September 9, 2017
Summary: Heavy thunderstorms are pounding portions of San Bernardino and Riverside Counties this afternoon as monsoonal moisture continues to be injected into the region. In the meantime, we have a cut off low off the central coast which is drifting southward and has already turned the winds aloft to out of the southeast over northern California. As the low continues to drift further south, those southeast winds aloft will become apparent over southern and central California. One model this afternoon is quite interesting. It shows a significant swath of moisture moving over the top of the low, just brushing the southwest portion of the valley and from the Coast Range to the coast. How this will all play out is still up in the air, literally, as cut off lows are notoriously difficult to forecast around.
The best chance of showers and thunderstorms over the valley floor will be Sunday afternoon through Monday as the east/southeast flow aloft will form showers and thunderstorms over the Kern County mountains and the Sierra Nevada. With the off shore low, it’s quite possible storms could sustain themselves even over the valley floor. Again, the best chance will be Sunday afternoon through Monday.
Interestingly enough, models indicate the low will move northward again Tuesday, moving inland through central California Wednesday. This presents a forecast challenge for Tuesday and Wednesday, as well, as the low will be directly overhead. It’s possible, especially Wednesday, that moisture will be wrapped around the northern flank of the low for a chance of showers and thunderstorms from mainly Fresno County northward.
By Thursday, the low will have shifted eastward as a westerly flow aloft develops. Also, a strong on shore flow will set up, moderating temperatures downward possibly as low as the 90 degree mark, as early as Wednesday.
Medium range models are showing just a weak trough along the west coast for near to slightly below average temperatures and dry conditions.
Forecast: Mostly clear tonight. Increasing cloudiness Sunday with a slight chance of showers and isolated thunderstorms Sunday morning but a greater risk Sunday afternoon through Monday, especially in the south valley. A slight risk of afternoon showers and thunderstorms Monday afternoon and Tuesday with a slight chance of isolated showers and thunderstorms Wednesday, mainly from Fresno northward. Becoming mostly clear Wednesday night with continuing mostly clear skies through Saturday.
Temperatures: Lows tonight will range in the mid 60s to the lower 70s. highs Sunday will generally range in the mid to upper 90s. Lows Sunday night will cool into the upper 60s to the lower 70s. Highs Monday will warm into the mid 90s to near 100.