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May 29, 2020 – A cooling trend begins today and continues into the weekend as a low pressure system moves through. This will bring widespread showers and thunderstorms tonight through Saturday, with the potential for heavy rain over northwestern Shasta County. Chances for some mountain showers and thunderstorms continues into early next week.

Discussion

The heat wave is ending as upper level ridging shifts eastward and onshore flow develops with a low pressure system off the California coast. Mainly cooler temperatures are evident across much of the area this morning. The biggest change has been for the Sacramento metro area, where temperatures are 10 degrees cooler than at 3 am yesterday. This is due to a decent marine layer and a moderate Delta Breeze, which at Travis AFB was gusting out of the southwest at 32 mph. For the Sacramento area, the 4 day consecutive stretch of triple digit temperatures ended Thursday, with highs forecast only into the mid 80s for this afternoon. The 4 day stretch of 100’s was the first on record for downtown Sacramento in the month of May. Triple digit temperatures were evident across the Valley. The northern Sacramento Valley hasn’t seen as much cooling yet this morning, but should be a little less hot today, with highs in mid to upper 90s.

The main weather issue shifts to showers and thunderstorms this weekend, beginning this evening over the mountains and becoming widespread tonight as the low moves inland. This is a wet system, with Precipitable Water more than double normal levels, around 1.2 inches. Abundant atmospheric moisture plus dynamic lift and steep lapse rates will combine to bring heavy rain showers over portions of NorCal, especially northwestern Shasta County. CAMs models show a band of heavy showers and thunderstorms moving northward through that area around 5 am to 11 am. This could bring localized flooding, as well as possible burn scar issues.

Scattered afternoon thunderstorms develop across the area in the wake of this band as the core of the low moves over Northern California. Surface CAPE levels around 200-500 j/kg are expected. Low level shear doesn`t look strong, with heavy rain the main threat. A convergence band sets up over Shasta County, bringing more heavy rain. A general storm total of around 1 to 2 inches is forecast, locally higher possible.

The shower and thunderstorm threat will diminish by late Saturday afternoon and evening, with a residual chance of showers and storms into Sunday over the Northern mountains and northern Sierra with the approach of a Pacific trough. This will continue onshore flow and cloud cover over the area on Sunday, with some lingering showers and afternoon thunderstorms limited to Shasta County, dry elsewhere. The trough will bring more widespread mountain showers and afternoon thunderstorms on Monday.

Temperatures on Saturday will be relatively cool, 7 to 15 degrees below normal levels. Valley and Delta highs will be in the 70s, into the mid 60s for the foothills. Sunday and Monday will see Valley highs a little warmer, into the upper 70s and lower 80s.

Extended Discussion (Tuesday through Friday)

A rather pronounced subtropical to mid-latitude blocking pattern unfolds next week which will stagnate features on the map. Most notably, an upper low pinches off in the vicinity of far northern Baja California. This system is forecast to stall through at least mid-week before being nudged eastward from broad upstream troughing. Depending on how far north this circulation center stays, embedded shortwaves may provide an impetus for scattered convection over the foothills and mountains each day. These should generally follow the diurnal heating cycle which favors these showers and isolated thunderstorms during the afternoon to early evening hours.

By late next week, broad troughing fills out the eastern Pacific with a gradual lowering of heights along the West Coast by next Friday/Saturday as shown by the well clustered ensemble means. Regarding temperatures, a gradual warm up is anticipated as a Four Corners ridge attempts to build heights north of the Baja California low. The 00Z ECMWF is decidedly warmer than other solutions which would support readings well into the 90s. For now, will continue to play the pattern conservatively with highs in the upper 80s to low 90s before cooling down by next Friday given the approach of a trough.