A "super" El Niño could bring heavy rain to Hillsboro in December and January, according to a winter outlook from KGW meteorologist Rod Hill published Oct. 27.

Hill wrote that ocean water temperatures off the coast of South America measure 3 degrees Celsius above the seasonal average. Weather models show the running average staying 2.5 degrees above normal for the rest of the year, crossing the threshold for a "super" El Niño. Hill defines that as anything above a 2-degree increase.

The rainy season begins Sunday, Nov. 1. Hill's confidence in the precipitation forecast is low, but historical data favors a somewhat dry November followed by a very wet December and January.

According to Hill's outlook, atmospheric rivers will most likely target California. But one or two streams of steady, heavy rainfall could shift north into Oregon, delivering 8 to 12 inches of precipitation during December and increasing the potential for flooding.

The ocean is expected to cool in the coming months. By January, the El Niño could weaken to moderate strength for the second half of the rainy season, according to Hill.

Temperature forecast carries more certainty

Hill wrote that each month of the rainy season will likely average 2 to 3 degrees Fahrenheit above normal, and he expressed high confidence in that projection. NOAA's U.S. Winter Outlook, released Oct. 15, also projects warmer-than-average temperatures for the Pacific Northwest from December through February.

Ken Graham, director of NOAA's National Weather Service, said a very strong El Niño will likely shift the jet stream southward and eastward. Hill wrote that El Niño seasons tend to favor sleet or freezing rain over snow in Portland, with strong south winds bringing an estimated three or four wind warnings in the valley.

Emergency preparedness in Hillsboro

The city's standing emergency guidance recommends residents keep enough water to sustain their household for 14 days, at one gallon per person per day. That guidance was developed for earthquake preparedness.

Oregon's Office of Emergency Management recommends checking home insulation, having heating systems serviced and clearing gutters before winter storms arrive.

NOAA plans to update its three-month outlook Nov. 19.