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September 21, 2019 – Dry and mild weather through the weekend. A few showers possible over the mountains Sunday. Breezy north winds possible early next week leading to elevated fire weather concerns. Above normal temperatures are likely Tuesday and Wednesday while a significant cool down and increasing precipitation chances return late in the week.

Discussion

Most Valley locations continue to experience light to locally modest northerly flow while foothill and mountain locales are seeing diurnally driven downsloping flow, both of which have contributed to keeping humidities lower than usual. With a positively-tilted trough juxtaposed over the Intermountain West, deep-layered northerly flow persists across the state.

Thus, one more day of this flow regime is expected, albeit with wind speeds likely to remain around 15 knots or below. Warm temperatures will prevail today as a progressive ridge moves through. This supports Valley highs into the upper 80s, very close to climatology.

For Sunday and into early next week, amplified flow descending from the Northeast Pacific will cool temperatures off by a few degrees. Forcing from the shortwave pushes through Shasta county late Sunday afternoon spreading light showers over the higher terrain. Eventually this activity swings into the southern Cascades and far northern Sierra, with perhaps a couple rumbles of thunder during the evening hours. Given sparse moisture, overall rainfall amounts should stay around 0.10 inches and below.

As this trough forms a closed low along the California/Nevada border early Monday, strengthening northerly flow ensues with notable drops in daytime/overnight relative humidities. The combination of the these factors in conjunction with curing fuels supports an elevated wildfire risk.

As such, a Fire Weather Watch will be in effect from Monday afternoon into Tuesday evening. This covers sections of the Sacramento Valley into the adjacent foothills as well as the northern Coastal Range. In addition to the wildfire threat, Valley highs will punch into the mid 90s on Tuesday as 850-mb temperatures rise into the 22-25C range. Relative to typical numbers, these high temperatures sit 6 to 10 degrees above average.

Extended Discussion (Wednesday through Saturday)

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With the parent upper low settling into northwestern Mexico by early Wednesday, broad ridging will hold on another day affording a warm day across the region. Valley high temperatures should reside in the low to mid 90s which includes a rather warm start to the day as well. Some foothills locations will see morning lows near 70 degrees which is 10 to 15 degrees above climatology. Based on the latest model guidance, this should mark an end to the brief round of heat as a pattern change looms.

A well advertised western U.S. trough sets up during the Thursday through Sunday (Sep 26-29) timeframe. Ensemble means even show the potential for this negative height anomaly to linger into the following week. Ultimately this shift in the pattern will support a significant cool down, particularly by Friday into next weekend.

Based on the latest forecast package, upper 60s to low 70s would be high temperatures across the Valley with surprisingly not too much spread in the 00Z GEFS ensemble members. Forecast highs would be around 10 to 20 degrees below average across interior northern California, especially over the foothills and mountains given precipitation.

It appears there will be daily chances for precipitation beginning Friday with snow levels falling to the 6,000 to 7,500 foot range. Details will likely change but there is building confidence in the colder, unsettled pattern to finish out the month.