Encyclopedia

The climate history of the Southwestern United States extends back thousands of years, far beyond the region’s written historical record, which begins only a few hundred years ago with the earliest Spanish expeditions into the region in the late 1530s and 1540s (American Southwest Virtual Museum, 2019). Human societies have inhabited the region for thousands…

CAPE is a measure, in joules per kilogram (J/kg), of the energy available to a rising air parcel once it becomes positively buoyant relative to its surroundings. It is the primary thermodynamic index used to estimate how strong thunderstorm updrafts could become on a given day. Forecasters treat it as a standard input to every…

CIN (also written CINH) is a measure, in joules per kilogram (J/kg), of the energy an air parcel must expend to overcome a stable layer in the atmosphere before it can rise freely. That stable layer is sometimes called a “cap.” Where CAPE (Convective Available Potential Energy) measures the energy available once convection starts, CIN…

The El Niño-Southern Oscillation (ENSO) is a natural, recurring swing in ocean temperatures, rainfall, air pressure, and winds across the tropical Pacific Ocean (NOAA Climate Prediction Center, ENSO FAQ). It has three phases. El Niño is the warm phase, when the ocean surface in the central and eastern tropical Pacific runs warmer than normal. La…

The Hot-Dry-Windy Index (HDWI) is a fire weather tool used by wildfire forecasters and fire managers across the United States. It judges how much the atmosphere itself, apart from the fuel on the ground or the shape of the land, is likely to make a wildfire harder to control on a given day. It does…

NZI is a seasonal climate index built from sea-surface temperature and atmospheric-pressure anomalies near New Zealand. It’s used to forecast winter precipitation in the southwestern United States (SWUS) months in advance. A statistical link like this is called a teleconnection: conditions in one distant part of the world reliably foreshadow weather in another. The NZI…

The Phoenix Dust Storm Scale (PHX-DUST) ranks the severity of dust storms (haboobs) in the Phoenix metropolitan area of Arizona, United States, on a five-category scale using post-event data. A coalition including Arizona State University researchers, the Phoenix office of the National Weather Service, regional air-quality and flood-control agencies, a utility, and local broadcast meteorologists…

The Rapid Refresh Forecast System (RRFS) is a computer weather forecasting model developed by the U.S. National Weather Service (NWS) to produce short-term, highly detailed forecasts across North America. It is built to update itself every hour with fresh weather observations and to simulate individual thunderstorms directly, making it especially useful for predicting fast-changing weather…

REFS stands for RRFS Ensemble Forecast System. It is not a stand-alone weather model in the way GFS (Global Forecast System), HRRR (High-Resolution Rapid Refresh), or RRFS (Rapid Refresh Forecast System) are. Instead, REFS is the built-in ensemble component of RRFS itself, a way of running RRFS many times with small variations and combining the…

Virga happens when rain or snow falls out of a cloud, then dries up on the way down and never reaches the ground. Rain evaporates; snow sublimates instead, turning directly from ice back into vapor without melting first (National Weather Service, Glossary; American Meteorological Society, Glossary of Meteorology). It is common enough in dry climates,…