Severe Weather Warnings: Land and Sea

Warnings of Severe Weather on Land

MetService issues a number of different warnings for severe weather expected over the New Zealand landmass:

• Severe Weather Warning

• Severe Weather Watch

• Special Weather Advisory

• Road Snowfall Warning

• Snow Otago Warning

Severe Weather Warning

MetService will issue a Severe Weather Warning whenever there is an expectation that any of the following weather conditions will occur within the next 24 hours:

• Widespread* rainfall greater than 50 mm within 6 hours or 100 mm within 24 hours;

• Widespread* snowfall below 1000 metres on the North Island or 500 metres on the South Island with a snow depth of 10 centimetres within 6 hours or 25 centimetres within 24 hours;

• Widespread* severe gales with a minimum mean speed of 90 km/hr or frequent gusts exceeding 110 km/hr.

*"Widespread" means over an area of 1000 square kilometres or more.

Severe Weather Warnings are issued as and when necessary, and once issued are updated on a twelve-hourly basis until cancelled.

Severe Weather Watch

MetService will issue a Severe Weather Watch whenever there is an expectation that conditions may deteriorate to the thresholds specified for the issue of a Severe Weather Warning:

• After the next 24 hours but within 48-72 hours, or

• If there is a high level of uncertainty within the next 24 hours.

Special Weather Advisory

MetService will issue a Special Weather Advisory whenever a weather event is likely to cause significant disruption to the general public or specific industry groups within the next 48 hours, but the weather is not expected to deteriorate to an extent that would require the issue of a Severe Weather Warning.

MetService may also issue a Special Weather Advisory following a severe storm that caused widespread disruption and damage in order to assist with any post-storm operations.

Road Snowfall Warning

MetService will issue a Road Snowfall Warning for the Desert Road, the Porter's Pass Road, the Lindis Pass Road, the Rimutaka Hill Road, the Milford Road, Arthur's Pass or Lewis Pass whenever there is a likelihood of snow settling on one or more of those roads within the next 24 hours.

Snow Otago Warning

MetService will issue a Snowfall Warning for South Canterbury and Otago whenever snow is expected to accumulate to a depth of 20 cm or more within the next 24 hours at or below an altitude of 1500 metres and conditions are not severe enough to warrant the issue of a Severe Weather Warning.

Snow Otago Warnings are primarily for the high country farming communities of South Canterbury and Otago.

Weather Event		Within the next 24 hours	Within the next 24 hours, but with a low level of certainty	Between 24 hours and 48-72 hours from now
1	Rain (over an area of 1000 square kilometres or more): 50 mm or more in a 6-hour period 100 mm or more in a 24-hour period	Severe Weather Warning	Severe Weather Watch	Severe Weather Watch
2	Wind — severe gales (over an area of 1000 square kilometres or more) with: Minimum mean speed of 90 km/h, or Frequent gusts exceeding 110 km/h	Severe Weather Warning	Severe Weather Watch	Severe Weather Watch
3	Snow — North Island, below altitude of 1000 metres (over an area of 1000 square kilometres or more): 10 cm or more in a 6-hour period 25 cm or more in a 24-hour period	Severe Weather Warning	Severe Weather Watch	Severe Weather Watch
4	Snow — South Island, below altitude of 500 metres (over an area of 1000 square kilometres or more): 10 cm or more in a 6-hour period 25 cm or more in a 24-hour period	Severe Weather Warning	Severe Weather Watch	Severe Weather Watch
5	Snow — South Canterbury and/or Otago, below altitude of 1500 metres (over an area of 1000 square kilometres or more): 20 cm or more in a 24-hour period And not as heavy or to as low an altitude as in 4	Snow Otago Warning
6	Snow — settling on the Desert Road and/or the Porter's Pass Road and/or the Rimutaka Hill Road and/or Milford Road and/or the Lindis Pass Road and/or the Arthur's Pass Road and/or the Lewis Pass Road	Road Snowfall Warning
7	Weather likely to cause significant disruption to the general public or specific industry groups but not expected to deteriorate to the extent of 1, 2, 3, or 4 (for example, following a severe storm that caused widespread disruption and damage)	Special Weather Advisory

Severe Weather Outlook

The aim of the Severe Weather Outlook is to provide a "heads up" of potential severe weather events in the day 3 to day 6 period.

