Closed Low to Bring Showers/Thunderstorms Through Tuesday

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An upper-level trough swinging south from the Pacific Northwest will close off into a closed low over the next 24 hours as it slides over California through the day Monday into Tuesday. As the low cuts off, the low and it’s associated jet strengthens and the trough it’s still loosely contained in becomes increasingly amplified. -20 to -22c 500mb temperatures atop valley temperatures in the mid-70s in the valley, 60s in the foothills, and 50s in the higher elevations will promote rather steep low-level lapse rates with moderately steep mid-level LRs (around 7c). This, combined with modest low-level moisture will promote CAPE values in the 200 – 500 j/kg range across the mountains and parts of the Sacramento valley Monday, mainly from around Jackson northward. This instability combined with pockets of accent courtesy of the upper-low off the northern California coast will support the development of convection through the day Monday, beginning across the sierra, northern and coastal mountains. As we head later into the afternoon, as the low continues it’s southeast trek, showers and storms may develop into the central and northern Sacramento valley, and by evening, perhaps into the southern Sacramento valley.

May 18th

Models aren’t in the best agreement nor show too great of confidence regarding coverage of convection, with some high resolution models indicating scattered showers and thunderstorms Monday evening while others indicate just a few isolated pockets of convection. In general, it’s possible for some showers or thunderstorms pretty much anywhere from (as aforementioned) Jackson to the Oregon boarder Monday/early Tuesday. Straight-line hodographs indicate storms will mostly be multi-cellular and/or linear, and pretty high based with LCLs in the 700 – 800mb range. Forecast soundings indicate inverted-v moisture profiles across much of the region Monday afternoon before evening RH recovery from a strong delta breeze, indicating potential for some strong downbursts and/or possibly microbursts. It may take some time before storms moisten the sub-cloud layer, and if lightning is produced before so, some dry lightning strikes could be possible.

18z (12km) NAM's forecast sounding between Sacramento and Yuba City late Monday afternoon, indicating a dry low and mid-levels under the LCL.

18z (12km) NAM’s forecast sounding between Sacramento and Yuba City late Monday afternoon, indicating a dry low and mid-levels under the LCL.

Showers and storms may continue (mainly in the mountains) into Tuesday morning before another wave develops Tuesday afternoon as the upper low continues it’s southward track. By Tuesday afternoon, the upper-low will have expanded in size and elongated itself from northern NV into central/southern California, with the cool air still aloft promoting even more widespread instability given somewhat warmer surface temperatures. However, due to the low being east of us, the northerly upper-level wind regime isn’t very favorable for widespread convective development. That said, some isolated to scattered showers and thunderstorms remain possible, especially in the mountains and parts of the foothills through Tuesday afternoon and early evening.

Behind the low, on Wednesday, the back edge of the trough and the exit jet will likely bring some breezy north winds, which will increase fire danger given the offshore flow rapidly drying us out again. Another effect of this offshore flow will be rebounding temperatures back into the mid to upper-80s in the valley, which will mark the beginning of another big warming trend through the week, with mid to upper 90s to possibly near 100 across the valley come the weekend and next week.

If the setup looks more, well, chaotic come Monday and more widespread/intense convection is looking possible, I’ll add an update to the beginning of this post or type out a full one.

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