Upper Low to Bring Potential for Severe Thunderstorms in Central Valley Friday

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An upper-low wandering about off the western U.S. coast this late week will move ashore into Oregon Friday, sending inland the cold front Friday morning. Currently, the system’s cold frontal precipitation band looks relatively insignificant, however as a plume of moisture gets infused into the jet overnight and the moisture and lift begin to run into land, increased convergence and friction will help precipitation generation/development become more widespread and efficient. After this burst of morning rainfall, there appears to be a shot at some clearing skies late in the morning into the early afternoon; around this time, the upper-low will still be just off the far northern California and southwestern Oregon coast, continuing to direct a westerly 50 – 60 knot mid-level jet overhead during the afternoon hours. Simultaneously, the surface low will also remain just off the Pacific Northwest coast, continuing to push a decent southerly low-level jet through the region in the afternoon, especially in the Sacramento valley where surface winds look to remain sustained around the 15 – 25mph range (gusts of 30 to 40mph possible). You’ll notice two different wind directions at those heights, and that’ll come into play in an explanation soon.

3pm Thursday visible satellite imagery with the NAM’s forecast 500mb heights overlaid to match up with the current time & actual position of the low.

Of course once a cold frontal passage occurs, colder air moves overhead — and when these cold pools move overhead in the daytime, certain layers of the atmosphere tend to destabilize. In this case, it looks like some clearing could occur in the valley in the late morning into periods of the afternoon depending on location. This could allow surface temperatures to climb into the upper 50s to low-60s in some cases, while up around 500mb (~18,000ft) they’ll be -22c (-8°F). This sets the stage for low and mid-level lapse rates of around 7c, which isn’t too shabby but not extreme. Enough moisture available in the low-levels due to morning rain and continuing westerly flow helping pool moisture in the valley in combination with those max surface temperatures could support surface based CAPE values anywhere from 400 to 1100 j/kg. Any of these values (preferably >600 j/kg. Values higher than this would suggest there was almost certainly some sunshine) could support some thunderstorm development by afternoon.

Forecast sounding near Corning in Tehama county, valid at 2pm Friday via the 4km NAM. The nicely curved hodograph (top left panel) suggests excellent turning winds with height in addition to increasing wind speeds with height. Additionally, the right main thermodynamic profile panel suggests a decent amount of CAPE in the low and mid-levels, extending somewhat narrowly into the upper levels.

As the upper-low approaches in the afternoon hours between the cold frontal passage and that potential brief period of partial clearing, bands of convective activity directly associated with the low are progged by high resolution, convective allowing models to begin rolling ashore. These would be a source of ignition of storms to develop ahead of and around them in the afternoon, although if enough instability is present in combination with bands of convergence in the valley, individual storms could develop without these bands’ help. Either way, I expect thunderstorms to blow up at least in portions of the Sacramento and San Joaquin valley Friday afternoon — the setup is quite supportive of convection, although strong storms will be dependent on increased surface heating in pockets of land that saw some sunshine for a while. These areas would have the best risk for robust storms to blossom.

As previously mentioned early on, the wind pattern with this system from the low levels to the upper-levels provides great directional shear as well as speed shear as winds increase with height. The strong l0w-level flow is also something quite a few of the last setups have skipped out on, and would help transport some warmer air and moisture into any potential storms that do develop from the south (inflow). However, the past several events have suffered from overclouding or overcrowding of showers/storms, cooling off the surrounding environment too fast. If storms manage to remain discrete and lonesome Friday afternoon, the setup would be ripe for classic Sacramento (or San Joaquin) valley mini-supercells. The areas with the greatest combination of needed ingredients looks to be in the central & northern Sacramento valley and the northern & central San Joaquin valley, as models seem to be suggesting cloud cover and showers from the delta into parts of the southern Sacramento valley slightly inhibiting potential. If this isn’t the case, the entire valley would have at least a chance at some strong storms.

Of course with a well-sheared environment comes the aforementioned supercellular thunderstorm risk, and all severe weather hazards typically ride along with supercells; this includes potential for a funnel cloud and/or tornado or two, hail, and strong to damaging winds. I’d give this setup a 7.5/10, with the uncertainty regarding sunshine holding back a higher risk value.

Aside from this, looking into the weekend and next week, a very wet pattern will resume; with an atmospheric river event likely from midweek into next weekend. I’ll try to carve out time this weekend for more on this looming risk, as it appears several inches of rain are likely in the valley next week alone, with over a foot of liquid precip expected in the adjacent mountains. With this falling atop already very moist soils, the flood risk will rise rapidly as rivers and streams remain easy to fluctuate. Stay tuned.

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