Mid-summer heatwave — what to do in the 72 hours when the forecast first turns red
When ECMWF and your national met office issue a heatwave alert 72 hours out, the household that prepares in those 72 hours handles a week of 36°C fundamentally differently from the one that improvises. Six concrete pre-event actions, then four during-event rules.
The 72-hour window
European national met services (DWD in Germany, IMGW in Poland, AEMET in Spain, Météo-France, Met Éireann, the Met Office in the UK, MeteoSwiss in Switzerland, KNMI in the Netherlands) all now issue heatwave forecasts 48-72 hours in advance with reasonable accuracy. The forecast you get on Monday for Thursday's 35°C peak is rarely wrong by more than 1-2°C.
That window is not a warning. It is a planning horizon. The households that handled the June 2022 and August 2023 European heatwaves well did six things in those 72 hours. None expensive. All boring.
Six pre-event actions
- Fill containers. Heat events frequently coincide with municipal water pressure drops. Fill every container you have — bath, large pots, 5-litre bottles, kettle. Aim for 30-40 litres in storage by the day the heat arrives. This is buffer, not survival reserve — you draw it down through the week.
- Identify your cool room. One room in your home will be cooler than the rest. Usually north-facing, on the ground floor or basement, with thick walls or external shade. Test it now (Wednesday afternoon) — close the shutters, close the door. Sleep there on the hot nights.
- Pre-cool the cool room before the heat arrives. On the night before the first hot day, open all windows wide between midnight and 06:00 and run a fan to draw cool night air through. Close everything at 07:00 and don't open again until the next midnight. A well-pre-cooled apartment in Central Europe stays 4-6°C below outside temperature for the whole day.
- Identify the most vulnerable person in your block. A neighbour over 75 who lives alone. A new parent with an infant. Someone with a heart condition. You don't need to do anything dramatic — just know who they are, and walk past their door once on Day 2 of the event. The 2003 French heatwave killed 14,000 people, the majority elderly and alone in apartments.
- Plan the medication schedule. Several common medications behave differently at 35°C+ (notably ACE inhibitors, diuretics, some antihistamines, lithium). If anyone in your household takes daily medication, check the heat-warning section of the package insert or call the pharmacist. Some need increased water; some need to be taken at different times.
- Pre-stage the cold food. Cook what you can eat cold for the next 4 days on the evening before the heat arrives. Cold pasta, cold rice, hummus, hard-boiled eggs, cold meats. Avoid using the oven during the event. A 200°C oven for 30 minutes adds 2-3°C to a small kitchen for the whole afternoon.
Four during-event rules
Once the heat is on:
- Drink before you're thirsty. Thirst is a 2-3 hour lagging indicator. Set a phone alarm for every 90 minutes and drink 250 ml of water with a tiny pinch of salt. This is not a prepper rule — it is what the WHO and Red Cross recommend for sustained 35°C exposure.
- Move physical work to 05:00-08:00 or after 21:00. If you must do anything that raises body temperature (gardening, sport, walking the dog far) — early morning only. Mid-afternoon walks in a 36°C heat event have killed otherwise-healthy 50-year-olds in the UK and France.
- Watch for heatstroke signs in others, not yourself. Heatstroke impairs the judgment of the person experiencing it. The person noticing it is usually a household member: confusion, slurred speech, dry skin (no sweat) at peak heat, body temperature above 40°C. This is a medical emergency — cool the person aggressively (wet sheets, ice packs at neck/armpits/groin) and call emergency services.
- Sleep where it's coolest, even if it's the floor. During the 2003 heatwave, indoor overnight temperatures of 30°C+ were the dominant predictor of mortality. If your bedroom is on the top floor and west-facing, move a mattress to the cool room. One night on the floor is not pleasant. Five nights of 31°C indoor sleep is dangerous.
What this is not
This is not advice for a 28°C summer day. It is the protocol for a forecasted 4-7 day event with daytime peaks at 33-38°C and overnight minimums above 22°C. If you live in Madrid, Athens, southern France or Italy, this is your normal July playbook. If you live in Berlin, Warsaw, Stockholm or Edinburgh, this is your once-or-twice-a-summer protocol — which is exactly when households underprepare.
One thing this week: identify your cool room. Walk into each room of your home in the warmest part of the afternoon. The one that is meaningfully cooler is your refuge. Note it.
— Systems Fail Lab