Dramatic weather changes are likely across all the West the rest of this week, through the weekend and well into next week. An unseasonably cool weather pattern now exists from the Northwest to the Southwest. Unusual rain has fallen in north-central California. It's only been in the 50s and 60s in the interior Northwest. The usual toasty deserts of the Southwest have been rather moderate with highs only near 100. All of this vanishes within a few days.
The culprit will be an upper-level ridge now over West Texas that increases in intensity and moves west. By Friday and Saturday, this high will be centered over the Arizona/Nevada/California borders. It should remain in place Sunday into Monday before moving even more to the north next week.
This ridge will set the mercury off to much higher readings with some of the hottest weather of the season likely by Friday or Saturday in the Southwest (away from the coast) and record highs will be challenged. Temperatures by the weekend will be a good 30 degrees higher than today east of the Cascades in the Northwest, well up into the 90s. It then could be even a little hotter early next week.
Initially, the high is too far south to allow any important monsoon moisture to move in. However, if our models are right with the center moving farther north next week, the door does open for at least some monsoon moisture from Arizona to Southern California and up the Sierra by early next week. It is awfully hard to tell how much right now, but there is at least the chance of scattered thunderstorms developing, especially over the higher terrain.
While it will get blistering hot for the coastal valleys, mountains and deserts and all the interior West, coastal sections will not see nearly as much heat. Without any important offshore pressure gradient showing up a marine layer, be it shallow, will still exist. This will keep the immediate coastal areas much more comfortable than farther inland. Of course, this is going to make forecasting high temperatures for some places very difficult. Some of those areas will be the areas between the 5 and 15 freeways in Southern California, especially San Diego County and up the inland coastal plain in Orange and Los Angeles counties. Also around the Bay Area, the East Bay to Napa Valley temperatures could be 5-10 degrees higher or lower with very minute changes in the weather. It's going to get very warm in these places, no doubt. But it's the difference between upper 80s to lower 90s or upper 90s to near 100 with the biggest chance for huge ranges around the East Bay and Napa Valley.
Here are some of my forecast high temperatures for Friday through the weekend. For a forecast for your particular city, just put your zip code in AccuWeather.com to get a detailed forecast for you.
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.