Sunday’s Storm Update

Posted 10:15pm Saturday evening

Active, Atmospheric River, Convection, Storm, Strong Storm, Thunderstorms, Unsettled, Wet, Windy

It’s been a pretty active past 48 hours across northern California as a potent jet stream has sagged south, directing multiple systems toward/into northern California, along with a very moist atmospheric river originating from the central/eastern Pacific east of Hawaii. This final system – the strongest of the three, is shaping up to be a fairly powerful mid-latitude cyclone, bottoming out down to as low as 984 – 986mb off the California coast Sunday afternoon & evening. As this low rapidly strengthens offshore, it’ll once again draw northward the very moist streak of moisture continuing to surge northeastward toward California in the warm sector of the system Sunday morning and afternoon, ahead of the cold front. A brief period of clearing may occur ahead of the storm’s cloud shield Sunday morning, allowing some surface temperature warming. Clouds are likely to quickly overspread northern California by late morning and afternoon… with frontal precipitation not too far behind.

Given the strong surface cyclone offshore Sunday, a strong southerly low-level flow will develop, transporting warm, moist surface air up the valley, which on top of perhaps a brief period of sun in the morning should allow surface temperatures to climb into the low to mid 60s across much of the lower elevations, from the inland coastal valleys to the central/Sacramento valley. This surface heat beneath increasingly cool mid/upper-level temps should boost mid-level lapse rates to around 6.5 – 7.5c (moderately steep), as well as when combined with relatively high surface moisture (dewpoints in the upper 50s to low 60s) should promote surface-based CAPEs in the 200 – 500 j/kg range on a widespread scale through the afternoon. This instability combined with strong lift and wind energy from the surface upward should promote potential for at least isolated thunderstorms embedded or ahead of the cold front during the morning & early afternoon. Behind the front, which may pass by early to mid-afternoon, a more showery/cloud-broken airmass slides overhead… potentially allowing surface temperatures to climb a few extra degrees locally and boost dewpoints further (locally, once again), and support increased CAPE values on a scattered scale from 500 – 1,000 j/kg, mainly in the Sacramento valley, delta, and coast.

0z 4km NAM’s forecast ML (mixed-layer) CAPE valid at 5pm. Light green hues are values in excess of 750 j/kg, and darker greens are 1,000 j/kg+.

Fairly strong south to southeasterly low-level to surface flow combined with a WSW to SW mid/upper-level flow create decent directional shear… not quite a 90-degree angle in difference of winds with height, but not a too far off. Thus, when you add in the strength of these winds from the low to mid-levels, shear in general is quite strong, and should be supportive of rotating storms through the afternoon and into the early to mid-evening before instability wanes. Behind the front, from mid to late-afternoon onward into the evening, is when the threat for the strongest thunderstorms is expected. Given the available shear and potentially decent instability as currently modeled, some thunderstorms could strengthen and take up supercellular structures, rotate, and become capable of producing tornadoes and large hail. It’s also possible that bands of thunderstorms could form, which would be more capable of strong/damaging straight-line wind, as well as hail.

Winds even without thunderstorms will be pretty strong from Sunday morning through the evening as the potent surface low slides northward up the coast toward the Pacific Northwest, with 900mb winds in excess of 45 knots across a good chunk of the region… with locally some 50  – 55kt values being indicated by the 4km NAM. At this level, not every thing makes it to the surface… but a strong thunderstorm could quite easily drag down equivalent speeds to possibly locally higher speeds.

0z 4km NAM’s forecast 850mb winds, valid at 4pm Sunday. Note: these winds are several thousand feet above the surface, thus, winds of this strength likely wouldn’t directly make it to the surface without the aid of a strong thunderstorm.

I’ll likely post another update Sunday morning or afternoon as the event begins to unfold with an update on thunderstorm potential.

Previous updates are below, with still relative information to this event.

Possible tornado activity on Sunday

It may get a lot more interesting around here on Sunday.We are tracking the potential for severe thunderstorms in the Northern Sacramento Valley. The Storm Prediction Center has given us a marginal risk of severe thunderstorms.

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