The high pressure air mass that has been consistently sitting over the Great Basin for the last five days will begin to shift eastward Thursday. With this being said, the heat advisory still in effect for Siskiyou County will expire at 11 p.m. Wednesday evening. The incoming shortwave is going to allow us to catch a break from the heat and humidity currently in place.
This heat wave was unusual for Southern Oregon & Northern California, in the sense of moisture levels. Dewpoints in the 60’s are very rare for this area, and now three consecutive days we have seen them climb into the low and mid 60’s. Tuesday afternoon the dewpoint reached 65!
Even so, it was difficult to thunderstorm development. Isolated thunderstorms developed Tuesday and again that is expected into Wednesday evening. Although we have instability present, strong surface heating and the aforementioned moisture present, a capping inversion (warmer temperatures aloft) will prevent a thunderstorm outbreak. The most susceptible locations for thundershower activity will again be in Siskiyou County and the Klamath Basin. A very slight chance remains for the Valley and mountains.
By Thursday midday and afternoon, a shortwave is going to move in from the West. This is going to bring in a cooler air mass and drop the humidities. The relief will be short-lived however. By Saturday, and more so Sunday, temperatures will be climbing again. They will remain above normal, but not nearly as hot as this past week. Expect afternoon highs to be roughly 10 degrees warmer than average July temperatures. Morning lows, on the other hand, should be close to normal as westerly flow will be in place, filtering in drier air.
With all of this being said, no rain is in the forecast with the exception of the slight chances for a late afternoon sprinkle & thunderstorm. This diminishes and becomes less likely through the remainder of the week.
If you are traveling for the 4th safe travels! Enjoy the holiday!
Meteorologist Alyssa Caroprese