Purpose

The Tropical Weather Discussion describes major synoptic weather features and significant areas of disturbed weather in the tropics. The product is intended to provide current weather information for those who need to know the current state of the atmosphere and expected trends to assist them in their decision making. The product gives significant weather features, areas of disturbed weather, expected trends, the meteorological reasoning behind the forecast, model performance, and in some cases a degree of confidence.

Content

The Tropical Weather Discussion is a narrative explaining the current weather conditions across the tropics and the expected short-term changes. The product is divided into four different sections as outlined below:

1. SPECIAL FEATURES (event-driven)
   The special features section includes descriptions of hurricanes, tropical storms, tropical depressions, subtropical cyclones, and any other feature of significance that may develop into a tropical or subtropical cyclone. For active tropical cyclones, this section provides the latest advisory data on the system. Associated middle and upper level interactions as well as significant clouds and convection are discussed with each system. This section is omitted if none of these features is present.
2. TROPICAL WAVES (event-driven)
   This section provides a description of the strength, position, and movement of all tropical waves analyzed on the surface analysis, from east to west. A brief reason for a wave's position is usually given, citing surface observations, upper air time sections, satellite imagery, etc. The associated convection is discussed with each tropical wave as well as any potential impacts to landmasses or marine interests. This section is omitted if there are no tropical waves present.


3. ITCZ
   In this section, the Intertropical Convergence Zone (ITCZ) is depicted by coordinates from east to west with an outline of all associated convection referenced to the axis, if possible. The ITCZ is depicted based on the following definition: "a zonally elongated axis of surface wind confluence in the tropics, due to confluence of northeasterly and southeasterly trade winds, and/or confluence at the poleward extent of cross-equatorial flow into a near-equatorial 'heat trough' or 'monsoon trough.'"


4. DISCUSSION
   The discussion section makes reference to middle and upper level features and provides connection to how these features are interacting with or influencing surface features. Convection is outlined with the features as warranted. This section will also provide short-term (up to 48 hours) trends and forecasts of features, especially if they are forecast to strengthen and/or affect landmasses. A discussion of model guidance may also be included. In general, this section is regionalized according to the current weather pattern (i.e. Gulf of Mexico, Caribbean, Western Atlantic, etc.)

Coverage and intensity for convective and non-convective weather is defined as follows:

COVERAGE FOR CONVECTIVE and NON-CONVECTIVE WEATHER:

COVERAGE FOR SIGNIFICANT CLOUDS:

INTENSITY OF CONVECTION (based on cloud top temperatures):

Coverage

The National Hurricane Center issues two Tropical Weather Discussions, as shown below:

1. Atlantic
   From the equator to 32°N west of the prime meridian including the Caribbean Sea, Gulf of Mexico, and adjacent land areas. These areas include the southeast United States [especially Florida], Mexico, Central America, northern South America, and western Africa
2. East Pacific
   From the equator to 32°N east of 140°W including coastal areas of Mexico, and Central and South America.

Issuance/Transmission

	WMO	NWWS
Atlantic	AXNT20 KNHC	MIATWDAT
East Pacific	AXPZ20 KNHC	MIATWDEP

Purpose

Purpose

The Offshore Waters Forecast provides forecast and warning information to mariners who travel the oceanic waters adjacent to the U.S. and its territorial coastal waters. This forecast serves customers who operate from the coastal waters to several hundred nautical miles from shore with 10 different products issued by the National Hurricane Center, the Ocean Prediction Center, the Honolulu Weather Forecast Office, the Anchorage Weather Forecast Office, and the Juneau Weather Forecast Office.

Content

The Offshore Waters Forecast includes a synopsis which describes significant surface weather features that may cause significant winds and seas over the forecast area during the forecast period. The synopsis identifies major weather systems and the strength, trend, and movement of each. The most detail is focused in the first 48 hours of the forecast. The synopsis also includes tropical cyclone forecast positions out to 120 hours.

