Are thunderstorm rain amounts hard to predict?

Posted on July 29, 2024 by WeatherGuys Editor

Meteorologists would generally agree that summer is the most challenging time of year for making accurate rain forecasts.

Neither scattered nor isolated stroms give any indication of whether the storm is severe or when it’s happening. These two terms only refer to how much of an area it is expected to cover. (Image credit: Weather Station Advisor)

During summer, there generally is enough moisture and atmospheric instability to help fuel the development of rain showers. Scattered thunderstorms are one of the most difficult weather events to forecast.

Thunderstorm development also is difficult to pinpoint in the summer. Summer thunderstorms sometimes develop from subtle outflow boundaries from previous thunderstorms, sea-breeze fronts, higher terrain or localized solar heating.

Numerical weather prediction models help define areas where conditions will be right for the development of thunderstorms.

These models are designed to calculate atmospheric conditions over a large area but cannot pinpoint the locations of a particular thunderstorm development. Summer thunderstorm forecasts are often broad, using terms like “isolated” and “scattered.”

“Isolated” corresponds with a 10% chance of measurable precipitation (0.01 inches or greater). “Scattered” indicates that there will be 30% to 50% coverage of convective weather, such as thunderstorms, in the forecast zone. “Widely scattered” is used for a forecast of 20% coverage. If you are planning outdoor activities and the chances of rain are 10% to 20%, it is a good idea to have backup plans for moving indoors.

Along with numerical models, satellites and ground based radars contribute a significant amount of additional information for forecasters as the storms develop. It is wise to have a weather radar app on your smartphone to see whether any thunderstorms are nearby.

The National Weather Service issues a severe thunderstorm warning when a thunderstorm is producing wind gusts of at least 58 miles per hour or hail of at least 1 inch in diameter.

If a tornado is detected, a tornado warning will be issued either in replacement of or concurrently with the existing severe thunderstorm warning. Severe thunderstorm warnings should not be ignored, as severe thunderstorms are dangerous and occasionally deadly.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic Sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather

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How does our late spring/early summer precipitation stack up historically?

Posted on July 22, 2024 by WeatherGuys Editor

If you think the first half of the summer has been unusually wet, it is not merely your impression — it is a measured fact.

Between May 1 and July 15, Madison received 21.85 inches of precipitation, punctuated by the 18th-wettest day in Madison history (3.3 inches on July 14) — which is 10.16 inches above normal.

In other words, since May 1 we have received nearly twice the normal rainfall in the wettest part of the year. In fact, this amount is the all-time record for May 1 to July 15 in Madison and one of only two such periods (the other was in 2000) during which 20 inches of rain or more has fallen. Perhaps unsurprisingly, in the 76 days of this interval, we have had only 30 completely dry days — well less than half. Consistent with this incredible total, only one time in this period have we had as many as five consecutive days during which not even a trace of precipitation fell — May 14-18.

Yet another interesting fact regarding this remarkable wet period is that over the same interval the daily average temperature has been 2.54 degrees Fahrenheit above normal. Historically, rainy late spring/early summers have been closer to average or even a bit cooler than normal as a result of the persistent cloud cover that usually accompanies such prolonged rainy periods. The four other rainiest May 1 through July 15 periods, for instance, averaged 0.55 degrees below average (2000), 0.44 degrees above average (2013), 1.76 degrees below average (2008) and 0.30 degrees below average (1993) — nothing at all like this year.

Since warmer air has a greater capacity for water vapor which is then available for conversion into precipitation, the combination of unusual precipitation amounts and higher than average temperatures is clearly suggestive of global warming.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: History, Seasons

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What happens to the wintertime cold pool in summertime?

Posted on July 16, 2024 by WeatherGuys Editor

We have commented a number of times in the past few years about the areal extent of the hemispheric cold pool of air at 850 millibars (about a mile above the surface) during the winter. As one might expect, that pool expands dramatically from October through February and then begins to contract as we move toward spring and summer.

