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Old 11-03-2000
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Reading Weather Fax Charts

A typical surface analysis for the North Atlantic is chockablock with information on a macro scale.

Prudent mariners can extract a wealth of useful information from weather fax charts.  Here's an overview of all the products available and how to utilize their information.

Surface Analyses    Surface analyses are generated four times per day, at 0000Z, 0600Z, 1200Z, and 1800Z (Zulu, UTC, or Greenwich Mean Time) for each ocean. Analyses consist of isobaric pressure observations at four-millibar (mb) contour interval spacing, labeled every eight mb. Central pressure mb values of low and high-pressure systems are depicted in bold three or four digits, underlined, and placed adjacent to or under the "H" or "L." Surface analyses also consist of abbreviated, automated ship plots of wind direction (eight points on the compass rose), wind speed in knots, present reported weather, and cloud coverage. The product is issued in two parts, which overlap by some 10 degrees of longitude (between 165W and 175W in the Pacific Ocean, and between 50W and 60W in the Atlantic Ocean). Both parts will project the low or high-pressure systems' forecast position with an arrow to the 24-hour position. These will be labeled with an "X" for lows and a circle with an "X" in the middle for highs, with a bold two-digit mb central pressure value underlined under or adjacent to the 24-hour position label (e.g.,1050-mb high would be written as a 50 and a 960-mb low would have 60). Significant weather systems have labels depicting whether the system has gale or storm conditions as noted by ship and buoy observations, Special Sensor Microwave Imagery (SSM/I), satellite, or computer model guidance. If 24-hour forecast gale or storm conditions are expected, the appropriate area has the label "developing gale" or "developing storm." Used in conjunction with a 500-mb analysis, a 24-hour forecast position of synoptic scale weather systems will aid in determining a weather system's motion and intensity trends.

In this surface analysis, the low-pressure system over North and South Carolina is forecast to move off to the northeast.
The hand-drawn surface analyses depict isobars, surface winds, frontal systems (occluded, stationary, cold, and warm), low and high-pressure center positions, and central pressure. The 24-hour track history and 24-hour forecast position of each synoptic scale system's position and central pressure are displayed on all 48-hour surface forecasts. Systems having or expected to have synoptic scale gale or storm conditions are labeled in bold capital letters. Similarly, systems expected to develop gale or storm conditions in 24 hours have labels of developing gale or developing storm. Surface low-pressure falls of 24 mb or greater during a 24-hour period are denoted in large capital letters as RAPIDLY INTENSIFYING. For tropical cyclones, the alphanumeric description of the analyses or forecast time are displayed in bold capital letters adjacent to the tropical cyclone's position with the appropriate cyclone symbol in the following manner:


24-Hour Surface Forecast    Surface forecast charts feature low and high-pressure center positions with bold three and four-digit central pressure values underlined under or adjacent to the L or H.  An arrow displays direction of movement, with the system’s speed shown in knots depicted adjacent to the head of the arrow. Significant systems have labels denoting whether the system is expected to have GALE or STORM conditions. If 24-hour forecast gale or storm conditions are expected, the appropriate area has the label DEVELOPING GALE or DEVELOPING STORM. Also displayed on this surface forecast chart are frontal systems (occluded, warm, and cold) and when appropriate, associated areas of fog, signifying areas of potentially restricted to visibility. Isobars are depicted in increments of four mb, except for deep systems which are depicted in eight-mb increments. The 1000-mb contour will be dashed to separate four-mb from eight-mb contour spacing.


This analysis shows a low-pressure forecast will move over Newfoundland in 24 hours.

48-Hour Surface Forecasts   
These surface forecast products are generated twice each day at 0000Z and 1200Z for each ocean. Products show surface isobars every four mb with labeling of two digits in increments of eight mb. Central pressure millibar values of synoptic scale lows and highs in bold three or four digits are underlined adjacent to or under the L or H. The 24-hour forecast position and future 72-hour forecast position of lows and highs have vector arrows with an X for low centers and a circle with an X inside by the head for high centers. An underlined bold two-digit mb central pressure value will be placed under or adjacent to the 24/72-hour position label (e.g., 1050-mb high would be written as a 50 and a 960-mb low would have 60). The 48-hour surface forecast depicts wind speeds in knots (wind barbs in increments of five or 10 knots), and frontal systems (occluded, warm, and cold). Significant systems have labels depicting whether the system is expected to have gale or storm conditions. If 72-hour forecast gale or storm conditions are expected, the appropriate area has the label DEVELOPING GALE or DEVELOPING STORM.

24-Hour Wind/Wave Forecasts    This forecast product depicts 24-hour forecasts of wind, in increments of five knots, and significant wave heights, in isopleths of combined sea and swell in three-foot intervals. During appropriate weather conditions such as Gulf Stream "North Wall" episodes, substantially higher wind/wave height values are highlighted. Arrows will point to a superimposed hatched area. Wave-height values are depicted by solid contours in increments of three feet. Superstructure icing, displayed by a half moon with one or two lines crossing through the center, will depend on the forecast for light or heavy accumulation of ice.

"Sea state analyses highlight where the most significant combined sea and swell wave heights prevail."

Sea State Analysis    Sea State Analyses are issued once a day per ocean at the time of the greatest number of ship observations (1200Z in the Atlantic and 0000Z in the Pacific) and analyze ship's synoptic reports for sea state in meters. The sea state analysis has solid one-meter contour intervals along with primary swell direction arrows. Where appropriate, maximum and minimum combined wave-height values (approximately 1/3 the height of the wind wave added to the height of the swell wave) are centrally depicted and underlined with the abbreviation of MAX or MIN under, or adjacent to, the values. The primary swell direction arrows are based on actual ship observation reports. During winter cold months, ice edges are displayed as a bold jagged line. Sea state analyses highlight where the most significant combined sea and swell wave heights prevail and when viewed with surface analyses, provide a complete picture of surface weather conditions.

