The cold, closed low I have been talking about for almost a week now is on schedule for Southern California and then into the Southwest later tonight and tomorrow. The low this afternoon was just off the coast of San Luis Obispo by mid-afternoon Thursday and will go south of Pt. Conception tonight, turn east tomorrow and move across the LA Basin tomorrow afternoon then to near the Colorado River by Friday morning. There will be two distinct maximum areas of rainfall with this storm. The first will be over Southern California especially from the mountains on west and in the Upper Deserts and the second in Arizona north into Nevada and Utah. Thunderstorms will be locally heavy, and there is the potential for locally severe weather in the form of strong, gusty winds, heavy downpours and hail. There is even the slight chance of a waterspout in the coastal waters of Southern California and a funnel cloud or weak tornado over land.
Precipitation will end tomorrow night in Southern California except near the Colorado River where a shower or thundershower could linger in spots late tomorrow night and to start on Friday. Precipitation in Arizona, southern Nevada and Utah continue on Friday with the heaviest activity in the morning hours.
This one storm is the only one the area will see for some time to come. Longer-range computer models show a big ridge building in California and the Southwest over the weekend and next week and temperatures rise back to above-normal levels.
These is the possibility that another surge of tropical moisture could move into Arizona and Southern California by the middle of next week.
I still believe the greatest amount of rain, at least partially relating to Norbert, occurs from southeast California to Arizona
The effects on Southern California are very much in question.
With low wind shear and warm water in its path for a while longer, it is likely that Norbert intensifies into at least a Category 2 storm.
For much of the last week the monsoon moisture flow into the Southwest has been completely shut down by dry westerly winds. This is about to change.
A big pattern change is underway, one that will completely shut down any chance of showers and thunderstorms for an extended time period.