Introduction
Weather doesn’t recognize geographic or political borders. In a rare example of international cooperation, the majority of countries with a national weather forecasting agency readily share their observations and forecasts. In order to coordinate the timing of observations and forecasts, meteorologists adhere to a standard timekeeping system.

The original standard timekeeping system was Greenwich Mean Time (GMT), a 24-hour clock system based on the local time on the prime meridian in Greenwich, England. Since the local time in Greenwich is the same at GMT, 1:00 am is 0100 GMT, noon is 1200 GMT, and 6:00 pm is 18 GMT.Although you may occasionally see a reference to GMT on weather graphics, it was replaced as the international standard in the 1970s with Universal Time Coordinated (UTC). 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 abbreviated as “Z” on their forecast maps and text products. UTC and Zulu are identical, so 1200 UTC and 1200 Z are the same time.

In order to interpret forecasts that use UTC/Zulu, you’ll need to convert from UTC/Zulu to your local time. Fortunately, converting is easy once you know the time difference (time zone offset) between your location and Greenwich, England and whether you are currently subject to Daylight Saving Time.

Daylight Saving Time
During Daylight Saving Time, if you live in the Eastern Time zone, you’ll need to subtract four hours from UTC/Zulu to adjust the forecast to your local time. For example, 1200Z is 8:00 am Eastern. Boaters in the Central Time zone must subtract five hours. For example, 1200Z is 7:00 am Central Time.

Standard Time
When Daylight Saving Time is not in effect, an additional hour must be subtracted. For example, 1200Z is 7:00 am Eastern and 6:00 am Central. Table 1, below, shows UTC/Zulu conversions for the Eastern, Central, and Pacific Time zones (click here for a larger version).

Converting From 0000 UTC
Converting to local time from 0000 UTC can be a little tricky because subtracting hours from 0000 UTC also changes the date. For example, during Daylight Saving Time in the Eastern Time Zone, Friday at 0000 UTC converts to Thursday at 8:00 pm local time.

Figure 1 – Time Conversion Table

Interpreting Images and Converting Time
The National Weather Service and other providers of forecast graphics display UTC/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 0958Z on Sunday, July 9, 2023. Surface weather forecasts are issued by the Weather Prediction Center and display the future position of frontal boundaries, areas of  precipitation, and the overall barometric pressure pattern 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 0600Z on Sunday, July 9, 2023. Valid is frequently abbreviated as “V”.

Figure 1: Surface Weather Forecast

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 15 to 30 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 1821Z on May 8, 2023. The reference to Band 2 indicates the image was created from visible light.

Figure 2: Visible Satellite Image

Convective Outlooks: The Convective Outlook from the Storm Prediction Center (SPC) shows the probability of organized severe thunderstorms during a 24-hour period. The sample Outlook (Figure 3) was issued at 0559Z on July 2, 2023 and was valid from 1200Z on June 2 to 1200Z on June 3.

Figure 3: Convective Outlook

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 requires that the user has a working knowledge of meteorological shorthand and timekeeping systems. Figure 4 is a sample High Resolution Wind Forecast for the northeastern United States.

The first half of the first line “3km NAM 10-m Wind (KTS)” indicates the image was produced by the NAM forecast model and has a horizontal resolution of 3 kilometers (smaller resolution is generally more accurate). In addition, the legend indicates it is a forecast for wind speed in knots at a height of 10 meters (10 meters is the standard height for wind forecasts).  The second half of the line “THU 231019/ 1800V012” indicates the forecast was valid at 1800Z on Thursday, October 19, 2023. The “V012” indicates the forecast was valid 12 hours after it was initialized. The second line indicates the forecast was initialized, or processed, at 0600Z on October 19, 2023 and is the 12 hour forecast.

Figure 4: High Resolution Wind Forecast

Conclusion
Translating legends on forecast graphics can be challenging, but the important step of converting from UTC/Zulu to your local time becomes straightforward with a little practice.