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Surface Weather Overview

Weather forecasts for Pacific waters are broken down into large-scale and regional segments. The regional segments are depicted by the smaller boxes lining the coast of the US.

For sailors, surface analysis and forecast charts are some of the most widely used products in making departure, arrival, and routing decisions. What follows is a just-give-me-the-facts overview of charts from the Marine Prediction Center (MPC) and the information they contain. I hope this overview whets your appetite for more detail, and if so, you can find extensive and very useful information on all the MPC charts in the Radiofacsimile Users Guide. This guide is available online at the MPC website (, where it can be viewed, downloaded, and printed. So here goes, let's jump into surface charts with both feet!

For starters, Surface Analysis charts are produced four times each day, along with two 48-Hour Surface Forecasts and a 96-hour forecast. Surface analyses depict isobars, surface winds, frontal systems (these include occluded, stationary, cold, and warm), low and high-pressure center positions, as well as central pressure.

Forty-eight-hour surface forecasts display both a 24-hour track history and 24-hour forecast position of each synoptic scale system's position and the central pressure. 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 36 hours are labeled "Developing Gale" or "Developing Storm." When surface low pressure dives 24 millibars (MB) or greater during a 24-hour period, it receives special treatment in large capital letters as "BOMB" or "RAPIDLY INTENSIFYING."

There are two types of base maps used for constructing surface charts; mercator and stereographic. For large-scale ocean regions, a Mercator projection is used having latitude and longitude marked in 10-degree increments. Coastal regions use a stereographic projection.

This image from February 19, 2001, shows a strong low-pressure system lining up to smack the mid coast of California. 

A 24-hour 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.

Surface Analyses    Generated four times each day at 0000Z, 0600Z, 1200Z, and 1800Z (Z is Zulu, UTC, or Greenwich Mean Time) for both Atlantic and Pacific Ocean, surface analyses consist of isobaric pressure analyses at four-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, and underlined, and placed adjacent to or under the "H" (High pressure) or "L" (Low pressure). Surface analyses also consist of ship reports of wind direction (eight points on the compass rose), wind speed, (in knots), and present reported weather.

Surface Charts for Atlantic and Pacific Oceans are issued in two parts, i.e. for eastern and western portions, which overlap in mid ocean by 10 degrees of longitude.

Both parts project low and high-pressure forecast positions by drawing an arrow to the 24-hour position labeled, which is identified as an "X" for lows and a circle with an "X" in the middle for highs. A bold and underlined two-digit MB central-pressure value is placed 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 the system as "GALE" or "STORM" conditions and are confirmed by ship and buoy observations, satellite, or computer model guidance. If a gale or storm is expected within 36 hours, then a label "DEVELOPING GALE" or "DEVELOPING STORM" is added to the charts.

This 24-hour forecast of the northeast Atlantic indicates a low-pressure system moving over Nova Scotia.

48-Hour Surface Forecasts    Surface forecast charts are generated twice each day at 0000Z and 1200Z for each ocean, based on the 0000Z and 1200Z model run outputs. Each 48-hour surface forecast depicts wind speeds in knots for areas of wind in excess of 33 knots (wind barbs are 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 a label "developing gale" or "developing storm."

96-Hour Surface Forecasts    Four-day extended surface forecast products are generated once each day, based on the 0002Z Medium Range Forecast (MRF) model for both Atlantic and Pacific Ocean with additional guidance from other government agencies such as the Navy's Operational Global Atmospheric Prediction System (NOGAPS), European Center for Medium Range Weather Forecasts (ECMWF), and Canadian Regional Model.

As do the 48-hour charts, these four-day charts show surface isobars every four-MB with labeling of two digits in increments of eight MB. Central pressure is shown using millibar values of synoptic scale in bold and underlined three or four digits adjacent to or under the "L" or "H." A 72-hour forecast position and future 120-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 is placed under or adjacent to the 72/120 hour position label (e.g., 1030-MB high would be written as a 30 and a 980-MB low would have 80). Wind speed is given in knots and shown only for winds of 34 knots (wind barbs in increments of five or 10 knots) and greater. Frontal systems (occluded, warm, and cold) are also shown. Significant systems have labels depicting whether the system is expected to have "gale" or "storm" conditions. If by 120 hours a forecast for gale or storm conditions is expected, the appropriate area has the label "developing gale" or "developing storm."

