SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0145 PM CST Tue Dec 23 2025 Valid 232000Z - 241200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF COASTAL CALIFORNIA... ...SUMMARY... An isolated threat for severe/damaging gusts and perhaps a brief tornado may develop late tonight/early Wednesday morning across parts of coastal California. ...20Z Update... Relatively minor adjustments to the general thunder forecast were made based on the latest observations and model guidance. The remainder of the forecast remains unchanged. See the previous discussion for additional information. ..Wendt.. 12/23/2025 .PREV DISCUSSION... /ISSUED 1020 AM CST Tue Dec 23 2025/ ...California... Within large-scale upper troughing over the eastern Pacific, an embedded mid-level shortwave trough will move quickly eastward and approach the CA Coast late tonight/early Wednesday morning. A corresponding strengthening and increase in the flow through much of the troposphere is anticipated late in the period, with a surface low also forecast to deepen along/near the coast of northern CA. A cold front attendant to this surface low will sweep quickly eastward, and should be approaching/onshore across parts of coastal northern/central CA by 12Z early Wednesday morning. Current expectations are for the thermodynamic profile to remain mostly saturated, with poor lapse rates generally limiting instability. Even so, it appears that a gradual increase in low-level moisture ahead of the cold front and slightly cooling mid-level temperatures with the approach of the shortwave trough may support weak but sufficient MUCAPE for an organized band of low-topped thunderstorms late tonight. Most guidance suggests this convection will not impact parts of coastal CA until after 24/06Z, and there is still significant uncertainty whether enough boundary-layer instability will be present to support surface-based convection. There is also a signal for pre-frontal convection in the low-level warm advection regime across parts of central into coastal southern CA. Given the strength of the low/mid-level flow forecast, and related strong low-level/deep-layer shear, it appears that convectively enhanced winds may be capable of reaching severe levels and producing damage on an isolated basis even if instability remains minimal. A brief tornado may also occur if convection can become truly surface based, as effective SRH will be enhanced by a 50-70 kt southerly low-level jet. Based on latest guidance trends, the Marginal Risk has been expanded southward along the coast to include more of central/southern CA. This isolated severe threat is expected to continue beyond 12Z Wednesday into the Day 2 period.
