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September 28, 2018 – Cooling trend begins today and accelerates over the weekend. Chance of showers this weekend and early next week.

Discussion

Scattered high clouds ahead of the closed low offshore cover the region early this morning. Surface pressure gradients are trending toward increased onshore flow and the marine layer has increased in depth to 1500-2000 ft with satellite imagery showing a solid area of coastal stratus extending inland through the Bay Area. Winds at Travis AFB have picked up and are presently gusting over 30 kts. Current temperatures are similar to 24 hours ago and range from the 30s in the mountain valleys to the mid 50s to around 70 across the Central Valley and surrounding foothill and Sierra thermal belts.

The cooling trend will initiate today and accelerate Saturday as the closed low begins to edge toward the coast and then lift out to the north over the weekend bringing synoptic cooling and increased onshore flow. Strongest cooling today (10-20 degrees will occur in the Delta and southern Sacramento Valley while areas further inland will see around 5-10 degrees of cooling. On Saturday, the situation will flip as the strongest cooling will occur across the northern half of the forecast area as the low brushes across northwest California. Stratus will likely intrude further inland extending well into the Central Valley.

Additionally, the low will bring a chance of showers and a few thunderstorms across the northern third of the forecast area Saturday afternoon into early Sunday. Uncertainly increases early next week with model differences leading to low confidence in track and timing of next week`s upper low. More likely, the slower solution(s) should be the ones to follow.

Extended Discussion (Tuesday through Friday)

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A Pacific upper-level low will approach interior NorCal early next week, resulting in below normal temperatures, and the possibility of precipitation across the region. However, model spread leads to low confidence in track and precipitation amounts of next week`s upper low. Latest model runs continue to show the low tracking further south and west of the forecast area.

Regardless, best chances of precipitation are expected over higher terrain. The upper system will drift into SoCal on Wednesday, with a few Sierra showers lingering into Wednesday afternoon. Model differences are evident for the second half of next week.

The ECMWF is showing dry conditions and warmer temperatures as ridging over the eastern Pacific builds into NorCal. However, the 00z GFS run indicates another system dropping into NorCal on Thursday, with some precipitation over higher terrain. Have decided to follow the ECMWF solution given better run-to-run consistency.