RSS Mesoscale Discussions from Storm Prediction Center
SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1145 PM CST Tue Dec 16 2025
Valid 171200Z - 181200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IN PARTS OF THE
NORTHWEST...
...SUMMARY...
Sporadic severe gusts may accompany low-topped convection across
parts of the Northwest this morning into midday.
...Northwest...
A vigorous shortwave trough will move quickly east along the
Canadian border from WA to the northern Great Plains by evening.
Intense tropospheric flow will accompany this wave, yielding
widespread high winds across much of the Northwest to the northern
Great Plains. Deep lee surface cyclone over southern AB should reach
the Red River Valley of the North. Attendant Pacific cold front will
sweep east with a tight pressure gradient in its wake.
Low-topped convection may be ongoing at 12Z along the Columbia Basin
portion of the front. Most guidance also suggests an increase in
low-topped convection should occur towards late morning across parts
of the Snake River Plain as the front pushes east. Surface-based
instability will remain scant in both regimes with morning
temperatures in the 40s to low 50s. Despite the cool temperatures,
this may be sufficient for sporadic lightning flashes amid strong
large-scale ascent. With very strong surface winds accompanying the
front, both regimes could be coincident with severe wind gusts.
The bulk of sustained severe wind speeds should become primarily
confined along/east of the Rockies in MT to the CO Front Range this
afternoon per HREF/REFS guidance. However, potential for severe
convective gusts appears too conditional with negligible
surface-based buoyancy anticipated.
..Grams/Wendt.. 12/17/2025
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SPC 0700Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1251 AM CST Wed Dec 17 2025
Valid 181200Z - 191200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS CENTERED OVER
KENTUCKY AND TENNESSEE...
...SUMMARY...
Isolated damaging wind gusts are possible across portions of the
Lower Ohio and Tennessee Valleys on Thursday.
...Synopsis...
A strong mid-level jet streak will round the base of the mid-level
trough across the central Plains early Thursday and emerge across
the Ohio Valley by Thursday afternoon/evening. This trough will
continue east and take on a more neutral to perhaps negative tilt by
early Friday across the East Coast. A strong (sub 990mb) surface low
will move from the Upper Midwest to southern Quebec with a trailing
cold front sweeping from the Midwest to the East Coast.
...OH/TN Valleys...
The cold front is expected to interact with the richer low-level
moisture early in the period across Illinois and Missouri with a
shallow line of thunderstorms expected to develop. In addition, some
weak elevated instability may develop ahead of this front as cooling
mid-level temperatures result in sufficient elevated instability for
storms. Low-level instability ahead of the front is expected to
remain quite weak (100 to 200 J/kg). A strong frontal circulation
should overcome the weak instability and produce sufficient lift for
lightning. Any updrafts deep enough to produce lightning, and with
minimal inhibition (seemingly around 55F surface dewpoint) may also
have at least some isolated damaging wind threat given the 60 to 70
knot low-level jet around 1km.
Greater instability is expected south of the TN/MS/AL border where
upper 50s to low 60s dewpoints are forecast. However, despite the
more favorable thermodynamics, the upper-level forcing will be
weaker. Therefore, it is unclear whether the line of storms will
continue that far south, and additional convection ahead of the
front will likely struggle given the weak lapse rates/minimal
heating.
...Eastern Virginia into North Carolina...
Strong storms and potential supercells are expected over the warm
Gulf Stream waters Thursday evening. Some guidance indicates surface
based instability may advect inland across far eastern North
Carolina which would allow some of these stronger storms to move
inland. Current expectation is for stronger storms and 60+F
dewpoints to remain mostly offshore, so no marginal risk will be
added.
Late in the period, as the cold front crosses the Appalachians,
some increase in storm strength is anticipated as mid 50s dewpoints
east of the Appalachians provide weak instability to fuel the
storms. If more aggressive inland moisture advection occurs, similar
to the scenario mentioned above, some marginal damaging wind threat
could materialize given the strong wind field.
..Bentley.. 12/17/2025
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