During the eastern Pacific hurricane system, there is usually at least one cyclone that moves inland and into North America, bringing moisture into the southwestern US. After all, it still is the wet season in this part of the country. Hurricane Rosa, currently located well off the western Mexican coast, is forecast to take a turn to the east and make landfall on the northern part of Baja California, possibly still as a tropical storm, early next week.
A deep and intense trough of low pressure will dig to the south and east off the West Coast of the US, which will pull Rosa up to the northeast. Typically as a cyclone reaches this latitude over the eastern Pacific Ocean it takes a turn to the west and out to sea. But in this scenario, these two components will team up and direct Rosa’s remnants and highly-anomalous moisture into the Southwest next week.
The rain will begin to become present across the region on Tuesday as showers and thunderstorms ramp up. As far north as California may even receive rain thanks to Rosa and the incoming trough of low pressure, so this may not solely be a Southwest event. The worst of the weather is currently expected to take place Tuesday night into Wednesday as the core remnants of Rosa moves into the Four Corners states, allowing for potentially very heavy rain to come down. No doubt flash flooding will occur in some areas. Given the fact that this is a desert region, river levels may increase rapidly and landslides and mudslides will become a threat. The weather for the region will remain unsettled the rest of next week, with occasional showers and potentially even some mountain snow in the highest of elevations.
As the European model illustrates, there will be copious amounts of moisture present in the atmosphere. In fact, it will come close to record levels for this time of the year in the southwest. This amount of moisture tells us two things: there will be heavy rain and flash flooding will very likely take place in some locations.
Next week, some locations, especially in Arizona, will likely near or exceed half a foot of rainfall while surrounding areas generally pick up 1-3 inches (which is still substantial).