How important was the weather forecast on D-Day?

Posted on June 1, 2026 by WeatherGuys Editor
ERA-CLIM, a ECMWF reanalysis, forecasts at 84 h (upper left), 60 h (upper right) and 36 h (lower left) range, all valid at 12 UTC 6 June 1944, and the 13 UTC analysis of surface pressure for 6 June (Image credit: Forecast for Overlord by J. M. Skagg, Gen Eisenhower's chief meteorologist).
ERA-CLIM, a ECMWF reanalysis, forecasts at 84 h (upper left), 60 h (upper right) and 36 h (lower left) range, all valid at 12 UTC 6 June 1944, and the 13 UTC analysis of surface pressure for 6 June (Image credit: Forecast for Overlord by J. M. Skagg, Gen Eisenhower’s chief meteorologist).

D-Day was the name given to the June 6, 1944, invasion of the beaches at Normandy in northern France by troops of the Allied Forces during World War II. The goal was to cross the narrow, often stormy, English Channel into Nazi-occupied France.

The successful D-Day invasion required a combination of environmental factors: initial invasion around sunrise and at at low tide; nearly clear skies; at least 3 miles of visibility; close to a full moon; relatively light winds; non-stormy seas; and good conditions persisting for at least 36 hours and preferably for four days. To make the task of forecasting even more difficult, the Allies needed at least two days advance notice of these conditions —and because the science of meteorology was only just beginning to modernize, at the time this was a nearly hopeless task.

Krick strongly advocated for analog forecasting. The British Royal Air Force team was led by Sverre Petterssen, who was an expert in the science of air masses, cyclones and upper air patterns. His co-leader was C.K.M. Douglas, who made forecasts by interpreting weather maps and pattern recognition. Reginald Sutcliffe, the British scientist who laid the groundwork of modern weather forecasting, also worked on the D-Day forecast.

While the weather forecasters initially suggested June 5, they realized that a one-day postponement would offer better conditions. This prompted Eisenhower to invade on Tuesday, June 6. German meteorologists did not predict the clearing on the morning of the invasion, adding to the Allies’ advantage.

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

Category: History, Severe Weather, Weather Dangers

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What is fire weather?

Posted on May 26, 2026 by WeatherGuys Editor

Wildfires need three essential elements: heat, oxygen and dry fuel. Fire weather refers to a mix of meteorological conditions that make it easy for wildfires to ignite and spread quickly. The main weather factors are low humidity, strong winds, warm temperatures and atmospheric instability.

Dixie Fire in 2021 burned 73,240 acres in Lassen Volcanic National Park. (Photo credit: Steve Ackerman)

When humidity is low, moisture is drawn out of plants, turning them into highly combustible fuel for fires. Extended dry spells or droughts remove the moisture that helps prevent ignition, making the physical environment more vulnerable. Strong winds provide fires with oxygen and carry hot embers ahead of the fire line, starting new fires. Warm temperatures heat dry vegetation, making it more likely to ignite. Unstable atmospheric conditions allow smoke plumes to rise rapidly, intensify fires, and create unpredictable shifting winds.

National Oceanic and Atmospheric Administration meteorologists use specific alerts to warn when dangerous conditions are present. A Fire Weather Watch is issued when conditions may develop in the next 12 to 72 hours, while Red Flag Warnings are issued when critical fire weather conditions are imminent or already occurring.

Thresholds for issuing Red Flag Warnings vary by region due to differences in vegetation, terrain and climate, but they generally include winds of at least 15 mph measured 20 feet above ground level, relative humidity below 25% and 10-hour fuel moisture (a measure of the water content of dead vegetation in the quarter-inch to 1-inch size range) at or below 10%. In rare cases, the Red Flag Warning may designate an area as being in a “Particularly Dangerous Situation,” such as when warning criteria are greatly exceeded.

During Red Flag Warnings, dry vegetation and strong winds make it easy for fires to spread rapidly, posing a serious threat to people and property. Under Red Flag Warnings, even a single spark can trigger a major wildfire, so it is important to avoid using open flames outdoors during these events.

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: Phenomena, Severe Weather, Weather Dangers

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Who or what triggers the decision to sound a tornado siren?

Posted on May 18, 2026 by WeatherGuys Editor

The National Weather Service is responsible for issuing forecasts, watches and warnings for a variety of weather and water hazards. A “warning” is issued when hazardous weather poses an immediate threat to life or property.

Dane County uses a zone-based method for activations, so only sirens directly affected by the areas included in a Tornado Warning or a Severe Thunderstorm Warning with the “Destructive” tag will sound. That warning area is set by the National Weather Service office in Milwaukee/Sullivan. (Image credit: EmergencyManagement@danecounty.gov)

A warning can be issued for a variety of hazardous weather, including tornadoes, thunderstorms and flash floods. These threats can form quickly, last a few minutes and impact a small area. Forecasters must rely heavily on observational data from Doppler radar, satellite, and other ground-based equipment and sensors to inform them during the warning process.