People cool off in misters along the las Vegas strip last week. the northern Hemisphere’s cold pool of air about a mile above the earth, which extends 27 million square miles in winter, often disappears entirely during the summer. (Photo credit: John Locher, Associated Press)

Our analysis uses the minus 5 Celsius isotherm (line of constant temperature) and has shown that the average winter cold pool area has systematically shrunk for at least the past 76 years. One might reasonably wonder if this cold pool survives at all during the height of Northern Hemisphere summer. As it turns out, some summers have a number of days in mid-July on which there is absolutely no air at 850 mb that is as cold as minus 5 Celsius. Roughly half of the last 76 years have had such a “vanishing” cold pool, with the pool getting very close to vanishing many of the other years.

The calendar date on which the smallest areal extent is observed in a given summer varies from around July 4 to as late as July 23. Thus, later this week we will likely be close to the day of the minimum area for the year.

With no intention to distract from the pleasant summer we are all currently enjoying, reaching the annual minimum this week means that by the beginning of next week the cold pool will begin its slow, inevitable expansion again — culminating in the coldest week of the year in late January when the cold pool will cover nearly 27 million square miles.

Enjoy the summer — it can’t last!

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Seasons

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Was Hurricane Beryl a special storm?

Posted on July 8, 2024 by WeatherGuys Editor

All hurricanes are special, given the extensive damage they can cause when they make landfall. Beryl was considered extra special because it was a record-breaking storm.

Hurricane Beryl gained 95 mph in less than two days, a process known as rapid intensification. (Image credit: NOAA)

Beryl rapidly intensified from a tropical storm to a major hurricane (Category 3 or higher) with wind speeds that increased to 95 mph in less than two days. Category 3 storms have sustained winds between 111 mph and 129 mph. Only six other Atlantic Basin storms have intensified this quickly and those storms all happened after August, the typical time of year with conditions favorable for hurricanes.

Beryl is the earliest Category 4 or 5 hurricane on record. It set the record for the earliest Category 4 (sustained winds of 130 mph to 156 mph) by more than a week, breaking the record set by Hurricane Dennis on July 8, 2005. It is the easternmost major hurricane to form in the month of June. Beryl is the only Category 4 hurricane in the Atlantic Basin to occur in June and only the third Category 3 Atlantic Basin hurricane to occur in June.

Beryl also broke the record for the earliest Category 5 hurricane by more than two weeks. Category 5 hurricanes have sustained winds of 157 mph or higher. Hurricane Beryl was deemed Category 5 on July 1, breaking the record set by Hurricane Emily on July 17, 2005. Beryl is the strongest July Atlantic hurricane ever measured.

Forecasters expect the 2024 hurricane season to be one of the busiest seasons on record. This is due to the ocean water temperatures being so warm at this time of year. Warmer waters support stronger hurricanes. A La Niña pattern is predicted to develop later in the season, which makes the atmosphere more supportive for hurricanes and could also steer more storms toward the U.S.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: History, Tropical

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What is a heat dome?

Posted on July 1, 2024 by WeatherGuys Editor

“Heat dome” is a term used by the news media to explain extreme heat conditions across large geographic regions.

The American Meteorological Society maintains a glossary of meteorological terms and added the term “heat dome” and this definition in March 2022: “An exceptionally hot air mass that develops when high pressure aloft prevents warm air below from rising, thus trapping the warm air as if it were in a dome.”

Heat domes involve high-pressure areas that trap and heat up the air below.  (Image credit: NOAA)

This is not the same as a heat wave, which is a spell of three or more abnormally hot days.

A heat dome develops when a ridge of high pressure builds over an area and resides there for a week or more. High pressure is associated with very few clouds and lots of sunshine, leading to warm temperatures near the surface. The sinking motion in the high pressure prevents warm air near the surface from rising. This motion causes further warming of the air by compression. Unless the upper atmospheric pattern changes, the high pressure will continue to exacerbate the hot conditions. The ground also warms and loses moisture, which can lead to drought conditions and the risk of wildfires. The term “heat dome” may also be used in describing drought events.

Hot and humid conditions can lead to heat-health issues. The heat index indicates how hot it feels. The index is calculated using an equation that is a function of air temperature and the relative humidity. The heat index is sometimes referred to as the “feels-like” temperature.

When our bodies get hot, we cool down by sweating. The sweating does not directly cool our bodies — it is the evaporation of the sweat that cools us down. If the air has a high humidity, then the rate of evaporation is reduced. This hampers the body’s ability to maintain a nearly constant internal body temperature.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ ssec.wisc.edu or jemarti1@wisc. edu.

Category: Meteorology, Seasons, Severe Weather

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