Heat advisory in New Castle County through Wednesday evening
NEW CASTLE County, in Delaware, is currently under a heat advisory effective from 11:00 a.m. on Tuesday, June 18 through 8:00 p.m. on Wednesday, June 19. After a gloomy and unseasonably cool Father’s Day, where highs barely reached 65°F (18°C), residents are now facing a sharp rise in heat and humidity.
The heat index is expected to climb to around 99°F (37°C) due to high temperatures near 86°F (30°C) and elevated humidity levels, increasing the risk of heat-related illnesses.
What to expect across Delaware
New Castle County: Temperatures on June 18 will peak around 86°F (30°C) with an 80% chance of showers and thunderstorms during the afternoon. By Wednesday, June 19, highs could reach 91°F (33°C) with another 80% chance of storms, especially after 2:00 p.m., some of which may become severe. The skies are expected to clear by the weekend, with sunshine and highs in the low 80s°F (27–28°C) on Thursday, June 20, the high 80s°F (31°C) on Friday, June 21, and the low 90s°F (32–33°C) by Saturday, June 22.
Kent County: No official heat advisory is in place, but the region will still experience highs in the low 90s°F (32–33°C) on June 18, followed by afternoon thunderstorms. On June 19, highs will stay around 93°F (34°C), again with the potential for showers and storms. Like New Castle, sunny and hot weather will dominate the weekend.
Sussex County: Similarly, Sussex is not under a heat alert, but temperatures will soar into the 90s°F (32–34°C) on Tuesday, with storms possible in the afternoon. The June 19 forecast mirrors that of Kent, with highs near 93°F (34°C) and another round of showers and thunderstorms. Expect sunshine and rising temperatures from Thursday through Saturday, with mid-80s°F (29°C) on June 20, upper 80s°F (31°C) on June 21, and low 90s°F (32–33°C) on June 22.
Understanding heat advisories and heat index
A heat advisory is typically issued within 12 hours of the onset of dangerously hot conditions, usually when the heat index surpasses 100°F (38°C) and nighttime temperatures remain above 75°F (24°C). While the specific criteria can vary by region, the general guidance is based on the risk of heat exhaustion or heat stroke.
The heat index reflects how hot it feels to the human body when relative humidity is added to the air temperature. On humid days, sweat evaporates more slowly, limiting the body’s ability to cool itself. This makes the air feel significantly hotter than the actual temperature.
The National Weather Service classifies heat index readings as follows:
Caution (80–90°F / 27–32°C): Fatigue possible with extended activity or exposure.
Extreme caution (90–103°F / 32–39°C): Risk of heat exhaustion, cramps, or stroke with prolonged exposure.
Danger (103–124°F / 39–51°C): Heat exhaustion likely, heat stroke possible.
Extreme danger (125°F / 52°C and above): Heat stroke is likely under extended physical activity or exposure.
Conditions across Delaware will continue to be oppressive through midweek, with daytime highs and humidity making it critical to understand how the heat index affects the body.