Hazardous weather also occurs on a larger scale and are slower evolving, such as low-pressure systems that produce ice storms. In these cases, forecasters use numerical weather models and statistical analysis to come up with a most likely forecast. The forecaster studies the data to determine the need to issue a warning. Once forecasters have high enough confidence there will be significant impacts, a warning is issued as far in advance as possible. The warning covers areas that are expected to see hazardous weather.

The broad reliance on the NWS is a good thing, as publicly accessible data that is standardized and updated regularly helps ensure accuracy of the forecasts.

Sirens are typically activated by city or county officials, usually a police or fire department or emergency management personnel. Individual counties’ emergency management teams or local municipalities own the sirens. The NWS will indicate where and when a tornado warning is in effect, and the local dispatch centers active the sirens. Each county uses its sirens differently. Some will sound sirens across the entire county. Other will only activate sirens within the warned area.

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, Weather Dangers

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Are we ahead of schedule with the spring greening?

Posted on May 11, 2026 by WeatherGuys Editor

We all just lived through a historic April here in Madison, and broadly across the region.

In Madison and Milwaukee, April 2026 was the wettest April since record keeping began in 1869 and 1871, respectively. Madison received 7.26 inches of rain during the month, more than half of that amount coming on just three days (1.34 inches on April 2, 1.53 inches on April 14 and 0.98 inch on April 17).

Rain fell on 22 of the 30 days of April in Madison, with measurable rain on 18 of those days. On two days we set new daily records for precipitation. The previous record for the month was 7.19 inches, set in 1909. Milwaukee shattered its old monthly record by 2.11 inches, as 9.49 inches of rain fell on the city this April, breaking the prior record set in 2013. As was the case in Madison, it rained on 22 of the 30 days, with 18 of them recording measurable rain — one was a daily record.

The extraordinary total featured six particularly rainy days, with nearly half of the monthly total coming on consecutive days from April 12 to April 17. Both cities also had relatively warm Aprils, as Madison came in 15th and Milwaukee ninth all-time for April warmth, that ddespite a record chilly end to the first week of the month.

The rainy, relatively warm April has put the spring greening well ahead of schedule in the southern part of the state. In fact, the regional spring leaf-out this year is running two to five weeks early, with southern Wisconsin on the high end of that range.

So, if you have been wondering if the trees look a bit ahead of schedule for the first week of May, your suspicion is correct. There really is some potency to those April showers.

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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Is global warming impacting bird migration?

Posted on May 4, 2026 by WeatherGuys Editor

Global warming refers to the rise in global temperatures due to the increasing concentrations of greenhouse gases in the atmosphere. One impact is that northern latitudes are experiencing warmer mean annual temperatures and experiencing earlier springs, milder winters and delayed falls.

 The American Robin population in WI is arriving ~13 days earlier in 2010 than it did in the year 1990.
FAD – First arrival date; MAD “Mean” or “population: arrival data. (Graph: Jones, G. M., B. Zuckerberg, A. T. Paulios. 2012. The early bird gets earlier: A phenological shift in migration timing of the American Robin (Turdus migratorius) in the State of Wisconsin. The Passenger Pigeon, 74:131-140.)

Bird migration is a natural phenomenon that involves the seasonal movement of birds from one place to another. Their evolutionary adaptation allows species to take advantage of seasonal resources and avoid harsh winter conditions. Because seasonal change is a dependable feature of our planet, migratory bird species have adapted to this seasonality as it coincides with the optimal conditions for feeding, breeding and raising their young.

Bird migrations are closely linked to mean annual temperature, which influences both departure and arrival times at breeding and wintering grounds. This dependence makes many migratory birds vulnerable to global and regional warming. Changes in seasonal conditions affect plant and insect populations, which serve as natural food sources for birds. Studies have shown that, in response to warming temperatures, many bird species are migrating earlier in the spring. For example, American robins, the state bird of Wisconsin, have advanced their spring arrival.

Changes in climate and weather that can arise from global warming include extreme precipitation events and extreme temperatures. Climate change can result in a multifaceted set of pressures on bird populations. A warmer climate system will accelerate Earth’s hydrologic cycle — the cycling of water from invisible vapor to liquid and frozen precipitation. Heavy rainfall and flooding threaten bird populations by destroying nests, drowning chicks and limiting foraging for aerial insectivores.

Global and regional warming are ongoing realities. Addressing these challenges and their impacts on bird migration requires collaborative efforts among researchers, policymakers and conservation organizations to ensure effective conservation and management of bird populations in the face of climate change.

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, Phenomena

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