Glossary
Terms used on this site. On other pages, the first mention of each term is underlined with dots: hover for the definition, or click to come here.
1991–2020 normals and medians
NOAA’s 30-year average for each calendar day at the Flagstaff airport (1991–2020 Climate Normals). SNOTEL comparisons use the NRCS 1991–2020 median for the date.
250-hPa wind and the jet stream
Wind speed at the 250-hPa level, about 10–11 km above sea level, near the level of the jet stream (a narrow band of strong westerly wind). Maps contour 40, 55 and 70 m s⁻¹.
500-hPa height
The altitude of the 500-hPa pressure surface (hPa = hectopascal = millibar), about 5.5–6 km above sea level. Low heights mark troughs and lows aloft, and the wind at this level blows nearly along the height contours.
ALERT gauge network
Automated Local Evaluation in Real Time: flood-warning rain gauges operated by the City of Flagstaff and Coconino County. Each gauge is a 1-mm tipping bucket that reports every tip by radio.
Atmospheric river
A long, narrow corridor of strong water-vapour transport. On this site, ‘atmospheric-river strength’ means IVT ≥ 250 kg m⁻¹ s⁻¹, a threshold widely used for western North America (e.g. Rutz et al. 2014).
Closed low
A low aloft enclosed by at least one height contour. Here: a local minimum of 500-hPa height over a 5° neighbourhood within 1,500 km of Flagstaff.
ERA5
The fifth-generation ECMWF global atmospheric reanalysis from the Copernicus Climate Change Service: a consistent reconstruction of past weather from 1940 on, combining observations with a forecast model. Used here at 0.25° and 6-hourly; it becomes available about 5 days after real time.
Event window
The UTC period chosen to contain an event’s rain. Gauge totals are summed over it; airport and SNOTEL values use the local (MST) calendar days it covers.
Integrated vapour transport (IVT)
The horizontal flux of water vapour summed through the depth of the atmosphere, in kg m⁻¹ s⁻¹: how much moisture the wind carries across each metre of a line on the ground, per second.
MST and UTC
Mountain Standard Time is UTC−7. Arizona does not observe daylight saving time, so Flagstaff is on MST all year. ERA5 and maps use UTC (Coordinated Universal Time).
Network median and percentiles
Statistics across all gauges in the network. The median is the middle gauge value; the 10th–90th percentile range spans the middle 80% of gauges, so a single gauge does not stand in for the city.
Network-mean rain
The average of all gauges’ hourly rain. Used to time an event: the ‘central 80%’ is the period between 10% and 90% of the network-mean total.
NHC best track
The National Hurricane Center’s record of each storm’s position and intensity every 6 hours. This site uses the operational files; the final best track is released after the season and may differ slightly.
Provisional and final pages
An event page is provisional until ERA5 covers the whole event window (about 5 days after it ends); it is then rebuilt with ERA5 maps and the atmospheric summary and marked final.
Remnant disturbance / remnant low
What is left of a tropical cyclone after it loses its tropical structure; NHC continues to track it as a disturbance (DB) or low (LO). Its moisture can still reach Arizona.
Shaded relief, 3DEP and NHD
Gauge maps show terrain as shaded relief computed from the USGS 3D Elevation Program (3DEP) digital elevation model, with 200-m contours, streams from the USGS National Hydrography Dataset (NHD) and highways from Census TIGER.
SNOTEL
Snow Telemetry: automated USDA NRCS mountain stations measuring snow water equivalent, snow depth, accumulated precipitation and air temperature. Sites used here: Fort Valley, Snowslide Canyon and Fry.
Snow water equivalent (SWE)
The depth of water that would result if the snowpack melted, in inches.
Tipping bucket
A rain gauge that funnels rain into a small two-sided bucket that tips each time it fills (1 mm here). Tips are counted and time-stamped; very intense rain can be undercounted and snow is not measured until it melts.
Total column water vapour
The mass of water vapour above a unit area from the ground to the top of the atmosphere, in mm (kg m⁻²); also called precipitable water.
Tropical cyclone
A warm-core low-pressure system that forms over tropical oceans. NHC categories: tropical depression (sustained wind < 34 kt), tropical storm (34–63 kt), hurricane (≥ 64 kt).
Trough axis
The line through the lowest heights of an elongated region of low 500-hPa height. Here: the longitude of the lowest 500-hPa height along Flagstaff’s latitude between 135°W and Flagstaff.
Water year
October 1 to September 30, named for the calendar year in which it ends (water year 2027 began October 1, 2026).