Most widespread thunderstorm chances today-Monday, then limited to the mountains the rest of the week. Afternoon and evening thunderstorms the next several days. Temperatures will remain below normal then start increasing to near to slightly above normal by mid-week.
Water vapor imagery shows the low is currently along the SoCal coast with plenty of moisture continuing to stream into the region from the east and southeast. Current temperatures are milder compared to 24 hours ago with readings in the 40s and 50s in the mountains, and in the mid 50s to lower 70s in the Central Valley. Much of the region will see a chance for thunderstorms today and tonight as the low moves toward southern Nevada.
Storms will initiate in the Sierra/Cascades, but steering flow will become more out of the northeast or east later today which will allow storms to move down toward the Central Valley by this evening.
The main threats with any thunderstorms that develop will be heavy rain, gusty winds, frequent lightning, and hail. Latest HREF guidance indicates a 25-35% chance of 1 hr rainfall rates exceeding 1 inch over the Sierra up to around the Mt Lassen area from mid-afternoon into the early evening.
Localized flooding and ponding of water will be possible with stronger storms, but storm motion should limit the overall flash flood potential. Forecast soundings indicate an increase in instability and bulk shear compared to Saturday, so some larger hail may occur.
The threat for thunderstorms will continue Monday in similar areas (Coast Range, Sacramento Valley and Sierra/Cascades), but will likely retreat to the Sierra/Cascades by mid-week as ridging from the eastern Pacific attempts to nose into NorCal. This will also result in gradually warmer temperatures.
Extended Discussion (Thursday through Sunday)
A rather persistent pattern continues through the extended period with ensemble guidance and cluster analysis supporting a troughing pattern to continue off the Southern California coast. This will result in continued afternoon and evening thunderstorm potential over the mountains. The best chances will be along the Sierra Crest, mainly south of Interstate-80 (10 to 25 percent probability of thunderstorm development).
Temperatures will trend warmer through the end of the week. The National Blend of Models (NBM) advertises a 40 to 85 percent probability of reaching 90 degrees for the daytime high temperature through Saturday for most of the Valley, excluding the immediate vicinity of the Delta.
Even with the warming temperatures, widespread minor Heat Risk remains over interior NorCal with a just few patches of moderate Heat Risk. This is due to highs only slightly above normal for this time of year.
The increasing potential of a closed low moving inland on Sunday suggests high temperatures gradually trending downward for early next week, though there remains quite a bit of uncertainty with the details at this point.