Shortwave to Bring Thunderstorm Chances to Valley Thursday

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While I didn’t have the time to make a post about the cold front of this system exiting north-central California as of Wednesday night, I am making the time now to post on the interesting post-frontal shortwave currently off the northwest CA/SW OR coast tonight. This shortwave, swimming around in a cold pool with 500mb temperatures below -30c, will swing into OR/WA early Thursday morning. As the wave impacts the coast to the north, it’ll have a several hour delay before its trailing band of convection and a secondary shortwave behind it into northern California — by Thursday afternoon. This band and secondary shortwave will bring inland the needed low-level lift and even maintain the low-level southerly flow up the backbone of the valley through the rest of the day (more on this below).

IR satellite imagery & radar from 11:30 PM Wednesday, with shortwave #1 front & center off the Oregon coast.

Ahead of this wave, in the morning hours, I anticipate at least some scattered pockets of clearing in the Sacramento valley — which will occur beneath the aforementioned -30c mid-level cold pool. Assuming surface temperatures reach the low to mid-50s as progged by high resolution/short-range models, this would result in very steep low-level lapse rates (the rate of cooling from one lower part of the atmosphere to a higher part). Mid-level lapse rates are progged in the 8c range, excellent for convection. The southerly surface flow, likely strongest from Sacramento to Red Bluff, could also aid in transportation of slightly warmer air from the southern reaches of the Sacramento/northern San Joaquin valley further north by a county or two’s distance or so, which means a lot when it comes to overall instability and maintaining storms.

The timing of storms will be a big deal when it comes to potential for strong or even severe storms. If the (secondary) shortwave comes in too late in the afternoon or especially after sunset, thunderstorm strength would be limited to sporadic lightning strikes and tiny hail. If the shortwave comes inland between 1 – 3pm, this would be go-time and allow for the most sunshine to occur ahead of it and come inland with the most instability to utilize. Models are all off slightly from each other, which is normal, but the slight variations in shortwave position greatly affect their forecasted convection. A blend of them all would certainly result in isolated to scattered thunderstorms from Stockton north, with the best robust thunderstorm chance in what appears to be two regions. Region one is the northern & central Sacramento valley, where convergence could be the kicker of some storms in the afternoon — and is also where the best overall shear is. Region two is from Yuba City to Sacramento, where models currently bring the convective band itself through in the mid to late-afternoon.

Given the southerly to locally even more backed southeasterly flow up the valley between 10 – 15 knots (stronger in the central & northern Sacramento valley past the Sutter Buttes) undercutting the strong mid-level jet out of the west to west-northwest, there is strong directional shear — that is, a lot of turning of angle of wind direction from the surface upwards into the atmosphere. Bulk shear is also on the order of 30 – 50 knots throughout the valley. Additionally, instability assuming there is at least scattered patches of sunny skies in the morning, is progged to be moderate for California’s standards. CAPEs in the 200 – 500 j/kg range are fairly normal in our severe setups. The shear could be slightly better, though, in that the northerly tilt of the jet due to it’s soon departure isn’t as favorable as a dew-west jet. That said, this setup is something like a 6/10. Nothing to either forget about or drink seven pots of coffee too, but certainly a setup capable of producing anything if everything comes together just right.

3km NAM’s (0z run) forecast MLCAPE & 10 meter wind direction, valid at 4pm Thursday afternoon. Much of the Sacramento valley is enveloped in an environment characterized by CAPE values >300 j/kg with southerly winds dominating the field.

Hail will be the biggest thing from any storms that can sustain themselves and become dominant in a region. If a storm can remain relatively isolated and dominant in a region, I would not totally rule out a funnel cloud or brief tornado given the favorable low-level wind regime out of the south to southwest, which can help quickly get a mesocyclone amped up to try to drop a quick tornado. Once again, the tornado and overall severe threat isn’t big, but it is something that has potential to come together if everyone else on the team pulls through.

Otherwise, at the very least, some heavy showers and isolated sporadic lightning in the strongest showers are likely Thursday on the immedite coast and in the Sacramento/northern SJ valley with small hail being very likely with most cells that sustain themselves.

Stay tuned & stay aware.

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