RSS Mesoscale Discussions from Storm Prediction Center
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
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SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1124 AM CST Tue Dec 23 2025
Valid 241200Z - 251200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
COASTAL CALIFORNIA AND THE CENTRAL VALLEY...
...SUMMARY...
Thunderstorms with marginally severe wind gusts, small hail and
potentially a brief tornado will be possible from Wednesday into
Wednesday night along parts of the California coast, and inland
across the San Joaquin Valley.
...California Coast...
West of an imposing upper ridge building over the central US, deep
troughing will intensify Wednesday and Wednesday night over the
eastern Pacific. Several embedded disturbances will track onshore
over parts of the West Coast. Strong ascent and strengthening
onshore flow will aid in modest destabilization, first near the
southern CA coast/LA Basin and eventually farther north and inland.
A band of storms will likely be ongoing over southern CA and
adjacent waters early Wednesday morning. Strong flow aloft (1km AGL
wind 50+ kt) may mix to the surface supporting strong gusts and the
potential for a brief tornado. A second more potent shortwave and
surface low (sub 985 mb) will intensify and move near the
central/northern CA Coast into early Thursday. A broken, low-topped
band of storms along the advancing cold front may eventually move
onshore overnight with a risk for occasional strong gusts.
...San Joaquin Valley...
Across central CA, cold mid-level temperatures (-24 to -27 C),
strong height falls and the left exit of a 100+ kt mid-level jet
will overspread 50s F surface dewpoints across the northern and
central San Joaquin Valley Wednesday afternoon and evening. Model
soundings show modest MUCAPE (max around 500 J/kg) amid veering wind
profiles and enlarged hodographs. Despite the limited buoyancy, a
few semi-organized supercells are possible. Damaging gusts, hail and
a brief tornado are possible with the strongest storms before
convection gradually moves eastward into the higher terrain and
weakens.
..Lyons.. 12/23/2025
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SPC 1930Z Day 3 Outlook
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0122 PM CST Tue Dec 23 2025
Valid 251200Z - 261200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
COASTAL CALIFORNIA...
...SUMMARY...
Thunderstorms with marginally severe wind gusts and perhaps a brief
tornado, will be possible on Thursday along parts of the central and
southern California Coast.
...California...
Large-scale mid-level troughing is forecast to persist across the CA
coast and move onshore Thursday and Thursday night. Within the
broader trough, several embedded impulses will move onshore within
strong southwesterly mid-level flow. One of these stronger impulses
will start the day moving onshore over northern CA before quickly
lifting north into southern OR. A strong surface low and cold front
associated with this impulse will move quickly toward the coast
prior to 12z Thursday. A band of low-topped storms along the front
may be ongoing at the start of the forecast period, and persist for
a few hours Thursday morning. Confidence in sufficient buoyancy for
a sustained severe threat farther inland is low. Still, strong low
to mid-level flow and modestly steep low-level lapse rates could
support isolated severe wind gusts and perhaps a brief tornado early
along the coast.
The severe threat will likely shift southeastward toward southern CA
coast through the day as a second mid-level impulse and the main
core of the upper trough approach. Strong onshore flow and
persistent low-level moisture transport will allow for weak
destabilization near the coast during the afternoon and continuing
into the evening. 90+ kt of mid-level flow and locally enlarged
hodographs could allow for occasional stronger storms capable of
damaging gusts or a brief tornado despite the weak buoyancy.
..Lyons.. 12/23/2025
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