The Southwest Monsoon Sets Off Weekend Thunderstorms

A 500 mb high around the Four Corners is expanding west and is bringing increasing moisture northwest into Arizona, California and parts of Nevada for the long holiday weekend. This is the beginnings of the summer monsoon pattern that typically starts around the first week in July.

The last couple of days already have seen afternoon thunderstorms over the higher terrain of Arizona. But with moisture levels rising I expect lower elevations will start to get into the act too.

Here is the progression of the precipitable water (how much total water vapor that is in the air) increases over time through Saturday.

Thursday Morning:

Friday Morning:

Saturday Evening:

The yellow and reds are higher levels of moisture, the blues lower, with the scale on the right giving amounts in inches. The more vapor available the more the daytime heating can turn that moisture into storms.

As the moisture increases so too will be the coverage of thunderstorms. Most will still be over Arizona this evening. Tomorrow we will see a few develop over southern California, mainly the San Diego and San Bernardino County mountains and maybe even one or two over the Lower Deserts. Saturday will see a big up tick in the amount of thunderstorms with a few likely anywhere over the higher terrain from Arizona to Southern California and the southern Sierra, including the high deserts. Also expect widely scattered storms in the Lower Deserts. Any time there are thunderstorms in the deserts there is always the chance of a blowing dust storm. Sunday I expect about the same kind of coverage as Saturday except with an increase in coverage farther north in the Sierra up to around Tahoe. Also noticeable on Saturday will probably be a big increase in the low level humidity, dew points, in the lower deserts. Its entirely possible that dew points rise well into the 60s in portions of southeast California and southwest Arizona. This would hold down the actual temperature some but of course with much higher humidity levels it will feel hotter and more uncomfortable.

A wild card in all of this is the potential for something west of the mountains late Friday night or Saturday. Some of our computer models have been forecasting a weak upper level disturbance to move northwest out of Mexico. If this is real then it could be a focal point where not only there could be a shower or thunderstorm no matter what time of day but maybe something survives west of the mountains too inot the populated areas.