August 2018 Atlantic Hurricane Outlook

Note: MDR refers to the Main Development Region for tropical storms.

Joe D’Aleo provides the explanation (H/T iceagenow) Then the Rains Came

It has been a changeable and at times extreme spring and summer. The cold and snow of March gave way to a cold April and some very chilly days well into the spring. Warmth with some very hot days followed in the early to mid summer. Then the rains came with strong thunderstorms. The wet August spell put an end to a borderline drought spell the last few years. It became very muggy, keeping nighttime temperatures up and air conditioners on.

The changes all have to do with the wind direction. The jet stream brought chilly air masses (and snow) into eastern Canada even into June. The winds around these cool air masses turn to the northeast here in New England coming in off cool land and water. Then increasingly warm air masses built north into the Canadian prairies and came east. The surface winds turned northwesterly. In summer, warm air crossing the Appalachians and sinking down into the Merrimack Valley and coast heats by compression 5F or more. Our hottest days come with these ‘downslope winds’. Historically all the 100F days come with a west to northwest wind.

When late July and August came, our surface winds turned southwesterly.

This change was caused by a sharp cooling of the subtropical Atlantic Ocean waters relative to 2017.

The Atlantic cool subtropical pattern leads to that stronger than normal Atlantic high pressure called the ‘Bermuda High’. In these patterns, this regular feature of our climate expands south and west and acts as a pump for moisture much like we see in the southeastern and eastern Asia monsoon flow. This causes nights to be warm and muggy, and days very warm and showery here in the east.

Uncomfortable yes, unprecedented heat no. You may be surprised that most of the extreme heat records for the region, country and world occurred in the early 20th century or earlier. The 1930s was the record decade in the United States as a whole. For the east, the 1950s was the warmest but extreme heat has occurred even in cold periods.

Summary: Effect on the Hurricane Season

As a general rule, when the subtropical Atlantic is warm, we have more hurricane activity, when it is cool, we have fewer storms.

One comment

  1. Hifast · August 26, 2018

    Reblogged this on Climate Collections.

    Like

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