Climate Monitoring

Remnants of Tropical Cyclone Mario brought three to four inches of rainfall over southern California between September 17 and 19. This amount of rain over a climatologically dry area is equivalent to receiving several months' worth of precipitation in under 48 Hours. One particular mesonet station, located in Forest Falls, recorded 1.77 inches of precipitation in just one hour—an extreme rainfall event that happens, on average, once every 25 years. This amount of rainfall in a short time period brought flash flooding and mudslides in numerous areas, including Riverside and San Bernardino Counties. One fatality was reported, and multiple vehicle rescues were performed by local emergency management. In total, over 30 flood reports were received by local National Weather Service offices.

Over the past seven weeks, July 29 to September 16, the Northeast has plunged into a historically significant drought, with conditions deteriorating at a rapid rate. According to the U.S. Drought Monitor (USDM), a region that was almost entirely drought-free in early August now has over 40% of its area in moderate to extreme drought. This rapid decline has led to record-breaking milestones across New England. For the first time in the 25-plus-year history of the USDM, a portion of Vermont has been classified with extreme (D3) drought. At the same time, neighboring New Hampshire is experiencing its largest area of D3 drought ever recorded. This severe lack of rainfall has led to mounting impacts, including failing wells, critically low streamflows, widespread agricultural losses, and an elevated wildfire risk across the hardest-hit states.

A rare September tornado outbreak extended from north-central South Dakota to central North Dakota on September 14. Of the 21 tornadoes reported across the region, 19 preliminary reports occurred in North Dakota. North Dakota typically experiences one tornado in September and an annual average of 30 tornadoes. Preliminary storm counts for 2025 indicate 68 tornadoes were reported in North Dakota through mid-September (227% of average), which, if confirmed, would be a new annual record for the state (61 tornadoes in 1999). The September 14 outbreak accounted for over one-quarter of this preliminary total. Notably, 2025's highest daily tornado count for North Dakota (21) occurred on June 20, when a derecho traversed the state.

The 2025 Atlantic hurricane season has been unusually quiet during its peak. No named storms formed between August 24th and September 15th, a rare lull last seen in 1992. Additionally, no hurricane has formed in the Atlantic since hurricane Erin in mid-August. Through mid-September, the season has produced only six named storms and one hurricane, making it one of the least active starts in decades. This lack of activity is attributed to unfavorable conditions, including dry stable air, wind shear, and dust, which have prevented tropical waves from organizing in the main development region of the Atlantic.