Flood Danger Moves North, But San Diego Isn’t Out of the Crosshairs

Update: The National Weather Service has issued a flood watch for all of San Diego County, from late Sunday through Wednesday morning. The projected crest of the San Diego River at Fashion Valley has been raised slightly to 10.6 feet early Tuesday.

San Diego County residents can count on another very wet stretch next week. Just how wet and precisely when — and whether more flooding is in store — has yet to be determined.

Forecasters in the National Weather Service’s Oxnard office, which covers Los Angeles, Ventura, Santa Barbara, and San Luis Obispo counties, are more certain. They say there is no doubt that the region is at high risk of damaging flooding from Sunday morning through Monday evening. 

The storm carries the potential in that area for 6 inches of rain at lower elevations by Tuesday or Wednesday. A foot of rain is possible in the mountains and foothills.

“Confidence is very high that the amount of water that will be hitting the ground will generate significant, widespread and possibly life-threatening hydrologic issues,” the office’s forecast states.

In San Diego County, the prospects appear less ominous but also less clear. Weather Service forecaster Miguel Miller said that by Thursday, San Diego could get another 2.5 inches, and Palomar Mountain could record 6 inches. 

But the odd nature of the storm, plus inconsistent computer model projections, have left local forecasters with more than the usual amount of uncertainty about timing and intensity of the approaching storm, Miller said. 

Forecasters often talk about atmospheric rivers or “pineapple express” storms that tap into the subtropics, or the region that starts 23 degrees north of the equator. Such storms have great potential because they transport long plumes of moisture. 

The approaching storm has a moisture source that extends even farther south — all the way into the tropics, or the area around the equator, Miller said. That’s rare for a storm that reaches Southern California.

“The reason we’re concerned about heavy rain rates (that can cause flooding) is we’re going to have a very saturated atmosphere,” Miller said.

But a super moist atmosphere alone is not enough to produce rain. The saturated plume needs some kind of atmospheric trigger to wring out the moisture.

 “It won’t take much of a trigger to bring heavy rain,” Miller said. “But there’s not really something you can hang your hat on. This is such a warm system. There’s not really a front or some organized feature we can point to and say, ‘here it comes.’ ”

The lack of an easily identifiable element that will induce the rain has probably contributed to the waffling of the forecast models, which have been predicting a wide range of potential outcomes locally, in terms of timing, intensity and location.

As of Friday afternoon, the models indicate rain might start in San Diego County Sunday, and possibly continue, off and on, into Thursday or even Friday.

“Tell people to be prepared for a lot of rain on Monday and Tuesday, but don’t be surprised if you get a little on Sunday,” Miller said.

Heavy rain appears more likely in North County than San Diego, Miller said, although no flood watches or warnings have been issued yet for the region.

Soils in and around San Diego, which has recorded more than 3.6 inches of rain in the last 11 days, are saturated, leaving the city prone to added runoff from the approaching storm.

The San Diego River is currently projected to crest in Fashion Valley at 10.6 feet Tuesday, considered the minor flood stage. The river reached 11.25 feet on Jan. 22, when some parts of San Diego and South Bay suffered significant flood damage. The 11.25-feet crest came after three hours of unusually intense rate rates.

Miller said flood levels on the river are notoriously hard to predict.

Before the Jan. 22 storm, the projected crest was way too low, and the predicted peak before Thursday’s storm was way too high.

“It’s a tough one,” he said. “It acts like a flash flood ravine. When they apply the normal river rules of thumb, they can underestimate how flashy it is.”