48-Hour Sea State Forecast    These forecasts are generated twice daily, at 0000Z and 1200Z, and are based on significant wave forecast model runs. Combined sea heights are depicted in solid contours of one-meter increments with a maximum or minimum combined sea state value underlined with the abbreviation of MAX or MIN, adjacent to the values. Ice edge is displayed as a bold jagged line during winter months. These forecasts provide a complete picture of surface conditions when used in conjunction with the 48-hour surface forecasts. Our tropical cyclone symbol forecast position will be depicted on all surface analyses. Both 24-hour and 72-hour tropical cyclone positions will appear on the 48-hour surface forecasts. Mariners are strongly advised to rely on the latest warnings from the Tropical Prediction Center's (TPC) National Hurricane Center (NHC) which covers the Atlantic and Eastern Pacific Oceans east of 140W; the Central Pacific Hurricane Warning Center (PHNL) covering the Eastern and Central Pacific Ocean west of 140W to the international dateline (180); and the Joint Typhoon Warning Center (JTWC), covering the Western Pacific west of 180.

Tropical Eastern Pacific Analysis    Streamline analyses covering the tropical East Pacific Ocean are issued twice daily for 1200Z and 1800Z. Coverage is from 30N to 20S and between 78W and 155W. Bold streamlines with arrows indicate prevailing general wind direction with actual or derived wind reports from ships and satellite. This chart includes all pertinent synoptic features (lows, highs, fronts, troughs) along with their direction and speed of motion. All tropical systems have internationally recognized symbols: a circle with a dot inside represents a tropical disturbance, a circle with an X inside represents a tropical depression, an open circle with two spiraling arms indicates a tropical storm, a closed circle with two spiraling arms denotes a hurricane. All tropical cyclones analyzed have the latest TPC advisory, location, intensity, and the direction and speed of motion.

Two types of surface wind data (barb format) are plotted on this analysis: ship reports, denoted by a circle at the end of the barb, and boundary layer winds from the latest numerical model run. Boundary layer winds are included to supplement the sparse data in the region. All regions of significant showers/thunderstorms on the Tropical Eastern Pacific Analysis are delineated by scalloped lines and labeled by coverage (SCT - scattered, ISOLD - isolated) and intensity (MOD - moderate, STG - strong). This analysis includes all appropriate synoptic features, i.e., lows, highs, fronts, and troughs. All tropical waves and tropical cyclones are displayed with the name and latest TPC advisory position of each tropical cyclone. This information is followed by a six digit tropical cyclone code with general system information.

This 500-mb analysis can help mariners see that a low-pressure system is apt to develop in the mid-Atlantic.

Regional Products—General information    Regional surface graphic charts cover both coastal and high-seas areas. There are two products—the first encompasses the western Atlantic Ocean west of 50W and north of 30N, which includes the US east coast, and the central Florida coast. The second covers the eastern North Pacific Ocean, from the Baja peninsula, south to Cabo San Lucas, and north to the Gulf of Alaska, which includes Prince William Sound as far west as 150W. Regional products consist of 24-hour surface forecasts and wind/wave forecasts and are issued twice daily per ocean for 0000Z and 1200Z.

These short-range forecast products depict synoptic and mesoscale features of surface low and high-pressure systems, isobars with frontal features, areas of reduced visibility, wind speeds, and significant wave height as generated by the synoptic and mesoscale weather systems within 1,000 miles of the coasts. The wind speeds are derived from Special Sensor Microwave Imagery (SSM/I) received from a US satellite from oceanic areas. This state-of-the art data input represents a significant enhancement in analyzing wind conditions in the marine environment. SSM/I is especially important in areas sparse in data, where ship and buoy reports are not available. SSM/I aids short-range prediction in the 24-hour forecast by enabling marine meteorologists to compare initial data from forecast model outputs and make necessary adjustments to near-term forecast solutions.

500 MB Upper Air Charts    The 500-mb charts are produced from a computer model of the atmosphere. These products are automated, unmodified computer outputs that depict height contours above the earth's surface (geopotential heights) at 60-meter intervals. Wind speeds of 30 knots and greater are shown with wind barb increments of five or 10 knots. Embedded within the 500-mb height field are short wave troughs, generally 50 degrees or less in longitude. These are drawn on the charts as bold dashed lines. These short-wave troughs assist the mariner in locating surface low-pressure systems or developing lows on frontal waves. The 500-mb winds approximate the speed of surface extra-tropical lows (often about 1/3 to 1/2 of the 500-mb wind speed) and surface wind force (approximately 50 percent), particularly in colder SW quadrants. The 5640-meter height contour is highlighted since this height contour is widely used by mariners for general surface storm track direction and the southern extent of Beaufort Force 7 (28 to 33 knot) or greater surface winds in the winter, and Force 6 (22 to 27 knot) winds in summer. Analyses are generated twice a day at 0000Z and 1200Z, and depict synoptic scale flow patterns, location, and amplitude of long and short waves. Synoptic scale features can be compared with previous analyses to determine the movement and trends of the upper air pattern. They can be used in conjunction with the surface analyses, sea state analyses, and meteorological satellite imagery, which are valid at the same synoptic time.

12-Hour 500-mb Forecasts     These products can be used to compare changes in flow patterns from the latest 500-mb analyses to follow the progression of short waves. A 500-mb 48-hour forecast should be used in conjunction with surface and sea state 48-hour forecast products, and comparison of previous 48-hour 500-mb forecasts with the most current 500-mb analysis can establish confidence in subsequent forecasts.