Severe Thunderstorms Possible in Central Valley Sunday Afternoon

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This will be a quick post on the potential for severe thunderstorm development in the central valley Sunday afternoon, after a potent system offshore swings its associated cold front ashore. Currently, offshore, a large mass of cold-core/open-cell convective cloudcover exists offshore within the circulation of a upper-level low/deep trough. Rounding this low is a 135 – 145 knot upper-level jet fueling a strongly divergent flow on the southeastern corner of the upper-low, where the low’s cold front should solidify a precipitation band overnight into Sunday morning as it works its way through interior northern California. Precipitation rates with this front could be fairly heavy at times given the associated strong mid & upper-level jet aided by frontal convergence/lift with precipitable water values around 1 – 1.3″ along the frontal band. It appears models are in decent agreement that the front will clear the valley by 10am to noon, leaving several hours of afternoon heating available to warm surface temperatures.

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NAM’s forecast 500mb wind and heights, valid 8am Sunday as the cold front begins rolling ashore. The strong jet streak off the central coast is the aforementioned streak enhancing the front.

High resolution models suggest surface temperatures warming into the low 60s across much of the Sacramento and northern San Joaquin valleys, which will be beneath a -25c mid-level cold pool steeping lapse rates significantly. Something to note about this trough is that it’ll be negatively tilted (meaning from the northern tip of the trough to the southern tip of the trough it’s slanted/oriented from northwest to the southeast on the south end of the trough), the most favorable trough tilt there is for convective weather. Negatively tilted troughs generally feature a strong mid-level jet out of the west in most cases (which it certainly has) and a decent southerly low-level flow, creating excellent directional shear. Forecast soundings in the Sacramento valley Sunday afternoon resemble something you’d expect to see in the plains during a low to medium end severe weather day — which in California exceeds our threshold for “severe weather” by quite a bit.

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4km NAM’s sounding in the central valley west of Chico Sunday afternoon, with an excellent hodograph and decent instability combined with an LCL essentially to the surface: meaning surface based convection is a good bet.

As mentioned, the surface temperatures rising into the 60s will promote steep low and mid-level lapse rates; but additionally dewpoints in the low to mid-50s being driven up the valley via a strong southerly surface flow, which when combined with the lapse rates will equate to quite a bit of instability. CAPE values are progged by most models to exceed 500 j/kg across much of the interior Sunday afternoon, with some models suggesting values upwards of 1000 j/kg — quite high for California’s standards. This amount of instability in proximity to the strong lift from the offshore low will certainly lead to convective development throughout the day, which in one way could be a problem: and that’s too much convective coverage. The problem of overdevelopment does happen in California quite often, when there’s too much instability for a systems own good to allow space between storms for some to mature. In these overdevelopment cases, showers and weak thunderstorms develop on a widespread scale, while in a scattered scenario thunderstorms can utilize untapped energy around them to strengthen and mature into strong storms.

Model guidance currently suggests Sunday afternoon’s setup being either scattered (prime) or between scattered and overdevelopment Sunday afternoon (decent, but not perfect). Either way, this system’s setup looks like one of the better setups we’ve had since last season or perhaps in the last couple of seasons when it comes to strong thunderstorm development. The only question is if we’ll have enough space between storms in the afternoon to allow storms to reach their full potential in what is looking to be a prime environment: especially if cloud breaks are widespread for a couple of hours late in the morning into the early afternoon behind the front.

Given the available instability and absolutely perfect shear, any strong thunderstorms that develop Sunday afternoon would likely take on the characteristics of supercells and be capable of a variety of hazards ranging from large hail to damaging winds. Tornado potential will exist with any of the strongest storms that are able to mature and hold onto their updrafts for extended periods of time and remain isolated from other storms to utilize the strong and moist low-level flow. The threat for severe storms will be maximized from Redding southward to Sacramento, which is essentially the entire Sacramento valley, as the best instability will be set in place here.

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NCEP WRF’s forecast mixed-layer CAPE, valid 2pm Sunday afternoon suggesting widespread 500 j/kg values (dark grey), with plenty of 700 – 1000 j/kg values (greens) in the central and northern valley.

Northern California storm chasers: keep your eyes and ears on the radar and be ready to jet.

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