SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1020 AM CST Tue Dec 23 2025 Valid 231630Z - 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. ...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. ..Gleason/Thornton.. 12/23/2025
