A rainy week is ahead for the Santa Clarita Valley as significant rain and mountain snow are forecast for Central and Southern California on Monday, with lingering showers possible through Thursday, the National Weather Service reported Sunday afternoon.

The system will bring a late-season February-like storm to the Santa Clarita Valley over the next few days.

There was a slight chance of thunderstorms Sunday night through Tuesday including across the coastal waters and some snow will fall on the major passes including the Grapevine by Tuesday morning.

There is the potential for many areas to receive rainfall far exceeding what we normally see for the entire month of April, including downtown L.A. where the normal April rainfall is 0.91 inch.

After more rain through Thursday, dry and warmer weather can then be expected Friday through Sunday.

A moist onshore flow out ahead of a Pacific storm system kept plenty of rain clouds and showers over portions of the forecast area into early Sunday afternoon.

A more organized band of rain was moving through San Luis Obispo and Santa Barbara Counties as well on Sunday.

It was increasingly rainy across those counties through Sunday afternoon, with additional showers expanding across Ventura and L.A. Counties toward evening.

Snow levels continued to be 6,000 feet through the afternoon with some light snow accumulation possible.

The region remains on track for significant and unseasonable amounts of precipitation over the next few days. This should help to go a long way to bringing seasonal rainfall totals for many areas closer to normal or possibly even above normal in some locations.

An unseasonably strong upper-level low-pressure system was centered just off the Northern California coast early Sunday afternoon. This upper-level low is forecast to very slowly move south to a position off the central coast on Monday, to just southwest of Point Conception on Tuesday, remain nearly stationary just south of the Channel Islands Tuesday night, then move east into far Southern California on Wednesday.

The focus of the heavier precipitation will move into L.A. County late Sunday night and Monday as the front edges head east, although additional showers will affect much of the rest of the forecast area.

Strong and gusty south to southeast winds can also be expected along and ahead of the front, bringing enhanced orographic precipitation to the south-facing foothills and mountains. Wraparound rainy conditions will bring additional showers across the region Monday night through Wednesday as the upper level low lingers nearby.

Additionally, there will be enough instability and upper-level diffluence to warrant a mention of a slight chance of thunderstorms for many areas tonight through Tuesday night, except for Monday night into Tuesday morning as the instability is marginal at that time.

Any thunderstorms that develop would be capable of producing heavy precipitation, gusty winds, small hail and dangerous cloud-to-ground lightning.

Rainfall totals through Tuesday are expected to generally range from 1.00-2.00 inches for the coastal/valley areas with 1.50-3.00 inches in the foothills and mountains. Local rain totals to 4.00 inches or more will be possible in some mountain areas. Rainfall rates of 0.25-0.50 inches/hour are expected with local rates up to 0.75 inches per hour. With any thunderstorm activity, hourly rainfall rates could even be higher.

With these rainfall totals, there will likely be widespread nuisance urban flooding as well as the potential for minor mud and debris flows near the recent burn areas.

Additional rainfall Tuesday night and Wednesday should be around 0.10 to 0.33 inch or so for the coast and valleys, and 0.25 to 0.75 inch in the mountains, except possibly up to 1.25 inches in the eastern San Gabriel Mountains.

Winter Storm Warning & Possible Grapevine Closure
Snow levels will continue near 6,000 feet through Monday morning then drop to 5,000-5,500 feet Monday afternoon, and to 4,000-4,500 feet Monday night and Tuesday morning. There will likely be significant snow accumulations, especially above 6,000 feet.

With the heavy snow and strong gusty winds expected, a WINTER STORM WARNING is in effect for the Ventura/LA County mountains from this evening through Tuesday afternoon. Wind gusts up to 45 to 55 mph are expected in the high mountains Sunday night and Mon. Snow accumulations of 15 to 30 inches or more will be possible above 6,000 feet, and about 4 to 12
inches between 4,500 and 6,000 feet.

In addition, lower snow levels late Monday night into Tue morning will likely result in travel delays along Interstate 5 over the Grapevine.

More snow is expected above 5,000 to 5,500 feet Tuesday night and Wednesday, with 3 to 6 inches or more possible especially above 5,500 feet. This should prolong wintry travel conditions in the higher mountain elevations through mid-week.

Needless to say, temperatures will remain significantly below normal in most areas Monday through Wednesday. Highs for the coast and valleys should reach only into the mid-50s to lower 60s.

Break for the Weekend
After a rainy week, dry and warmer weather is expected Friday through Sunday as some mainly flat upper-level ridging moves into the area. Temps will continue to be a few degrees below normal Thursday, then warm to slightly below normal for many areas Friday, and near normal to slightly above normal for Saturday and Sunday.