Moisture has increased in portions of the Southwest over the past 24 hours. This is in response to an easterly wave moving across northern Mexico. The water vapor satellite picture from AccuWeather.com Pro site shows the moisture (in green) and the easterly wave (over the northeast Baja)
This easterly wave will progress west slowly tonight and tomorrow taking moisture with it into southern California. The increase in moisture is mainly in the middle layers of the atmosphere and will produce some clouds as it moves west. It will also be responsible for scattered showers and thunderstorms. The more widespread activity will be over the higher elevations of Arizona and Utah, but spotty thunderstorms can occur in the lower deserts as well. This will be mostly tonight and tomorrow in Arizona and mostly tomorrow and tomorrow evening in southeastern California. There also can be an afternoon thunderstorm in portions of the mountains of Southern California tomorrow and maybe a stray one in the deserts.
Moisture will start to be forced back to the east on Wednesday. However, it will not be quick enough to rule out another afternoon thunderstorm over portions of the mountains and deserts of Southern California. A shower and thunderstorm will be scattered around Arizona Wednesday just like tomorrow. By Thursday, expect more a southwest flow and Southern California and even western Arizona should dry out some more.
As of the end of June there had been no named storms in the Eastern Pacific basin.
This is some serious and dangerous heat. Outdoor activity is just not at all recommended during the daytime.
A strong ridge of high pressure in the West brings the highest heat of the season so far to a large area.
Combine the cold with the wind and some precipitation and there is a real danger of hypothermia.
Any shower and thunderstorm can contain heavy downpours, heavy enough to cause temporary, low-lying ponding.
According to all long-range models, the warmest area in North America compared to average will be over the Northwest.