The offshore waters product provides a forecast for winds and seas over the affected area for roughly the next 5 days. The forecast also includes significant weather which would pose a hazard to navigation, such as precipitation or restrictions to visibility. Warnings are issued when conditions are expected to meet the specific warning criteria during the first two periods of the forecast. Thereafter, a headline is used which shows that the conditions are expected to occur at some time after the first two periods.

Coverage

The National Hurricane Center issues two separate Offshore Waters Forecasts, as shown below:

Gulf of Mexico offshore waters:

1. Northwest Gulf (north of 25°N west of 90°W)
2. Southwest Gulf (south of 25°N west of 90°W)
3. Middle Gulf (between 85°W and 90°W)
4. Eastern Gulf (between 81°W and 85°W)

Southwest and Tropical North Atlantic and Caribbean offshore waters:

1. Northwest Caribbean (north of 15°N west of 75°W)
2. Southwest Caribbean (south of 15°N west of 75°W)
3. Eastern Caribbean (east of 75°W to Leeward and Windward Islands)
4. Tropical North Atlantic (from 7°N to 22°N between 55°W and 65°W)
5. Southwest North Atlantic (south of 31°N west of 65°W including the Bahamas)

Issuance/Transmission

Offshore waters products are transmitted under World Meteorological Organization (WMO) and NOAA Weather Wire Services (NWWS) headers as shown in the following table:

Offshore waters products are issued on a regular six hourly schedule as shown below:

Purpose

The Marine Weather Discussion is a semi-technical product, analogous to the Area Forecast Discussion (AFD) issued by the local National Weather Service Forecast Offices, primarily used as a means to explain the scientific rationale behind the Offshore Waters Forecast and to summarize the warnings in effect. The Marine Weather Discussion is used to convey forecast and warning information to federal agencies, weather-sensitive officials, and the media. Part of this goal is to enhance coordination with the coastal Weather Forecast Offices.

Content

The Marine Weather Discussion describes synoptic and mesoscale features expected to affect areas in and adjacent to the offshore waters of the Gulf of Mexico, Caribbean Sea, and Southwest and Tropical North Atlantic Ocean. The narrative describes weather, wind speed, and seas with primary focus on the next 48 hours. The product emphasizes timing and issuance of warnings, includes future trends of wind and sea conditions, discusses the effects of currents such as the Gulf Stream on winds and seas, and addresses how the latest computer model guidance is handling features of significance to the mariner.

Coverage

The National Hurricane Center issues one Marine Weather Discussion to convey forecast reasoning over both its offshore forecast areas. This area includes the Gulf of Mexico, the Caribbean Sea, the Tropical North Atlantic, and the Southwest North Atlantic Ocean south of 31°N west of 55°W. The area between 55°W and 65°W is not specifically covered by the Offshore Waters Forecast products, but conditions in this area are discussed since they often affect the offshore waters in the future.

Issuance/Transmission

The Marine Weather Discussion is transmitted under World Meteorological Organization (WMO) and NOAA Weather Wire Services (NWWS) headers as shown below:

In general, the Marine Weather Discussion is issued twice a day (with updates as needed) shortly before the issuance of the scheduled Offshore Waters Forecast products. Forecasters will issue a brief discussion to provide information of an impending offshore forecast update. The scheduled transmission times are shown below:

Purpose

The Pan-American Temperature and Precipitation Table is a collection of temperature and precipitation data and present weather for selected cities in Mexico, Central and South America, and the Caribbean area. It provides general weather conditions two times per day for general public use domestically and internationally.

Content

This product provides general weather conditions, high and low temperatures in degrees Fahrenheit (°F) and degrees Celsius (°C), and measured precipitation in the previous 6, 12, or 24 hours, as indicated, for selected cities in Mexico, Central and South America, and the Caribbean area. The morning product contains the low temperature for the current day and the high temperature for the previous day. The evening product contains the low and high temperature for the current day.