At days 3 to 6, the broad features of weather systems (depressions, anticyclones, major flows of air) are reasonably predictable. However, the fine detail of the weather — especially weather of a severe nature — that may result from the passage of weather systems through the New Zealand area in 3 to 6 days' time is less predictable. For this reason, MetService

• Only issues Severe Weather Outlook forecasts for broad areas of the country

• Assigns a level of confidence to each forecast made in the Severe Weather Outlook.

If MetService considers that the chance of a severe weather event occurring in days 3 to 6 is ...	... then the confidence assigned to it in the Severe Weather Outlook is ...
< 10%	None: the potential severe weather event is not significant enough to mention
11-30% (i.e., about one chance in five)	Low
31-50% (i.e., about two chances in five)	Moderate
51-70% (i.e., about three chances in five; more likely than not)	Good
> 71%	This level of confidence is unlikely to be reached further into the future than day 3. If it is reached on day 3, MetService will issue a Severe Weather Watch.

So, a confidence of "Low" assigned to a potential severe weather event means that: about four out of five times it is forecast, the event will not occur. Similarly, a confidence of "Good" assigned to a potential severe weather event means that: about three out of five times it is forecast, the event will occur.

For example, suppose the Severe Weather Outlook forecasts heavy rain for the East Coast of the North Island with a low confidence. Then, about one out of five times this forecast is made, rainfall on the East Coast of the North Island will reach Severe Weather Warning criteria (100 mm in 24 hours or 50 mm in 6 hours). On the other four occasions, it will probably rain, but not as heavily.

The trend in the confidence expressed in a Severe Weather Outlook forecast over successive days provides a good lead-up to an event — or to a non-event. For example, suppose that over four successive days, the confidence that heavy rainfall would occur on a given day somewhere increased gradually from "Low" to "Good. This trend indicates that, as the event approaches, MetService is becoming increasingly sure that heavy rainfall will occur. On the other hand, if confidence that heavy rainfall would occur was not increasing over successive days, then MetService is less sure of — but still not completely ruling out — heavy rain.

Land Beaufort Wind Scale
B.No.	Description	Knots	Km/hr	How to recognise
0	Calm	0 -1	0 -2	Smoke rises straight up
1	Light Air	1 - 3	3 - 6	Smoke drifts
2	Light Breeze	4 - 6	7 - 11	Wind felt on face; leaves rustle
3	Gentle Breeze	7 - 10	12 - 19	Flags flap; twigs move all the time
4	Moderate Breeze	11 - 16	20 - 28	Papers blow; small branches move.
5	Fresh Breeze	17 - 21	29 - 38	Small trees sway
6	Strong Breeze	22 - 27	39 - 49	Large branches move, wind whistles
7	Near Gale	28 - 33	50 - 61	Whole trees sway
8	Gale	34 - 40	62 - 74	Twigs break off, Gale warning on radio
9	Strong gale	41 - 47	75 - 88	Large branches break, some damage
10	Storm	48 - 55	89 - 102	Trees uprooted; major damage
11	Violent Storm	56 - 63	103 - 117	DANGER - TAKE SHELTER
12	Hurricane Force	64+	118+	DISASTROUS

Warnings of Severe Weather over the Sea

Oceanic warnings

New Zealand is the issuing service for oceanic forecasts and warnings in METAREA XIV. For that part of METAREA XIV south of latitude 25 S, New Zealand is also the preparing service. For that part of METAREA XIV north of latitude 25 S, Fiji is the preparing service.

