Warm with a chance of thunderstorms through the weekend

May 18, 2018 – Afternoon and evening thunderstorms possible over higher terrain into next week. Temperatures will gradually warm through Saturday, and then cool slightly on Sunday.

Discussion

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Weak ridging will build over the CWA today with WNwesterly flow limiting the convection to the Sierra Crest. Then more areal coverage of storms expected as the weekend wears on as a weak trof is progged to move through on Sat, followed by a digging trof over the Western States on Sunday leading to the development of a closed low over the southern half of CA on Monday.

There are still some clouds moving Swd along the Sierra Crest in the lingering cyclonic flow from the Great Basin upper low. This flow will back to westerly as the weak ridge builds and this cloudiness should erode. Some cumuliform cloudiness is likely to develop over the mountains as the flow turns upslope. As the flow turns westerly, the convection will move off/over the Sierra Crest.

A stronger onshore pressure gradient is leading to a stronger Delta Breeze this morning. Ft Ord profiler shows the marine layer is slightly elevated and has spread into the Wrn portion of Solano County. It looks doubtful the stratus will advect inland as the T/Td spreads are too wide and RHs lower than yesterday, although the stratus scheme suggested patchy stratus possible (mainly east side Sacramento Valley) given the elevated depth to 3 kft and strong enough Delta Breeze overnight.

Generally, the SREF MUCAPE plumes show nil CAPE over the Coastal Mtns and RDD area this afternoon, modest values Sat, and higher on Sun. However, the values are similar each day over the Sierra today and through the weekend. The synoptic pattern would suggest stronger forcing/ascent with the approaching trof on Sat and then with the digging trof on Sun. A marked increase in instability is likely over the Shasta County and Coastal Range mtns with the digging trof on Sun, with an increasing chance of convection over the N end of the Sac Vly.

Modest warming is expected over NorCal through Saturday, before a cooling trend on Sunday. Onshore flow should yield a Delta Breeze Saturday and Sunday with the two trofs, then modest warming again on Monday as the low moves into SoCal.

Extended discussion (Tuesday through Friday)

While model discrepancies exist in the extended period, there is broad agreement that a closed upper low will remain over the Pacific Southwest Tuesday, possibly into Wednesday. This will continue to bring the chance of showers and thunderstorms mainly over the higher elevations through at least the middle of next week.

Model solutions diverge toward the end of next week. The ECMWF remains more progressive, with another closed low approaching the West Coast. The GFS amplifies an upper ridge over the West Coast, bringing warmer and drier weather. Without a clear or preferred solution, our forecast leans heavily on persistence, with continued chances of showers and thunderstorms over the mountains.