 1 PM PDT) but several hours later (around 5 PM) in summer.    Furthermore, there is thermal inertia of the atmosphere and the surface that further delays the warming (takes a while for the soils, water, and air to warm up).

So one might conclude that having the warmest day of the year in July or early August might be expected.

But why are things so different around the country, with the warmest day ranging from near the solstice (June 21st) to the equinox (Sept 21)?

I think I can offer an explanation.

First, consider the very early dates from western Texas to Arizona.  I think those are due to the North American or Southwest Monsoon, a period of clouds and thunderstorms over the interior SW U.S. that develop in late June and extends to early September.  With all the clouds and moisture moving in during late June, the high temperatures are lowered during mid to late summer, pushing the highs into June.

But why are the high temperatures so late in the summer in steamy eastern Texas, Louisiana and Arkansas?    Because they have a lot of thunderstorms and clouds during late spring and early summer, which delays the onset of high temperatures.  This precipitation characteristic is illustrated by the monthly precipitation in Houston, Texas (see below), with June being their wettest month.

Moving back to the western U.S., most of the interior has a late July maximum, which makes sense from the radiation viewpoint discussed earlier.   But what about the craziness along the thin coastal strip, where many locations don't hit their high temperatures until late August or September?

The reason has to do with the cool Pacific Ocean and the eastern Pacific high that dominates the region from late spring into mid-summer.  The Pacific Ocean is cool year around (roughly 50F near the West Coast).   During spring and most of the summer, high pressure exists offshore, with lower pressure over Arizona (see surface pressure map for June), which creates a large onshore pressure gradient along the West Coast that pushes the cool air and low clouds into

the coastal zone, keeping things cool (see satellite image for Monday afternoon).

By September, the situation changes (see below), with pressure building over the Pacific Northwest and the coastal pressure gradient weakening.

Occasionally, high pressure builds further over the inland western U.S., as cold air starts to move southward, producing offshore flow that is much warmer than the cold onshore flow off the Pacific.  Furthermore, the offshore flow can be warmed by compression as it sinks along the West Coast mountains, producing high temperatures along the coastal strip.  These situation produce the high temperatures during late summer near the coast.

In short, the variations in the dates of the high temperature days actually make sense with a little meteorological sleuthing.

Escaping the Southwest Heat

Residents of the Puget Sound region can breath a sigh of relief that they are not living farther south, since an heat wave is now frying the southwest U.S.  As shown below, high temperatures east of the California coast were generally above 100F today, with the area stretching from Palm Springs to Las Vegas soaring about 110F.  Needles, California rose to 123F.  None of these are all-time records, but several daily records have fallen.

Here in the Pacific Northwest, the temperatures aloft (say at 5000 ft)are quite warm over the entire region, but the influx of cool air and considerable low clouds kept it cool over NW Washington (mid to upper 60s).  Middle 70s over the south Sound, but near 90F in Portland.   Cross the Cascades to escape the marine influence and 90s are the rule in the Columbia Basin.

The cause of the SW heat?  A upper level ridge of high pressure that brings sinking air aloft (and sinking causes compressional heating)--see 500 hPa (around 18,000 ft) weather map for 5 AM this morning.

A weak cold front will move through western WA tomorrow, bringing clouds and some light showers...temps only getting up into the mid-60s in western WA.   If you want to escape the cool, moist weather, you might head down to Palm Springs, where it will be a sizzling 121F tomorrow and around 115F for the rest of the week (see weather.com forecast below).