Coverage

The Pan-American Temperature and Precipitation Table includes the following stations (as shown in the map above):

Issuance/Transmission

The Pan-American Temperature and Precipitation Table is transmitted under World Meteorological Organization (WMO) and NOAA Weather Wire Services (NWWS) headers as shown below:

	Valid Time	Issuance Time
Morning MIATPTPAN	700 AM EST (800 AM EDT)	by 950 AM EST (1050 AM EDT)
Evening MIATPTPAN	700 PM EST (800 PM EDT)	by 950 PM EST (1050 PM EDT)

Purpose

Satellite rainfall estimates are provided for tropical cyclones or other significant convective systems (mainly in the tropics) when these systems threaten land. The product is issued when convection systems are affecting the WMO RA-IV area. The product is also issued when system affect the west coast of Mexico. These estimates are based on geostationary infrared satellite imagery using a method called the Woodley-Griffith Technique.

Content

The satellite rainfall estimate product provides an estimate of the maximum rainfall based on the last four geostationary satellite images six hours apart (24 hours), as well as the estimated maximum rainfall based on the latest geostationary satellite image. The product also provides a rainfall distribution within 4° (240 nautical miles) to the left and right of the observed system motion in 1° (60 nautical miles) increments.

Coverage

One of three satellite rainfall estimate products is issued for each convective system affecting or expected to affect land based on the system.s initial location:

1. Eastern Caribbean (areas between 40°W and 67°W)
2. Central Caribbean (areas between 67°W and 80°W)
3. Western Caribbean / Mexico (areas between 80°W and 120°W)

Issuance/Transmission

The Satellite Tropical Disturbance Rainfall Estimate product is transmitted under World Meteorological Organization (WMO) and NOAA Weather Wire Services (NWWS) headers as shown below:

The product is issued four times a day as needed near the synoptic times of 0000, 0600, 1200, and 1800 UTC. The exact issuance time is based on the time of the last satellite image used.

Purpose

The Tropical Surface Analysis is created every six hours to depict the current state of atmosphere, specifically the sea level pressure field and any relevant synoptic surface features. The Tropical Surface Analysis is attached to surface analyses over North America, the North Atlantic Ocean, the North Pacific Ocean, and the Western Pacific Ocean to create the National Weather Service Unified Surface Analysis.

Content

The Tropical Surface Analysis and NWS Unified Surface Analysis depict the sea level pressure field by showing lines of equal pressure, usually in increments of four millibars (mb) but often in increments of two millibars where the pressure gradient is weaker (especially in the tropics). The analysis also depicts important surface features that affect the weather, including areas of high and low pressure, frontal systems (cold, warm, stationary, and occluded), troughs, tropical cyclones, tropical waves, the Intertropical Convergence Zone (ITCZ), drylines, and squall lines.

Coverage

The Tropical Surface Analysis is created by the National Hurricane Center and covers the area approximately from 30°N to 20°S between 0° and 140°W. This area includes the tropical and subtropical areas of the Atlantic and East Pacific Oceans, Mexico, Florida, the Caribbean, South America, and West Africa. For radiofax purposes, the analysis is split into three different areas:

1. Atlantic (East Half from 5°S to 50°N between 0° and 70°W)
2. Atlantic (West Half from 5°S to 50°N between 55°W and 125°W)
3. Eastern Pacific (from 20°S to 40°N between 70°W and 150°W)

The NWS Unified Surface Analysis is created by merging the separate surface analysis produced by the National Hurricane Center, the Hydrometeorological Prediction Center, the Ocean Prediction Center, and the Honolulu Weather Forecast Office. The total area spans from the equator northward nearly to the North Pole, from 20°E westward to 130°W. This covers the area from Europe and Central Africa westward across North and South America to East Asia.

Issuance/Transmission

The Tropical Surface Analysis is transmitted by radiofax via New Orleans, Pt. Reyes, and Honolulu according to the following table:

The analyses are also updated on the NHC webpage by 0300, 0900, 1500, and 2100 UTC for the 0000, 0600, 1200, and 1800 UTC surface analyses, respectively.