Tropical Storm Hilary forms, takes aim at Southern California, Southwest

• The storm, which is forecast to reach major hurricane intensity of Category 3 or above, will likely lose its tropical characteristics by the time it nears California this weekend.
• It will bring a significant threat of flooding rains, high surf and possibly high winds from San Diego to the inland deserts of California northeastward into Nevada and parts of Arizona.
• Los Angeles could be affected by rain and there's outside chance of strong winds as early as Sunday evening, and the storm's impacts could spread further inland next week into Utah and Colorado, in the form of showers and thunderstorms.

Between the lines: Under some forecast scenarios, Tropical Storm Hilary could bring a year's worth of rain to typically dry locations in just two to three days, which would cause extensive flooding.

• The NWS forecast office in Phoenix said the amount of atmospheric water vapor surging into the Southwest ahead of the storm may reach levels "almost never experienced this time of year."

The big picture: The overall weather pattern that is likely to sling-shot Tropical Storm Hilary into the Southwest features a building heat dome over the Central U.S., along with a low pressure area expected to move close to the Central California coast.

• The heat dome, which the National Weather Service described Wednesday as "anomalously strong/massive" is likely to lead to an extended heat wave across the Central U.S. this weekend into midweek.
• The heat dome will help deflect the jet stream far to the north, across Canada, while a dip in the jet stream forms near the West Coast.
• The unusual steering flow between these two weather features will open a corridor that will allow the tropical weather system to threaten Southern California, and steer a potent atmospheric river into the Southwest.

By the numbers: Models are showing the potential for 2 to 4 inches of rain to fall in some areas of Southern California, Nevada and Arizona, including in desert areas and places where there are currently drought conditions.

Context: It is extremely rare for a tropical storm or hurricane to make landfall in Southern California, since the region is protected by both relatively cool ocean waters as well as conditions in the atmosphere that can rip such storms apart.

• According to meteorologist Kieran Bhatia, only two tropical storms have made it within 100 miles of San Diego, most recently in 1963.
• However, in this instance, computer models are showing that a landfall in the state as a tropical storm or hurricane is possible.
• Human-caused climate change is leading to more frequent and intense extreme precipitation events, and is also leading to wetter tropical storms and hurricanes.

Between the lines: The NWS has heightened its wording on this threat, stating Wednesday: "Confidence continues to to increase on a heavy rainfall and potentially high impact event to unfold and focus across parts of the Southwest and California Saturday to Monday."

• A portion of the rain may spread inland before Hilary's arrival from the tropical eastern Pacific Ocean.
• History suggests that the storm will arrive as it weakens and loses its tropical characteristics, becoming a dangerous deluge-producer at a typically dry time of year.

The bottom line: A high-impact tropical storm-related event is increasingly likely for Southern California and other Southwestern states beginning this weekend, but uncertainties remain over how intense the storm will be at that point and its precise path.

• "This is unlikely to be, at this point, just a nuisance event and could actually produce some major impacts," said Daniel Swain, a climate scientist at UCLA, in a video briefing.