Sunday showers to be eclipsed by Tuesday downpour

Moderate rainfall expected to soak the North Bay starting early Sunday afternoon will be a mere warm-up act for the main event. Following a mostly dry Monday, the region is going to get clobbered by what’s shaping up to be the biggest storm of the season, according to the National Weather Service.

Sunday’s system, described by weather service meteorologist Brayden Murdock as “a cold front trying to pass through,” will bring rainfall ranging from a tenth to a quarter of an inch, with heavier precipitation along coastal areas.

Much heavier precipitation is expected Tuesday night, when a massive plume of water vapor, a so-called atmospheric river, will push into the Bay Area. Up to 6 inches of rain is expected at higher elevations between Tuesday and Thursday night, said Murdock.

While Santa Rosa will not catch the brunt of this front, the city can expect 3 to 4 inches, with some of the heaviest rain falling Wednesday morning, he said.

The moisture is badly needed: Santa Rosa has logged less than 6 inches of rain since Oct. 1, almost a foot less than its average, by this time of the official rain year. While cattle ranchers, dairy operators and grape growers will cheer the atmospheric river, it also brings a measure of peril.

The upcoming deluge will create a risk of flooding, particularly on burn scars denuded of vegetation by last year’s wildfires. The National Weather Service office based in Monterey has calculated “thresholds” for those burn scars. If rain exceeds 0.4 inch within 15 minutes, 0.6 inch within 30 minutes or 1 inch inside an hour – “if we’re exceeding those rainfall rates in burn scar areas, that’s when we’re very concerned,” Murdock said.

“Thursday night looks like when things will start to subside,” he added, although there could be lingering showers into Friday.