Warning Messages — Page 526, Lesson 497

I want to walk you through the warning messages associated with tropical cyclones. This is a critical part of aviation meteorology because these storms pose serious hazards to flight operations, and we need to understand how the information gets to us. Let's start with the centres that produce these warnings. We have what are called Tropical Cyclone Advisory Centres, abbreviated as TCAC. Six regional specialized meteorological centres, or RSMCs, have been established specifically to monitor, track, and advise on tropical cyclones within their assigned areas of responsibility. So each RSMC covers a particular region of the world where tropical cyclones can form. In addition to those six RSMCs, around the Australia area there are six tropical cyclone warning centres, or TCWCs, that exist to provide the same function. So in that part of the world, you have additional centres doing the same monitoring and advisory work. Now, what do these centres actually produce? They issue warning messages. These messages are important because the meteorological watch offices — the offices that monitor weather conditions for aviation — may reissue these warnings as SIGMET messages. Specifically, they would be issued as SIGMET with the code WC, which stands for tropical cyclone. So the chain is: the RSMC or TCWC issues the advisory, and the local meteorological watch office can then put that information into a SIGMET for pilots. Let me show you an example of what one of these messages actually looks like. I want to walk you through it line by line so you understand the structure. This particular example was originated by the National Hurricane Centre in Miami, and the book notes that it does have minor differences to the WMO/ICAO recommended format — so the World Meteorological Organization and International Civil Aviation Organization have a standard format, but this one is close enough to teach from. The message starts with a header line: ZCZC MIATCMAT3 ALL. That's a transmission header used for routing the message over meteorological data networks. Next we have: TTAA00 KNHC DDHHMM. This is a bulletin header. TTAA00 is the bulletin type, KNHC is the station identifier for the National Hurricane Centre in Miami, and DDHHMM would be the date-time group of the bulletin's issuance. Then the actual advisory begins: HURRICANE MICHAEL FORECAST/ADVISORY NUMBER 18. So this is the 18th advisory issued for Hurricane Michael. It comes from the NWS NATIONAL HURRICANE CENTER MIAMI FL — that's the National Weather Service's National Hurricane Center in Miami, Florida. The identifier AL132012 tells us this is the 13th tropical cyclone of the 2012 Atlantic season. The time of the advisory: 1500 UTC FRI SEP 07 2012. That's 1500 Zulu time on Friday, 7 September 2012. Then we get a critical operational statement: THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT. So no coastal warnings are active for this storm at this time. Now we get into the storm's position and characteristics. HURRICANE CENTER LOCATED NEAR 31.2N 41.1W AT 07/1500Z — that's the centre of the hurricane at latitude 31.2 degrees north, longitude 41.1 degrees west, at the time of the advisory. The POSITION ACCURATE WITHIN 15 NM — so the centre position is known to within 15 nautical miles. PRESENT MOVEMENT TOWARD THE NORTHWEST OR 320 DEGREES AT 3 KT — the storm is moving toward 320 degrees true, which is northwest, at a speed of 3 knots. ESTIMATED MINIMUM CENTRAL PRESSURE 970 MB — the pressure at the centre of the hurricane is estimated at 970 millibars. Lower pressure generally means a stronger storm. EYE DIAMETER 15 NM — the eye of the hurricane is 15 nautical miles across. MAX SUSTAINED WINDS 90 KT WITH GUSTS TO 110 KT. So the maximum sustained winds are 90 knots, with gusts up to 110 knots. Then we have the wind radii. These tell us how far from the centre the winds of certain speeds extend, broken down by quadrant — northeast, southeast, southwest, and northwest. For 64 KT winds — that's hurricane-force — the radii are 20 nautical miles in each quadrant: 20NE, 20SE, 20SW, 20NW. For 50 KT winds, the radii are 30 nautical miles in each quadrant. For 34 KT winds — tropical storm-force — the radii are 60NE, 60SE, 50SW, 50NW. So the storm is not perfectly symmetrical; the strongest winds extend farther in the northeast and southeast quadrants. Then we have 12 FT SEAS — that's the area where wave heights reach 12 feet. The radii are 180NE, 180SE, 120SW, 150NW nautical miles. The message includes an important note: WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT. So these are the maximum distances you'd expect those conditions to extend in each quadrant — not the average. The message then repeats the centre location for emphasis: REPEAT...CENTER LOCATED NEAR 31.2N 41.1W AT 07/1500Z. It also gives a previous position: AT 07/1200Z CENTER WAS LOCATED NEAR 31.1N 41.0W — so three hours earlier, the centre was slightly south and east of its current position. Then we get the forecast positions. FORECAST VALID 08/0000Z 31.5N 41.4W — that's 9 hours from the advisory time. The forecast maximum wind is 90 KT...GUSTS 110 KT, with wind radii given: 64 KT winds extend 25NE, 20SE, 20SW, 20NW; 50 KT winds extend 40NE, 40SE, 30SW, 30NW; 34 KT winds extend 70NE, 70SE, 60SW, 60NW. And then FORECAST VALID 08/1200Z 32.0N 41.9W — that's 21 hours from the advisory time, with the storm forecast to move slightly north and west. So this is the kind of detailed information that gets produced by the RSMCs and TCWCs, and it's what the meteorological watch offices use to issue SIGMET WC messages for aircraft operating in or near tropical cyclone areas.