vary slightly by county warning area) for two consecutive days. Warning criteria may be lowered by the issuing WFO if the heat event occurs early in the season, during a multi-day heat wave or during a widespread power outage occurs during a heat event due to severe weather phenomena (such as high winds, severe thunderstorms, or a derecho). Excessive heat watch (NPW) – Conditions are favorable for an excessive heat event with extreme Heat Index values during the day, combined with nighttime low temperatures of or higher that limit perspiration recovery, are forecast to occur to meet or exceed local Excessive Heat Warning criteria in the next 24 to 72 hours. Extreme cold warning (NPW) – Forecast shelter temperature of or colder and air temperature remains below up to the level for three or more consecutive days. Only issued by Weather Forecast Offices in Alaska. Elsewhere, an Extreme Cold Warning can be issued by local Weather Forecast Offices as an experimental product using locally appropriate thresholds, usually with little or no wind. (The product was first used experimentally by Weather Forecast Offices in North Dakota, South Dakota and Minnesota from January until April 2011.) The Extreme Cold Warning and the Wind Chill Warning products will be consolidated into a singular Extreme Cold Warning product for use by WFOs in the rest of the conterminious United States in 2021, accounting for NWS software upgrades. Extreme cold watch (NPW) – Conditions are favorable for an extreme cold event to meet or exceed local Extreme Cold Warning criteria. Only issued by Weather Forecast Offices in Alaska. Freeze warning (NPW/FZW) – Minimum shelter temperatures are forecast to be or below over a widespread area within 12 to 36 hours, regardless of whether frost concurrently develops on plants, vegetation and other exposed surfaces. Freeze Warnings are usually issued to highlight freezes occurring during the locally defined starting and ending periods of the growing season (as determined by the average dates of the first freeze of Autumn and the last freeze of Spring), and will be intermediately issued when appropriate until the end of the growing season based on criteria set by the Weather Forecast Office. Freeze watch (NPW) – Issued during the locally defined growing season when conditions are favorable for a freeze event that may damage or kill crops to meet or exceed Freeze Warning criteria within 24 to 48 hours. Frost advisory (NPW)