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May 6, 2020 – Dry and warm through this week. High temperatures warm to around 15 to 20 degrees above average by Thursday into the weekend with some locally gusty north to easterly wind late in the week. Cooler weather returns next week with widespread showers.
An upper level shortwave passing through to north is flattening out the upper level ridge over the region. Dry weather will continue, though, with any precipitation limited to the Pacific Northwest. Afternoon high temperatures today will be maybe a degree or so cooler than on Tuesday.
A moderate Delta breeze overnight has diminished as the San Francisco to Sacramento surface pressure gradient has weakened. The north to south gradient across the area has strenthened and was 5.6 mb between Redding and Sacramento at 3 am. By late this afternoon, the gradient is projected to be around 10 mb. Northerly winds across the Valley in the afternoon will be breezy, gusting to 20 to 30 mph. These winds will continue into the evening then gradually decrease overnight for areas east of I-5. Locations to the west will remain breezy into Thursday, with northeasterly winds developing in the mountains. Winds will decrease by late Thursday.
Afternoon humidity levels will be quite low with the northerly winds, dropping into the teens over the next few days. This will likley increase curing of grasses in the Valley.
A strong buildup of ridging will occur Thursday in response to an upper low centered over the Gulf of Alaska. Temperatures will rise to well above normal levels due to strong subsidence, with Valley highs in the low 90s. The ridge builds further Friday with upper 90s widesprad across the Valley and Delta on Friday.
The current forecast has increased Friday highs a bit, with a few locations in the central and northern Sacramento Valley reaching around 100 degrees. Some records are likely to fall, especially for Redding, Red Bluff, and Sacramento Executive Airport. The heat risk will run in the moderate range both days, especially on Friday. Outdoor workers should prepare for hot conditions and stay hydrated during this period of well above average heat.
Saturday will see some cooling as the upper low drops southward and the ridge shifts eastward. This could also bring potential additional cooling from a Delta breeze. High temperatures should generally be about 2 to 5 degrees lower than the peak on Friday, perhaps as much as 5 to 8 degrees cooler in Delta influenced areas.
Extended Discussion (Sunday through Wednesday)
A deep upper level trough will push high pressure eastwards this weekend. This will bring some much needed relief to the region as onshore flow lowers temperatures. Sunday, highs look to be 5 to 10 degrees lower than Saturday, with the decreasing trend continuing into next week. Come Monday, a slow moving cold front boundary will bring increasing shower chances to at least the higher terrain for the afternoon/evening. The latest NBM guidance and deterministic models hint at the opportunity for rain in the Valley as well, with Tuesday having slightly better chances. While precipitation amounts are highly uncertain, the cool down appears quite likely with highs crashing down to the mid 70s by next Tuesday. This is approximately 3 to 6 degrees below climatology.