This 24-hour forecast shows relatively calm conditions for the US East Coast.
If a tropical system such as a hurricane or a typhoon is anticipated, the 96-hour position will be indicated as a "Low" with the pressure as "XXX" for an unknown pressure. The 72-hour position will correspond to the latest official forecast from the responsible Typhoon Warning Center in the Pacific or the National Hurricane Center in Miami, FL. A 120-hour forecast position corresponds to a Marine forecasters estimated position.

Regional Surface Forecasts    Twenty-four-hour regional surface forecasts cover adjacent and offshore waters of the eastern and western US. These surface forecasts are prepared twice daily and depict the centers of synoptic scale high and low-pressure centers, with central pressure denoted by three or four digits in millibars.

Winds of 34 knots (39 mph) to 47 knots (54 mph) define a gale; otherwise winds are storm force if 48 knots or greater. Also, included is the forecast position of any tropical system with pressure depicted as "XXX" for unknown. Surface pressure is depicted with a four-MB contour interval. Also depicted are the direction and speed of movement in knots for the next 12 hours. The forecast includes the location of fronts, troughs, ridges, areas of fog, and likely areas of freezing spray.

Regional Wind/Wave Forecasts    Charts based on regional surface forecasts depict winds (in knots) and significant wave heights (in feet). Maximum significant wave heights analyzed do not take into account higher individual wave heights. Significant wave height is defined as the average height of the highest 1/3 of wave heights observed.

96-Hour Surface Forecasts    Produced once a day from the 0000Z computer model, these charts rely on guidance based on the NCEP Medium Range Forecast Model (MRF). Additional guidance from the ECMWF and UKMET are used to adjust inconsistencies and to fine-tune the final product. Surface forecasts depict the centers of high and low-pressure centers with central pressure depicted by three or four digits.

Tropical cyclone central pressure is represented by an "XXX" for unknown. Due to uncertainty concerning the strength of tropical cyclones, an "L" is used to represent tropical systems. The 72-hour and 120-hour forecast positions of highs and lows are depicted by a circle with an "X" inside for highs and an "X" for lows with two digits representing the pressure. For example, a "74" would be a pressure of 974 MB and a "20" represents 1020 MB. The 72-hour and 120-hour positions for Tropical cyclones are defined with an "X" for the forecast positions with "XX" representing an unknown pressure.

96-Hour Wind/Wave Forecast    Four-day forecasts for wind and sea state are produced once a day. These charts depict the expected significant sea state with a one-meter-height contour interval. Where appropriate, relative maximum and minimum wave heights are centrally depicted by a sea state value in a box. Also included are forecast winds (knots) greater than 25 knots.

So take the time to learn how to read and understand weather charts. All charts produced by the MPC are available both on the Internet and via USCG Radiofax. Charts can easily be downloaded from the Internet using a cellular phone when coastal cruising and using satellite systems such as Inmarsat and Globalstar when on the high seas.

Understanding the Code

Just like understanding information from the IRS, digesting the weather data made available by NOAA requires that you understand the code, or in this case the abbreviations. Here's a list of the most commonly used labels and their explanations.

DSIPT - Dissipate
STNRY - Stationary
WKNG - Weakening
RPDLY - Rapidly
FRMG - Forming
MOVG - Moving
INLD - Inland
DVLPG - Developing
COMB - Combined
DCRS - Decrease
INCRS - Increase
INTSFY - Intesify
Q-STNRY - Quasi-Stationary

For tropical cyclones, a description of the analyses or forecast time is displayed in bold capital letters adjacent to the tropical cyclone's position and cyclone symbol.


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