Narrow corridors of concentrated water vapor flux that transport tropical moisture to mid-latitudes. The "Pineapple Express" carries moisture from Hawaii to California.
AR threshold: IVT > 250 kg/m/s. Extreme events exceed 1000 kg/m/s.
• California winter flooding
• Sierra Nevada snowpack
• Carries 7–8× Mississippi River flow
• Cat 4–5 ARs: extreme damage
Science: ARs carry up to 90% of poleward water vapor flux in extratropical latitudes in a band just 400–600 km wide (Zhu & Newell 1998). They are associated with pre-cold-frontal low-level jets at 850 hPa. AR scale 1–5 (Ralph et al. 2019) rates intensity (IVT) and duration — Cat 1–2 beneficial for drought, Cat 4–5 hazardous flooding.