Tropical Storm Vance sits about 315 miles off the southwestern coast of Mexico, crawling at 7 mph with maximum sustained winds of 70 mph, according to AOL.

Beginning Thursday, the storm's deep tropical moisture will interact with a developing frontal system, setting the stage for heavy rain and flash flooding in Texas before spreading into a much broader area of the country.

The unsettled pattern threatens to disrupt Halloween celebrations and outdoor plans for millions of Americans across the central and eastern United States.

According to the FOX Forecast Center, cited by AOL, Texas faces the greatest flooding threat, with repeated rounds of showers and thunderstorms capable of producing widespread rainfall. Some areas may see 2 to 3 inches of rain, with higher amounts of 4 to 5 inches possible.

A Level 2 out of 4 flash flood threat has been issued, meaning flooding is very likely in some areas — a notable escalation for a holiday weekend when families are outdoors.

By Friday and Saturday, the threat of rain will expand across the Plains, Mississippi and Ohio Valleys, Great Lakes and Southeast, AOL reports.

As an upper-level trough moves across the U.S. by Sunday, rain and gusty winds could spread from the Mid-Atlantic to the Northeast, delivering a wet and unsettled end to the holiday weekend.

The areas most affected will depend on how the front and the tropical moisture interact, but Texas remains the primary focus early in the event.

Vance is not the only tropical system loading the atmosphere with moisture.

In the Central Pacific, Tropical Storm Olana — formerly Tropical Depression Three-C — is drawing deep tropical moisture across Hawaii from Tuesday through Thursday, with 5 to 8 inches of rain possible over two to three days, FOX Weather reports.

The heaviest downpours are expected on south- and west-facing slopes, and flash flooding raises the risk of mudslides in high-elevation areas. FOX Weather adds that another storm system could develop west of Hawaii for Halloween weekend.

Both forecasters point to the same backdrop: a warm El Niño Pacific that keeps spinning up tropical systems.

ABC7 Los Angeles notes that with El Niño's warm water, tropical storms like Marie have formed repeatedly in the Pacific this season, some close enough to threaten the Southern California coast with rain, gusty winds and high surf.

That persistent supply of tropical moisture is what allows a storm like Vance, far offshore, to push flood-producing rain deep into the interior of the country just as Halloween arrives.