July 15, 2020 – Dry weather with hot temperatures over the northern half of the Sacramento Valley today, while Delta influenced areas see a break from triple digit heat. Not quite as hot for the remainder of the week, then hotter weather returns over the weekend. Isolated thunderstorms will be possible near the Sierra crest the next several days.
Clear skies cover interior NorCal early this morning. Satellite imagery indicates extensive stratus along the coast south of Cape Mendocino, and profiler data indicate its depth is similar to yesterday. A robust Delta Breeze continues with gusts over 30 mph at Travis AFB. Current temperatures are running very similar (within 3 degrees) across most of the area to readings from 24 hours ago.
Heights aloft and thickness will increase a bit today across the far northern portion of the state while remaining relatively unchanged further south as a weak remnant low attempts to close off along the central California coast. The result will likely be the hottest temperatures of the week across the northern half of the Sacramento Valley (the Redding area will see highs around 110 degrees which will be short of the daily record of 116 from 1972) and across higher terrain, while continued marine influence will moderate temperatures further south.
Overall airmass moisture has been increasing across the area since last weekend when a very dry airmass was in place. Daytime heating may be enough to pop a few late day storms over the high Sierra later today to the south of I-80. Activity will likely be capped further to the north by warmer mid-level temperatures.
Temperatures are forecast to cool a bit on Thursday into Friday as a short-wave trough passes to the north through the PacNW. This will bump heights aloft down slightly across NorCal and help to induce stronger onshore flow. Late day deep convection may spread further north to around the Mt. Lassen area on Thursday as the cap weakens.
The large strong high pressure area over the southern CONUS is forecast to build westward beginning this weekend which will result in another warming trend across the region as onshore flow diminishes. Main threat for diurnal deep convection will be over the Sierra Nevada south of Lake Tahoe.
Extended Discussion (Sunday through Wednesday)
Ensembles continue to indicate the monsoon high shifting slightly westward into California this weekend and into early next week. At this point, models suggest there could be enough moisture and instability for a slight chance of high elevation Sierra thunderstorms every afternoon & evening in the extended period.
Otherwise, this high pressure will keep conditions hot and dry through midweek. Valley highs will remain in the upper 90s to 106 with highs in the 70s to 90s in the mountains. The strong Delta Breeze will be dampened, though not completely.
Weaker onshore flow will prevent temperatures from rising substantially under the high pressure. This will also keep overnight temperatures palatable in the low to mid 60s for much of the Valley except for the northern Sacramento Valley and thermal belts where lows will only fall to the upper 60s to low 70s.