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August 31, 2017 – Very hot weather returns with record highs likely over the Labor Day Holiday weekend. Increasing risk of showers and thunderstorms next week, mainly over the mountains.

Discussion

Northerly winds bringing some smoke from wildfires down into the northern Sacramento Valley. Elsewhere, skies are clear as high pressure builds over the region. Current temperatures are a little cooler compared to 24 hours ago and range from the 40s in the mountain valleys to the lower 70s across the milder portions of the Sacramento Valley.

Hotter weather returns today and is forecast to peak on Friday and Saturday as strong high pressure covers the area and the marine layer is suppressed. Maximum temperatures will likely be around 110 or higher throughout the Central Valley on Friday and Saturday, though the amount of smoke from area wildfires may affect the forecast.

Southeasterly flow aloft returns Saturday and increases Sunday as the ridge moves over the interior West allowing monsoon flow to return. Elevated moisture and instability is forecast make it into the Sierra south of Tahoe later Saturday for a slight chance of thunderstorms, then spread further north and west early next week.

Temperatures may edge down a bit beginning Sunday as some mid and high clouds work their way north, and a Delta Breeze may bring a little additional cooling beginning Saturday night.

Extended discussion (Monday through Thursday)

Phasing of the Ern Pac High and the westward displaced 4-Corners High likely to occur on Sun with models forecast the strong blocking pattern to continue over the Wrn Conus. Again some westward displacement may occur Tue/Wed as a downstream trof gets carved out over the Ern Conus. This will continue the above normal temp pattern through the first half of next week as well.

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There is some certainty that beginning on Sun and continuing through the extended period that monsoon moisture and eventually remains of T.S. Lidia (off the srn tip of Baja) will advect into Norcal as the steering flow turns SEly and Sly around the strong Great Basin area High. This will increase the chance for thunderstorms in our area, mainly over the mtns, but would not rule out nocturnal storms or even storms in the Valley sometime during the EFP. The increased cloud cover will likely moderate maxes some, but keep mins elevated at night along with an increase in humidity. It may feel more humid than usual for most Norcal residents. Looking at the WR Tool Kit we noted the NAEFS 850 mbs temps will not change appreciably Mon-Wed (around 24-28 over our CWA) and then moderate to around 24 on Thu. This is more likely the influence of an approaching Pac trof from the GFS/GEM, while the ECMWF retrogrades the Great Basin High. This is the hotter, drier scenario. The previous models also handle a weak cut off a little differently than the ECM as it slides Nwd just off the CA Coast during the period. This scenario would bring some modest cooling, mainly to the Delta influenced areas. Our forecast temps generally followed the 850 mbs return intervals (RI) of the WR Toolkit. Mon going with a 2-5 yr RI, and then backing off to near normal Tue-Thu. This will still correlate with maxes above 100, but variable cloud cover could moderate temps a couple of degrees. But, the increased humidity may make it feel just as hot.