In part one (click here), I introduced surface weather maps, meteorological time-keeping systems, the difference between Issued and Valid, and barometric pressure. In part two, we’ll look at the symbols and meteorological shorthand used on surface weather maps.
The solid black lines winding across the country on the forecast map valid at 00Z on Thursday, February 23, 2017 (figure 1) are isobars, which are contours of constant sea level barometric pressure measured in millibars (mb). Isobars are typically drawn at 4 mb intervals on NWS maps and are labeled somewhere along the contour. The 1012 mb isobar, for example, starts northeast of the Bahamas and crosses the US coast near the border of Georgia and South Carolina (click here for annotated image). After heading west for a bit, the 1012 mb isobar makes a right turn and heads northeast parallel to the Appalachian Mountains. At any point along this isobar, sea-level pressure is forecast to be 1012 mb. Isobars allow forecasters to understand the overall pressure pattern and quickly identify areas of low and high pressure, along with other surface features such as troughs and ridges. Continue reading
Surface weather maps offer a wealth of information to the weather-savvy boater. However, the key to unlocking the vast treasure of information displayed on these maps is understanding the terminology and symbols used by forecasters to portray current and future weather patterns. In this first installment of a two-part series, I’ll introduce surface maps, some key terminology, meteorological time systems, and barometric pressure.
Types of Surface Maps
Surface weather maps come in two varieties — analyses and forecasts. These two types of maps are very similar in appearance but their purpose is quite different. Analyses show recent surface weather observations and features (Figure 1) and are published every three hours by NOAA’s Weather Prediction Center (WPC) (http://www.wpc.ncep.noaa.gov/). The observations used to prepare these analyses such as temperature, dew point, barometric pressure, and wind speed/direction, are collected by thousands of automated weather stations across the country. These stations, which comprise the Automated Surface Observing System (ASOS), are usually located at airports. Continue reading
Competing in the Chicago-Mac is never easy – after all, it is at least 333 statute miles to Mackinac Island. And by all accounts, the 108th running of Mac was unusually challenging. Light easterly winds on Saturday afternoon made progress toward the Island difficult for the racing fleet. The most significant challenge, however, appeared on Saturday evening, delivered by prolonged periods of thunderstorm activity. The storms repeatedly battered the fleet, hampering progress and prompting the withdrawal of several competitors due to minor crew injuries and equipment issues.
Light Winds Hinder Progress
By early Saturday afternoon, Lake Michigan was under the influence of a large area of high pressure centered just north of Lake Superior. This high was bisected by a stationary front originating from an area of low pressure near North Dakota and extending east across southern Lake Michigan (click here for surface analysis). This pattern resulted in light easterly winds across the southern half of Lake Michigan, impeding the progress of the racing and cruising fleets. Continue reading
Doppler Weather Radar is your best defense against a hair-raising and wind-blown encounter with thunderstorms. The 155 stations in the National Weather Service’s (NWS) network provide overlapping, ground-based coverage of the nation’s inland and coastal boating areas. With an effective range of approximately 120 nautical miles, data from the NWS radar network is not accessible if you are well offshore. (Regardless of how you obtain your radar imagery, you are viewing NWS data as theirs is the only national radar network.)
Scanning The Atmosphere
Strong thunderstorms may be several miles high, and so the radar station must collect data from the Earth’s surface up into the upper reaches of the atmosphere in order to completely analyze the storm. Stations use a variety of scanning strategies, called Volume Coverage Patterns (VCP) to accomplish this goal. The antenna makes an initial, or base, scan by making one complete revolution at an elevation of 0.5° above the Earth’s surface, alternating between emitting and collecting backscattered energy pulses. When this base scan is complete, the antenna completes additional scans, repeatedly increasing the elevation by about one degree, until the highest elevation of the VCP is reached. The highest elevation scanned by NWS radar is 19.5°.
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.