On Friday Hurricane Hermine made landfall in Florida, making it the first to make landfall in the state in more than a decade.
Hermine, which weakened to a tropical storm shortly after landfall as is commonly the case with weak hurricanes, was poised to redevelop and pose a threat to the Mid-Atlantic states and possibly southern New England into the middle of the week.
Eighty-one years ago, nearly to the day, the famous Labor Day Hurricane of 1935 struck Florida with greater intensity than any other landfalling hurricane in the country’s history.
While Hermine had sustained winds of about 75 mph and dropped nearly 17 inches of rain in the Tampa/St. Petersburg area, the Labor Day Hurricane made landfall with sustained winds near 185 mph.
Both storms developed within days of Sept. 12, the date of the average peak frequency for such storms in the Atlantic Basin. Though the number of tropical cyclones in an average year numbers in the low teens, these storms are deserving of attention because they can wreak enormous damage to life and property.
They also play a substantial role in ventilating the tropical ocean and mixing heat from the tropics, where an annual surplus accumulates, to the high latitudes where an annual deficit stands in need of remedy.
It may seem surprising given our location in the middle of the North American continent, but hurricane research, employing both satellite observations and theoretical advances, at UW-Madison has led the way toward improved understanding and forecasting of these powerful storms.
For southern Wisconsin, this has been a rainy summer.
This August, the precipitation that fell over southern Wisconsin is about 200 percent of normal.
Total precipitation was also above normal for June and July. With the summer coming to a close, we are about 6 inches above normal.
On Aug. 19, 2.74 inches fell on Madison, setting a daily record, beating the old mark of 2.13 inches set in 2007. In particular, August has been exceptionally rainy compared to the prior five Augusts, as the only above-normal August in that stretch occurred in 2014 (plus-1.14 inches).
In Madison, the official measurement is made at the Dane County Airport. Even given the convective nature of the storm systems, all of Wisconsin was above normal in terms of the average precipitation for the month of June.
Because of the good rain, there is no area in Wisconsin that currently is classified as experiencing a drought condition. The soil moisture content for most of southern Wisconsin is above normal. The water levels in Lake Mendota and Lake Monona are above their average maximum summer levels.
The convective nature of summertime precipitation makes the amounts vary widely across the county and even across the city. So it is important to make as many observations of rainfall as possible. If you are interested in contributing observations, you can join the Community Collaborative Rain, Hail and Snow networks, or CoCoRaHS. This community includes over 15,000 volunteers who help measure and report precipitation type and amounts every day.
Observations of precipitation by a large group of volunteers are critical to understanding storms as precipitation varies widely from place to place even in a single storm. Such observations are useful for assessing flooding hazards. You can join CoCoRaHS at www.cocorahs.org.
As we head into the second half of August a subtle transition in our weather begins to occur — one that is probably hard to detect at first but that eventually becomes very obvious and then lasts for approximately eight months.
We are not talking about the gradual reduction in daytime high temperatures or the increasingly cooler to cold nights, though these are also beginning to invade.
Instead, we are talking about the nature of the storms that deliver our precipitation.
Throughout the summer, most of our precipitation comes in the form of thunderstorms, wherein large amounts of precipitation fall in a short amount of time from what we call convective clouds.
Most often these storms have life cycles of only a few hours and drop precipitation over a relatively small area.
These characteristics also make the exact timing and location of summertime precipitation difficult to forecast.
As we transition to late summer/early autumn, the thunderstorm frequency abruptly decreases and precipitation tends to occur in persistent, light to moderate rain events that will sometimes last an entire day.
This mode of precipitation is associated with the passage of what are known as mid-latitude cyclones — storms that live for over a week, during which time they can cover an area the size of 10 states and characteristically take on a comma-shaped appearance in satellite imagery.
As they progress across the country, these mid-latitude cyclones can drop precipitation (rain or snow) over enormous portions of the country.
Though not entirely missing from summertime precipitation, such events are definitely the exception rather than the rule in the summer.
This past week, the first really well developed such storm of the season paraded across Hudson’s Bay in Canada and another one like it is poised to do the same late this week.