Millcreek trick-or-treaters may need umbrellas with their costumes Saturday, Oct. 31, as two storm systems move toward northern Utah over Halloween weekend.

The National Weather Service in Salt Lake City said in an Oct. 28 forecast discussion that an influx of tropical moisture will bring scattered showers and thunderstorms on Friday, Oct. 30, followed by a stronger system late Saturday through Monday, Nov. 2, producing widespread rain across the region.

The second storm is the bigger concern. The NWS said the Wasatch Front is favored for higher rainfall totals. At the 25th percentile of model estimates, the agency projects a half-inch to an inch. At the 75th percentile, totals reach 1.5 to 2.5 inches. The agency attributed the heavier moisture to a strong atmospheric river feeding into the system.

For Halloween night specifically, The Salt Lake Tribune's Andy Larsen reported that current modeling shows scattered rain during peak trick-or-treating hours, with a large spike in precipitation much later in the evening. The forecast temperature for Halloween night is 54 degrees.

That's cold enough for a jacket but well above freezing. Temperatures along the Wasatch Front will stay above freezing, according to the Tribune, so snow is not a valley-level concern.

Snow in the mountains, cold air behind the front

A cold front arriving late Sunday, Nov. 1, will drop snow levels to between 6,500 and 7,000 feet by early Monday morning, the NWS said. The agency's forecast calls for 3 to 6 inches of snow above 8,000 feet and 8 to 14 inches above 9,000 feet.

Precipitation should taper off through Monday morning, with low temperatures early next week among the coolest of the season so far.

Flooding risk and Millcreek's flood-prone areas

The NWS noted an increased flash flood threat on Friday, mainly across southwest and west-central Utah, where instability will be highest. The agency also flagged a "considerable forecast bust scenario," noting about 33% of models suggest the northern stream trough could remain decoupled from the tropical low-pressure system. That scenario would essentially eliminate the flash flood risk on Friday, the NWS said.

For Millcreek residents near Neff's Creek in the Olympus Cove area, the storm is worth watching. According to Millcreek's floodplain information page, the Utah Division of Emergency Management (DEM), the Federal Emergency Management Agency (FEMA), Millcreek and Salt Lake County have been working to update flood maps for Neff's Creek, which is designated as an active alluvial fan prone to sudden, unpredictable flood events.

Zone A areas along the creek carry the highest flood risk, while Zone AO areas carry moderate risk. Properties in both zones face federal flood insurance requirements. Millcreek participates in the National Flood Insurance Program (NFIP), and flood policies carry a 30-day waiting period before taking effect.

Forecast uncertainty remains

The NWS cautioned that models still show notable spread in potential rainfall amounts, and hour-by-hour forecasts several days out carry limited reliability. The Tribune reported that forecasters could not yet say with certainty whether it would be raining during peak trick-or-treating time Saturday.

Historically, about 24% of Halloweens in Salt Lake City have had measurable precipitation, according to the Tribune's analysis. Snow on Halloween is rare: it has measurably snowed on Oct. 31 in Salt Lake City only five times in the past century.

No storm-specific guidance from Millcreek or Salt Lake County emergency management had been issued as of Oct. 28. Residents in Neff's Creek flood zones can find floodplain maps and insurance information on Millcreek's floodplain page, linked above.