Weather recognizes no geographic or political borders. In order to coordinate their observations and forecasts, meteorologists around the world use a standard timekeeping system. The original standard timekeeping system was Greenwich Mean Time (GMT) a 24-hour clock system based on the local time in Greenwich, England. For example, 1:00 am in Greenwich is 0100 GMT, noon is 1200 GMT, and 6:00 pm is 18 GMT.
Since GMT is technically a time zone, it was replaced with Coordinated Universal Time (UTC) in the early 1970s. Similar to GMT, UTC is a 24-clock system that doesn’t recognize local adjustments such as Daylight Saving Time. The National Weather Service’s version of UTC is called Zulu, which is typically abbreviated as “Z” on their forecast maps and text products.
In order to use forecast products based on Zulu, you’ll need to convert from Zulu to your local time. Fortunately, converting is easy once you know the time difference (time zone offset) between your location and Greenwich, England. If you live in the Eastern Time zone, subtract four hours from Zulu to arrive at your local time. Residents in the Central Time zone must subtract five hours. For example, 1200Z is 8:00 am Eastern and 7:00 am Central Time. When Daylight Saving Time is not in effect, an additional hour must be subtracted — 1200Z is 7:00 am Eastern. Table 1, below, shows Zulu-to-local-time conversions for the Eastern and Central Time zones.
Interpreting Images and Converting Time
The National Weather Service and other forecast providers display Zulu time in a variety of ways. Let’s look at a few examples.
Surface Weather Forecasts: The sample surface weather forecast below (Figure 1) was issued, or published, at 1852Z on Friday, June 10, 2016. Surface weather forecasts are issued by the Weather Prediction Center and display the future position and nature of weather features such as barometric pressure, frontal boundaries, and precipitation at a specific time in the future. This future date and time of a forecast is known as its valid time. The forecast shown in Figure 1 is valid at 1200Z on Saturday, June 11, 2016. Valid is frequently abbreviated as “V”.
Satellite Imagery: Satellite imagery, particularly visible and infrared, can help a weather-savvy boater assess cloud coverage or the extent of severe weather. Most websites update satellite images every 30 to 60 minutes. Since conditions can change dramatically between updates, it is prudent to determine the age of the image by comparing the image’s time stamp to the current time. The sample visible image (Figure 2) was published at 2130 UTC (same as Zulu) on June 11, 2016.
Thunderstorm Outlooks: Thunderstorm Outlooks from the Storm Prediction Center (SPC) show the probability that thunderstorms will occur during a particular four-hour period. The sample Outlook (Figure 3) was issued at 1633Z on June 6, 2016 and was valid from 2000Z on June 7 to 0000Z on June 8.
High Resolution Wind Forecast: Penn State University (PSU) produces a wide variety of cutting-edge products that are useful to the recreational boater. However, the imagery presumes the user has a working knowledge of meteorological shorthand and timekeeping systems. The sample high-resolution wind forecast (Figure 4 is available in 1-hour increments, and its legend contains a lot of information (Figure 5).
The first half of the first line “4km NAM 10-m Wind (KTS)” indicates the image was produced by the NAM forecast model, and has a horizontal resolution of 4 kilometers (smaller resolution is better). The image is a forecast for winds at a height of 10 meters (this is standard) and the units are in knots. The second half of the line “SAT 160611/0200V002” indicates the forecast was valid at 0200Z on Saturday, June 11, 2016. The “V002” indicates the forecast was valid 2 hours after it was initialized. The second line indicates the forecast was initialized, or processed, at 0000Z on June 11, 2016.
Translating legends on forecast graphics can be challenging, but the important step of converting from GMT/Zulu/UTC to your local time becomes straightforward with a little practice.