July 16, 2020 – Not as hot today and Friday, then hotter weather returns for the weekend. A few late day thunderstorms are expected along the Sierra crest the next several days. Keep an eye to the sky if in the mountains.
Discussion
Clear skies cover the region early this morning. Remnant clouds from Wednesday afternoon’s thunderstorms over the northern Sierra Nevada is moving east across Nevada while IR difference imagery is showing extensive stratus along the coast extending well inland, and now spreading through the Carquinez Strait. Coastal profiler data indicate the marine layer has deepened to around 2k ft, and theres a chance a little stratus will make it inland into the Sacramento region around sunrise.
Stronger short-wave trough currently approaching the PacNW will begin to lower heights aloft, and thickness, today across NorCal leading to a few degrees of synoptic cooling. The feature will also induce stronger onshore flow by this afternoon which will result in several more degrees of cooling to portions of the Central Valley and foothills. Onshore flow will continue on Friday with a few more degrees of cooling expected across the area.
Another round of isolated late day deep convection is expected across the northern Sierra today, possibly extending about as far north as the Mt. Lassen area. Weak westerly flow aloft will likely keep storms near and east of the Sierra crest. Storms are forecast to be retreat to areas mainly south of Lake Tahoe on Friday.
Hotter weather returns over the weekend as high pressure builds northwestward from the Desert Southwest. This will likely suppress the marine layer, but not completely wipe it out. Most of the Central Valley will see highs near or above the century mark, especially the northern Sacramento Valley where moderate heat risk will return. Isolated late day deep convection will likely be limited to the far southeast corner of the forecast area over the Sierra Nevada to the south of Lake Tahoe.
Extended Discussion (Monday through Thursday)
Current ensemble runs indicate the Four Corners high pressure will shift east out of California Monday and Tuesday as a weak upper level low approaches from the eastern Pacific. Temperatures will begin the week hot, especially in the northern Sacramento Valley, before slowly cooling each day to near or slightly below normal by Wednesday as the upper low makes its way onshore. The chance of high elevation Sierra thunderstorms is weakening as guidance moves the monsoon high further to the east than previous runs. However, current models indicate there may still be enough moisture and instability for isolated thunderstorms Monday and Tuesday afternoon.