Normal barometric pressure is about 29.9 inches of Mercury. A "normal" range is about 29.6 - 30.2 inches Hg (752-767 mm Hg) at sea level. Rarely (at sea level) do readings exceed 30.4 inches Hg (773 mm Hg) except for occasional arctic highs in January. Rarely (at sea level) do readings fall below 29.5 inches Hg (749 mm Hg).
But during a hurricane, pressure can drop precipitously. For each category of hurricane, the pressure can drop further:
2 — Moderate: 28.50 to 28.91 inches of Hg3 — Extensive: 27.91 to 28.47 inches of Hg4 — Extreme: 27.17 to 27.88 inches of Hg5 — Catastrophic: less than 27.17 inches of Hg