RSS Mesoscale Discussions from Storm Prediction Center
SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1237 AM CDT Tue Apr 21 2026
Valid 211200Z - 221200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
...SUMMARY...
Thunderstorms are expected today over much of Texas, from Illinois
into Ohio, and over portions of central California. Severe weather
is not currently forecast.
...Synopsis...
An upper ridge will move into the Plains as an upper trough moves
inland over the West, with a strong cyclonic speed max moving across
CA and into the Great Basin. Meanwhile, northwest flow aloft will
extend from the Great Lakes into the Mid Atlantic. Farther south, a
weakening disturbance will move out of TX and toward the lower MS
Valley.
At the surface, high pressure will remain over much of the eastern
states and across the Gulf Coast. Southerly winds across the Plains
and around this high may support mid to upper 50s F dewpoints as far
north as IA and IL by 00Z, while higher dewpoints remain relegated
to southern TX. Here, scattered showers and thunderstorms will
likely be ongoing early on Tuesday where weak elevated instability
will remain.
...IL/IN/OH...
A surface trough is forecast to develop from Lower MI into southern
WI and northern IL by late afternoon, beneath the northwest flow
regime and with cool midlevel temperatures. Southwest surface winds
and daytime heating should lead to 40s and 50s F dewpoints, with a
relatively dry boundary layer overall. Models suggest at least
isolated late day and evening thunderstorms will develop ahead of
this trough, though instability levels are uncertain. Despite
marginal dewpoints/moisture, the cool temperatures aloft may support
a few cells capable of small hail from IL into IN by late afternoon.
Wind gust potential is more uncertain and dependent on storm
coverage/outflow production, with the dry boundary layer
conditionally helping strong gust potential into the evening and
perhaps overnight.
...Central CA...
Cold temperatures aloft north of the midlevel jet will support weak
instability over parts of the central Valleys. Given early
precipitation and clouds, as well as marginal low-level shear,
severe storms appear unlikely. However, a few strong storms may
occur late in the afternoon if sufficient heating occurs behind the
early day rain. Small hail appears most probable with the strongest
cells.
..Jewell.. 04/21/2026
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SPC 0600Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1253 AM CDT Tue Apr 21 2026
Valid 221200Z - 231200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE HIGH PLAINS...
...SUMMARY...
Severe thunderstorms capable of hail and gusty winds may develop
during the late afternoon and evening on Wednesday across portions
of the High Plains.
... Synopsis ...
A closed upper low across the western US will devolve into an open
wave as it lifts northeast toward the northern Plains. At the same
time, a surface low will gradually deepen as it moves across
northern Montana east into North Dakota by Thursday morning.
Lee troughing will strengthen during the day with the approach of
the western trough. The surface wind field within this lee trough
will function to sharpen a dryline from the surface low south across
much of the High Plains. To the east of the dryline, strengthening
southerly winds will draw Gulf moisture northward into the
strengthening cyclone. The quality of this moisture return remains a
significant source of uncertainty regarding thunderstorm chances
with northward extent.
... Northern High Plains including areas of eastern Montana...
By late afternoon increasing large-scale ascent, driven by modest
height falls, will overspread northern portions of the dryline.
Despite moisture concerns, isolated to scattered thunderstorms
should develop across portions of the dryline during the afternoon.
Large temperature/dewpoint spreads may support strong wind gusts
with the strongest storms.
The best synoptic scale forcing for ascent will move across portions
of eastern Montana during the late afternoon and evening. Should
boundary layer moisture be richer than currently anticipated (e.g.,
closer to 40F than 25F), a threat for marginally severe wind gusts
may materialize. This area will continued to be assessed in
subsequent outlooks for an upgrade to Level 1/Marginal.
... Southern High Plains ...
Midlevel height falls will not be as great here as compared to areas
farther north. However, richer boundary layer moisture and strong
dryline circulations aided by temperatures approaching 90F to the
west of the dryline should support at least isolated thunderstorm
development. Around 40 knots of cloud-layer shear should support at
least a marginal hail risk with any storm that develops.
..Marsh.. 04/21/2026
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