Oceanic warnings advise of existing or forecast gale, storm or hurricane force winds:

• Gale: 34 — 47 kt

• Storm: 48 — 63 kt

• Hurricane: 64 kt or more, provided the wind is generated by a tropical cyclone. Otherwise wind speed of 64 kt or more is classified as a storm warning.

• Ice accretion: icing rate of greater than 2 centimetres per hour.

They are valid for 12 hours. Because METAREA XIV contains several time zones, Co-Ordinated Universal Time (UTC) is used instead of any particular local time.

Format

1. Warning type (Gale, Storm or Hurricane).

2. Warning number (resets to 1 at the beginning of each UTC month).

3. Oceanic area(s) affected (Pacific and/or Subtropic and/or Forties and/or Southern).

4. Time of analysis/observations on which warning is based.

5. Description of area over which warning is valid, and if appropriate, the movement of that area. Distances are usually described in multiples of 60 nautical miles (1 degree of latitude).

6. (a) Wind speed and direction, or (b) mention of heavy ice accretion in warning area, and if appropriate, the movement of that area.

7. Changes in the wind speed in the warning area, if appropriate.

8. The number of the warning which this warning cancels and replaces (if any).

Wind Direction

Wind directions given are the eight points of the compass (that is, north, northeast, east, southeast, south, southwest, west, northwest).

Wind Speed

Wind speed is described in multiples of 5 knots. A given wind speed implies a 10-knot range; for example, if the wind speed is forecast to be 40 knots, wind speeds between 35 and 45 knots should be expected.

Issue Times

0200, 0800,1400 and 2000 UTC.

Coastal Warnings

Coastal warnings advise of gale, storm or hurricane force winds observed or forecast within New Zealand coastal forecast areas. Coastal warnings are updated after each issue of the coastal forecast and with amendments to coastal forecasts when necessary. Wind direction and wind speed in coastal warnings is the same as described for oceanic warnings above.

For example, if there is a storm warning in sea area Conway, the Conway forecast will be of the format:
Conway
*STORM WARNING IN FORCE*
Forecast
...

Marine Beaufort Wind Scale
B.No.	Description	Knots*	How to recognise	Wave Height	Avg (m)	Max (m)
0	Calm	0 -1	Sea like mirror	Calm	0	0
1	Light Air	1 - 3	Ripples with the appearance of scales are formed but without foam crests	Calm	<0.1	<0.1
2	Light Breeze	4 - 6	Small wavelets, still short but more pronounced; crests have a glassy appearance and do not break	Smooth	<0.2	0.3
3	Gentle Breeze	7 - 10	Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses	Smooth	0.6	1
4	Moderate Breeze	11 - 16	Small waves, becoming longer; fairly frequent white horses	Slight	1.0	1.5
5	Fresh Breeze	17 - 21	Moderate waves, taking a more pronounced long form; many white horses are formed. (Chance of some spray)	Moderate	2.0	2.5
6	Strong Breeze	22 - 27	Large waves begin to form; the white foam crests are more extensive everywhere. (Probably some spray)	Rough	3.0	4.0
7	Near Gale	28 - 33	Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind	Very Rough	4.0	5.5
8	Gale	34 - 40	Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind	High	5.5	7.5
9	Strong gale	41 - 47	High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility	Very High	7.0	10.0
10	Storm	48 - 55	Very high waves with long overhanging crests. The resulting foam in great patches is blown in dense white streaks along the direction of the wind. On the whole, the surface of the sea takes on a white appearance. The tumbling of the sea becomes heavy and shocklike. Visibility affected	Very High	9.0	12.5
11	Violent Storm	56 - 63	Exceptionally high waves. (Small and medium-sized ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected	Phenomenal	11.5	16.0
12	Hurricane Force	64+	The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected	Phenomenal	>14.0	>16
* measured at a height of 10m above sea level ** Probable (significant) height of waves (in metres)