SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0150 PM CST Wed Dec 24 2025 Valid 242000Z - 251200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF COASTAL CALIFORNIA AND PORTIONS OF THE CENTRAL VALLEY... ...SUMMARY... An isolated risk for severe/damaging gusts and perhaps a brief tornado will exist today into tonight along much of the California Coast and portions of the Central Valley. ...20Z Update... ...Central/Northern California... Water vapor imagery shows the shortwave trough already moving inland. In the wake of this activity, visible satellite shows potential for cloud cover to reduce within the Central Valley from west to east. Some forecast soundings show destabilization is possible by late afternoon. Given the southeast flow within the Valley and strong flow aloft, a conditional threat for a marginal supercell/brief tornado could develop. Greater confidence exists with activity later in the evening into Thursday morning associated with a secondary, stronger shortwave trough. Another narrow band of convection is expected ahead of this shortwave and will impact parts of the central/northern California Coast and eventually parts of the Central Valley as well. Isolated damaging winds and a brief tornado would be possible with this activity. ...Southern California... A spatially limited threat for wind damage and a brief tornado will exist for another few hours this afternoon. A low-topped band of convection recently moved through much of the LA Basin and will generally continue eastward. There may be a narrow sliver of dewpoints near 60 F just east of this activity. Dewpoints closer to San Diego are only in the upper 50s F, however. Strong wind shear at low levels will maintain this marginal risk, though decreasing buoyancy southward and inland should lead to a downward trend in intensity. ..Wendt.. 12/24/2025 .PREV DISCUSSION... /ISSUED 1025 AM CST Wed Dec 24 2025/ ...California... A shallow band of pre-frontal convection may continue to pose a threat for locally severe/damaging winds (especially across higher terrain/ridgetops) and perhaps a brief tornado this morning across parts of coastal southern CA. This activity is being supported by strong low-level warm advection and large-scale ascent associated with an enhanced mid/upper-level jet. Current expectations are for this activity to continue moving eastward through the afternoon across the remainder of coastal southern CA in a strongly sheared environment. However, various NAM/RAP forecast soundings across this area suggest that even with low 60s surface dewpoints and modest/filtered daytime heating, the overall severe threat will likely remain isolated due to poor lapse rates aloft and limited boundary-layer instability. Reference Mesoscale Discussion 2269 for more details on the near-term severe threat across southern CA. Additional rounds of convection are expected later this evening and overnight into early Thursday morning over portions of northern/central CA, as multiple mid-level shortwave troughs embedded within persistent large-scale upper troughing over the eastern Pacific overspread these areas. One such shortwave trough is forecast to advance over coastal central CA and vicinity around 25/00-06Z this evening, with cool temperatures aloft (less than -20C at 500 mb) supporting weak MUCAPE. Both low-level and deep-layer shear are expected to gradually increase this evening as the shortwave trough approaches, with some updraft organization possible. Strong to locally severe winds and a brief tornado or two appear possible with this activity, but uncertainty is still substantial whether sufficient instability will be present to support surface-based thunderstorms. A stronger mid-level shortwave trough will move east-northeastward from the eastern Pacific and approach the northern CA Coast by 12Z Thursday morning, along with a deepening surface low and attendant cold front. A 70-80+ kt mid-level jet will accompany this shortwave trough, along with a 50-60 kt south-southwesterly low-level jet. A broken band of low-topped thunderstorms will likely develop in association with this shortwave trough along/ahead of the front, and overspread coastal portions of northern/central CA late tonight through early Thursday morning (25/06-12Z). A strongly sheared environment and weak but sufficient MLCAPE may exist to support at least an isolated threat for severe/damaging winds and a tornado or two as this activity spreads inland through the end of the period. Still, the overall severe threat should be tempered by the lack of stronger instability. Finally, some chance for low-topped/mini supercells may exist late this afternoon/evening across parts of the Central Valley of CA. If these thunderstorms can develop and be sustained, they could pose a threat for both damaging winds and a brief tornado. But, confidence in this scenario occurring remains rather low.
