On an ordinary summer night in Cessnock, the air cools by roughly 12 to 14 degrees from the afternoon high. That range is our estimate, worked from the Bureau of Meteorology's averages for Cessnock Airport, which use all years of data from 1968 to 2026. January's mean maximum is 30.4 °C and its mean minimum 17.1 °C, a drop of 13.3 degrees. December (29.2 °C and 15.2 °C) gives 14.0 degrees, and February (29.4 °C and 17.0 °C) gives 12.4.
That overnight drop is when a house, and the people in it, get their breath back. A heatwave is the run of days when the drop shrinks. The bureau defines one as a period when the maximum and minimum temperatures are unusually hot over 3 days, measured against the local climate and recent weather.
So a heatwave warning is not a forecast of a hot day. It is a forecast of a hot run, nights included. Below is how the bureau tests for one, how it grades them, which ones get a warning, and how much notice you get.
What counts as a heatwave?
A hot spell
- Day 1: hot
- Night 1: cools down
- Day 2: hot
- Night 2: cools down
- Day 3: hot
- Night 3: cools down
Hot days, but the nights cool back down
A heatwave
- Day 1: hot
- Night 1: hot
- Day 2: hot
- Night 2: hot
- Day 3: hot
- Night 3: hot
Days and nights both unusually hot for this place, over 3 days
BoM compares the 3-day average with what is hot for that location and with the past 30 days.
Source: Bureau of Meteorology, What is a heatwave?, Bureau of Meteorology, Heatwaves: what you need to know
A heatwave is not one hot day, and not just a high maximum: it's not a heatwave if it cools down to normal at night.
"Three hot nights" is a handy way to remember the bureau's 3-day definition. It is not an official rule or test. The days have to be unusually hot as well, and "unusually" is measured against your own area.
The nights matter because cooler nights let people recover from each day's heat. The bureau adds that a high overnight temperature can mean the next day heats up quickly, with more hours of extreme heat.
To grade a heatwave, BoM uses the excess heat factor, which compares the 3-day average temperature with what is hot for that location and with the past 30 days. Because the yardstick moves, it takes higher temperatures to meet heatwave criteria at the end of summer than early in the season. An early-summer run can therefore count as a heatwave at temperatures that might not register in late February.
What's a normal summer night in Cessnock, and what's forecast?
Cessnock Airport summer averages, all years of data (1968 to 2026)
Mean minimumMean maximum
- Decembermean minimum 15.2 °Cmean maximum 29.2 °C
- Januarymean minimum 17.1 °Cmean maximum 30.4 °C
- Februarymean minimum 17.0 °Cmean maximum 29.4 °C
Cessnock's summer nights usually settle in the mid-teens. The mean minimum is 15.2 °C in December, 17.1 °C in January and 17.0 °C in February. Those are the nights a heatwave pushes up.
The bureau's outlook issued 22 October 2026 leans warm at both ends of the day, for November to January:
- Nights: much of the country has a very high chance (over 80%) of unusually high minimum temperatures, "more than 4 times the normal likelihood".
- Days: the same outlook gives eastern parts of NSW over a 70% chance of unusually high maximum temperatures.
That is our reading of two separate odds side by side. It is not a forecast of any particular heatwave in Cessnock. The outlook is reissued monthly, and a newer one covering December to February was issued on 5 November, so check the current outlook before you plan around it.
Low, severe or extreme: which heatwaves get a warning?
Which heatwaves get a warning?
| Level | What BoM says about it | Warning issued? |
|---|---|---|
| Low | Frequent in summer; most people cope | No: warnings cover severe and extreme only |
| Severe | Harder for vulnerable people, including older people with medical conditions | Yes, up to 4 days ahead |
| Extreme | Rare; a problem even for healthy people who don't take precautions | Yes, up to 4 days ahead |
Warnings give expected maximum and minimum temperatures and list affected towns.
Source: Bureau of Meteorology, What is a heatwave?, Bureau of Meteorology, Heatwave services
BoM grades every heatwave at one of three intensity levels: low, severe and extreme:
- Low: frequent in summer, and most people cope.
- Severe: harder on vulnerable people, including older people with medical conditions.
- Extreme: rare, and a problem even for healthy people who don't take precautions.
Warnings are issued only for severe or extreme heatwaves. A low-intensity heatwave can be on the forecast map without a warning attached.
Don't treat any level lightly. The bureau describes heatwaves as Australia's most deadly natural hazard.
How much notice does a heatwave warning give?
When could the warned heat arrive?
Typical answer: a warning issued on a Tuesday
A warning issued at 3 pm on Tuesday can cover heat as far out as Saturday.
- TueWarning issued, 3 pm
- Wed
- Thu
- Fri
- SatLatest day covered
In NSW, warnings are issued at 3 pm local time, and only for severe or extreme heatwaves. Separate heatwave forecasts look 7 days ahead.
A warning comes up to 4 days ahead. It gives the expected maximum and minimum temperatures and lists the affected towns. Forecasts of heatwaves likely in the next 7 days are published as well.
In NSW, warnings are issued at 3 pm local time.
Warnings show up in BoM Warnings and alerts, the BOM Weather app and a telephone audio service. BoM says heatwave forecasts and warnings are generally issued from October until the end of March.
The forecast map changes daily, so look at BoM's live heatwave forecast rather than relying on anything written here. To see whether a low-intensity pocket or a warned area touches the Hunter, that page is the one to check.
For health advice in the heat, go to NSW Health or call healthdirect on 1800 022 222. In an emergency, call Triple Zero (000).
What to do at the windows
A warning gives you up to four days' notice, and windows are a major way heat gets into a house. We cover that, and which way each window faces, in our El Niño summer guide. To see which of your windows take the most sun, try the Shutter Aspect Planner.