Past “peak heat” this summer? Iselle threatens Hawaii

Have we already had the hottest day this summer in Minnesota?

Statistically the odds say yes. The weather maps seem to agree. The hottest day so far in 2014 in the Twin Cities was 92 degrees on July 21.

1805 2014 temps
NOAA Now Data

That's close to the sweet spot for the average dates of the hottest temperatures of the year in Minnesota.

Climate Central has a nice interactive graphic showing "peak heat" for cities in the United States based on National Oceanic and Atmospheric Administration data. It turns out the Twin Cities is statistically most likely to see the hottest day of the year between July 11 and 15.

Climate Central/NOAA

It's interesting how different climate zones in the United States play a role in determine the dates of "peak heat" each summer. A cool Pacific Ocean delays peak heat until fall in the West. Arizona bakes under a June sun, and the dry desert atmosphere responds to peak solar intensity more quickly.

Here's more from Climate central on why some areas warm more quickly that others in the summer season.

Statistically, the hottest time of year in the continental U.S. comes in mid-July, a few weeks after the sun reaches its highest point in the sky on the summer solstice. This year the solstice was on June 21.

But that mid-July peak heat is just a national average: if you look at smaller regions, the peak temperature might come earlier (that’s true for southern Arizona and New Mexico), or as late as September (most of the extreme Pacific Coast).

Of course, any given year could see temperatures peak at a different time due to short-term variability. But if you move from, say, Tucson to Los Angeles, you’re going to notice temperatures peak much later in the summer than what you’re used to seeing. The difference can be dramatic even within a single state: El Paso’s hottest weeks come in late June, while Houston’s come in August.

The reason for all of this spread has to do with local climate variations: in parts of the Southwest, the North American monsoon makes July, August and September relatively rainy and cool. Along the West Coast, by contrast, cool, moist air from the Pacific Ocean nudges its way ashore in late spring. Depending on where you live, it’s called either “May Gray” or “June Gloom”.

Iselle threatens Hawaii

A still strong but weakening Hurricane Iselle continues to chug westward across the open waters of the Pacific Ocean, headed for the Hawaiian Islands.

1805 iselle sat 2

Iselle will likely weaken to category 1 hurricane or a strong tropical storm as it approaches the Big Island of Hawaii Thursday night.

1805 iselle 2

Iselle has been a well formed, tightly wrapped storm. In fact, Iselle is so well developed it's reached the top 4 percent of all hurricanes which make the designation of an "Annular Hurricane." That makes the forecast for weakening a little more dicey for Hawaii, and Iselle may maintain strength a little longer than your average hurricane.

Weather Underground hurricane guru Jeff Masters has a look at Iselle and an excellent description of Annular Hurricanes.

Hurricane Iselle began a gradual weakening process overnight, falling from a Category 4 storm with 140 mph winds to Category 3 with 125 mph winds at 11 am EDT Tuesday. Ocean temperatures beneath the storm are about 26°C, which is marginal for maintaining a hurricane, and plots of Maximum Potential Intensity from the Center for Ocean-Land-Atmosphere Studies show that the Iselle should only be able to maintain Category 2 strength with these ocean temperatures and the current atmospheric background conditions. Iselle is headed westwards at 9 mph towards Hawaii, and will begin affecting the Hawaiian Islands Thursday night. Satellite images show an Iselle is an impressive storm with a large eye and intense eyewall clouds with very cold cloud tops, but the storm is no longer symmetric, due to wind shear and dry air eating away at its southwest side. The relative lack of spiral bands and large, thick eyewall qualify Iselle to be a rare breed of hurricanes known as "annular". Annular hurricanes are a subset of intense tropical cyclones that are significantly stronger, maintain their peak intensities longer, and weaken more slowly than average tropical cyclones. Only 4% of all hurricanes are annular hurricanes. 

1805 iselle nasa
True-color MODIS image of Hurricane Iselle from 19:40 UTC (3:40 pm EDT) August 4, 2014. At the time, Iselle was a Category 4 hurricane with 140 mph winds. Iselle was showing an annual structure--a lack of spiral bands and large, thick eyewall. Image credit: Weather Underground/NASA.

Stay tuned.

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