TUCSON, Arizona — A low pressure system swirling over the central Gulf of California is being watched for possible tropical development, with forecasters placing a 20 percent formation chance through both 48 hours and 7 days as of September 16, 2026. While development chances are decreasing as the system is expected to move inland, heavy rains will likely continue across portions of Sonora, Mexico during the next day or so and could cause isolated flooding that spills northward toward the Arizona border communities of Tucson, Nogales and Agua Prieta. For residents in southern Arizona, this system is worth watching closely even with a low formation chance.
What a 20 Percent Formation Chance Means for Southern Arizona
A 20 percent tropical formation chance is considered low but not zero. What matters most for residents of Tucson, Nogales, Safford and Silver City, New Mexico is not whether this system becomes a named storm — it is the heavy rainfall it is already producing regardless of its official classification. The low pressure system over the central Gulf of California is already bringing heavy rains to portions of Sonora, Mexico, and that moisture is positioned to push northward toward the Arizona and New Mexico border region over the next 24 to 48 hours.
Where the Heavy Rain and Flooding Risk Is Focused
The satellite image captured at 1:01 PM on September 16, 2026 shows the low pressure system centered over the central Gulf of California with green arrows tracking its expected inland movement toward Hermosillo and Ciudad Obregon in Sonora, Mexico. Here are the communities on both sides of the border that should monitor this system closely:
- Tucson, Arizona sits directly north of the Sonora moisture plume and is in the most likely path for heavy rainfall spillover as the system moves inland from the Gulf of California over the next 24 to 48 hours.
- Nogales, Arizona straddles the Mexico border and falls within the zone most vulnerable to heavy rainfall as the system pushes northward from Sonora through the next day or so.
- Agua Prieta and the Douglas, Arizona border corridor are visible on the satellite image within the moisture circulation from the Gulf of California system.
- Safford, Arizona and Silver City, New Mexico are visible on the northern edge of the satellite image and fall within the broader rainfall environment as moisture from the Gulf of California system spreads inland.
- Florence, Buckeye and Rocky Point are visible on the satellite image within the broader circulation zone being monitored as of September 16, 2026.
- Las Cruces and Lordsburg, New Mexico are visible on the eastern portion of the satellite image and could see moisture from this system as it tracks inland through Sonora.
Why Southern Arizona Residents Should Pay Attention Even With Low Formation Odds
The Arizona border region sits directly in the path of moisture from any system that moves through the Gulf of California and into Sonora. Even a disorganized low pressure system that never reaches tropical storm status can dump several inches of rain across Tucson, Nogales and surrounding communities within hours of the heaviest moisture arriving. Desert terrain in southern Arizona is particularly vulnerable to flash flooding because hardpan soils cannot absorb heavy rainfall quickly, sending water rushing into washes, arroyos and low-lying streets almost immediately. The isolated flooding risk for Sonora identified in the source data is a direct warning for the border communities of southern Arizona as well.
What to Watch Over the Next 48 Hours
The system is expected to move inland from the Gulf of California later today, which decreases the chances of tropical development but does not eliminate the heavy rainfall threat for southern Arizona and New Mexico. Monitor local radar closely over the next 24 to 48 hours if you are in Tucson, Nogales, Safford or Las Cruces. Never cross a flooded wash or road during monsoon-related rainfall events, as water levels in desert terrain can rise from zero to dangerous in minutes even when skies overhead appear clear…