Ontario, CA Weather

Synopsis

Onshore flow will begin to spread cooling inland today with a return of coastal low clouds and fog likely for tonight into Tuesday morning. A low pressure system from the north will bring greater cooling for Wednesday and Thursday along with showers and a chance of thunderstorms, most likely for Wednesday night through Thursday afternoon. Then dry and warmer for next weekend.

Discussion

FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

...UPDATED AVIATION DISCUSSION...

Onshore flow ahead of a low pressure system approaching from the northwest will help to spread the low clouds into the coastal areas tonight. A developing coastal eddy, cyclonic flow aloft and falling heights will cause the marine layer to deepen and the clouds to lift overnight into Tuesday morning.

We can expect a general cooling trend through Thursday as the low pressure system moves into SoCal. Thursday will likely be the coolest day, with temperatures from a few degrees to as much a 15 degrees below normal. Ahead of the system, moisture from the low latitudes will likely be drawn into socal with the flow aloft. The system will use this moisture to produce light precipitation as early as Wednesday morning. Precipitation will become more widespread, progressing from northwest to southeast across the region and increasing in intensity on Thursday as the system moves through Socal. Conditions become most unstable on Thursday as colder air moves in aloft from the northwest bringing a chance for thunderstorms for Thursday morning through Thursday afternoon. Precipitation will taper off by Friday morning as the upper level trough axis moves east and conditions become more stable.

For Orange and southwestern San Bernardino Counties, rainfall could range from around 1 inch for the Orange County coastal plain to around 1.5 inches on the coastal slopes of the mountains with amounts decreasing from north to south into San Diego County. For the 24 hr period ending at 5 pm Thursday, there is a 35 to 65 percent chance of greater than 1 inch of rainfall in San Diego County from the coast to the mountains. There is about a 30 to 45 percent chance of 1 inch or more in the Inland Empire and about a 45 to 65 percent chance in Orange County, the Santa Ana Mountains and on the slopes of the San Bernardino Mountains.

The snow level will drop to around 8,000 ft on Thursday afternoon and the highest peaks of the San Bernardino and San Jacinto mountains could get small amounts of snow.

Aviation

200530Z...Coast...Gradually increasing low clouds based around 1000- 1200 feet MSL overnight. Vis reduced 3-5SM on mesas. Cigs lifting some before scatter out 16-18Z Tuesday. Low clouds based around 1500- 2000 feet MSL to redevelop after 03Z Tuesday evening.

Otherwise...mostly clear and VFR conditions through Tuesday morning, until SCT-BKN clouds based around 10 kft develop over inland areas, especially mountains 19-01Z. Isold SHRA 21-01Z Tuesday.

Marine

No hazardous marine conditions are expected through Wednesday morning. A storm system moves into the region later Wednesday through Thursday, bringing gusts at or exceeding 20 knots and associated choppy seas, along with a 10-20% chance of TSRA.