September 23, 2026

IVT < 250
Final
Network median 0.08 in (range 0.00–2.36 in); peak IVT 68 kg m⁻¹ s⁻¹ from the SW
Published

September 23, 2026

Summary

Across 39 gauges the event total ranged from 0.00 to 2.36 in (median 0.08 in). The central 80% of the network-mean rain fell between Sep 23 11:00 MST and Sep 23 15:00 MST (4 h). Moisture transport over Flagstaff stayed below atmospheric-river strength: ERA5 IVT peaked at 68 kg m⁻¹ s⁻¹ from the SW at Sep 23 05:00 MST, and total column water vapour reached 18 mm. At the IVT peak, 500-hPa flow over Flagstaff was from the SW at 9 m s⁻¹, and the 500-hPa trough axis along 35.25°N was at 115.2°W, 327 km west of Flagstaff (5,824 m, against 5,834 m over Flagstaff). A closed 500-hPa low (5,815 m) was centred 1,060 km to the SW, at 28.0°N 119.0°W. The strongest 250-hPa wind within 1,000 km was 49 m s⁻¹, 887 km to the NNE. The 250-hPa wind over Flagstaff peaked at 31 m s⁻¹ (Sep 23 05:00 MST), at the same time as the IVT peak.

Rain across the network

Event rain total at each city and county gauge (circle area proportional to total, labels in inches) over shaded relief (USGS 3DEP), 200-m elevation contours, streams (USGS NHD) and highways (Census TIGER). Orange triangles: SNOTEL sites with event precipitation (in). Square: Flagstaff airport.

Hourly and cumulative rain across the gauge network (median and 10th–90th percentile; cumulative from the start of the event window). When ERA5 is available: IVT, 250-hPa wind speed and 500-hPa height at the ERA5 grid point nearest Flagstaff. Times UTC.
  • Network rain (39 gauges): median 0.08 in; 10th–90th percentile 0.03–0.61 in; lowest 0.00 in (Rio de Flag at Butler), highest 2.36 in (Peak View Park).
  • Peak hour: network median 0.08 in (hour beginning Sep 23 14:00 MST); highest single-gauge hour 1.61 in.
  • SNOTEL event precipitation: Fort Valley 0.0 in; Snowslide Canyon 0.3 in; Fry 0.1 in.
  • ERA5 at Flagstaff: peak IVT 68 kg m⁻¹ s⁻¹; maximum 250-hPa wind 31 m s⁻¹; minimum 500-hPa height 5,834 m; maximum total column water vapour 18 mm.

Atmospheric setting

ERA5 atmospheric setting, animated
Sep 23 12 UTC · Sep 23 05:00 MST

ERA5 every 6 hours, from 12 h before to 12 h after the event window, opening at the time of peak IVT at Flagstaff. Shading: integrated vapour transport (IVT); blue contour and arrows: IVT ≥ 250 kg m⁻¹ s⁻¹. Black: 500-hPa height (60-m interval). Orange: 250-hPa wind 40, 55 and 70 m s⁻¹. ★ Flagstaff; ✕ NHC best-track position. Lower strip: network median hourly rain, with the event window shaded and the map time marked in orange. Step with the buttons, slider or ← → keys; space plays and pauses.

Climate context

  • Airport precipitation: 0.12 in over 1 local day, against a 1991–2020 normal of 0.05 in for those days (2× normal).
  • Rank: the 117th largest 1-day airport total in September among 129 years with data since 1898 (each year’s largest 1-day total ending in September).
  • Airport temperature: highs averaged +0.0 °F and lows +4.0 °F relative to normal.
  • Water year to date (Oct 1, 2025 – Sep 23, 2026): airport 19.6 in (97% of normal); SNOTEL Fort Valley 18.2 in (100% of median); Snowslide Canyon 36.3 in (105% of median); Fry 29.6 in (116% of median).

Station data

Flagstaff airport (local days; normals 1991–2020; M = not yet reported)

Precip (in) Normal (in) Snow (in) High (°F) High vs normal Low (°F) Low vs normal
Sep 23 0.12 0.05 0 72 +0 44 +4
Total 0.12 0.05 0

SNOTEL (water-year precipitation from Oct 1; medians 1991–2020)

site Event precip (in) Water-year precip (in) Median to date (in) % of median SWE at end (in) Max snow depth (in)
Fort Valley 0 18.2 18.2 100 0 0
Snowslide Canyon 0.3 36.3 34.7 105 0 0
Fry 0.1 29.6 25.5 116 0 0