No watches are valid as of Tue Apr 21 18:37:01 UTC 2026.MD 0494 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PORTIONS OF THE SACRAMENTO AND SAN JOAQUIN VALLEYS
Mesoscale Discussion 0494
NWS Storm Prediction Center Norman OK
0134 PM CDT Tue Apr 21 2026
Areas affected...Portions of the Sacramento and San Joaquin Valleys
Concerning...Severe potential...Watch unlikely
Valid 211834Z - 212030Z
Probability of Watch Issuance...20 percent
SUMMARY...A few stronger storms are possible this afternoon across
portions of the Sacramento and San Joaquin Valleys. Isolated gusty
winds and small hail will be the main hazards, although a tornado or
two cannot be ruled out.
DISCUSSION...A closed upper-level low is currently located off the
coast of northern California, with a trough axis extending
southeastward along the coast. This trough axis is currently
pivoting northeastward -- per current water vapor imagery -- over
the central California coast. Broad synoptic forcing has yielded
extensive showers streaming northward across the region, limiting
temperatures to the mid-50s. Some clearing is occurring in the wake
of these showers with temperatures increasing into the low 60s
across the western portion of the delineated area.
As the early-day showers and cloudiness moves eastward, continued
insolation and ascent in the left-front quadrant of the landfalling
jet will yield convective development. Current MLCAPE values are
marginal (< 100-200 J/kg) but are expected to increase to around 500
J/kg through the afternoon. Upper-level flow will back and remain
strong (around 50 kts at cloud top), yielding long and relatively
straight wind profiles. This should support continued thunderstorm
development through the afternoon and evening. Some stronger storms
are possible, yielding an attendant threat for scattered gusty
winds. A tornado or two is possible with any transient supercell
structures, although this threat will be limited by generally meager
low-level shear.
..Flournoy/Lyons/Hart.. 04/21/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...HNX...STO...MTR...
LAT...LON 36742057 37372148 38172157 38552105 38292048 37651964
37051917 36601915 36391992 36742057
MOST PROBABLE PEAK TORNADO INTENSITY...UP TO 90 MPH
MOST PROBABLE PEAK WIND GUST...UP TO 60 MPH
MD 0493 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR FAR SOUTHEAST WI INTO SOUTHWEST LOWER MI AND NORTHERN PARTS OF IL AND IN
Mesoscale Discussion 0493
NWS Storm Prediction Center Norman OK
0107 PM CDT Tue Apr 21 2026
Areas affected...Far southeast WI into southwest lower MI and
northern parts of IL and IN
Concerning...Severe potential...Watch unlikely
Valid 211807Z - 212000Z
Probability of Watch Issuance...5 percent
SUMMARY...Isolated occurrences of hail approaching severe levels
and/or locally strong wind gusts are possible with the strongest
storms this afternoon. Given the marginal nature of the environment
and expected storm coverage, a severe thunderstorm watch is not
expected.
DISCUSSION...A small cluster of elevated thunderstorms has persisted
into early afternoon in southeast WI, aided by forcing for ascent
attendant to an upstream shortwave trough. 12z observed and model
forecast soundings suggest the ongoing storms are rooted in a moist
plume atop a capping inversion at the base of an elevated-mixed
layer, which has spread east into the region. To the south of the
ongoing storms, strong daytime heating coupled with a gradually
moistening boundary layer from the w/sw may allow a subset of the
elevated convection to become progressively more surface-based
amdist a marginally unstable, but steep-lapse-rate environment.
Additional, surface-based storms are also possible in 20-21z time
frame along a weak surface front advancing southeast through the
region.
The current MKX VWP indicates a largely unidirectional wind profile
yielding around 30 kt of effective bulk shear. Wind profiles improve
with southward extent across northern part of IL/IN owing to the
presence of a 35-40 kt low-level jet. Marginally severe hail appears
to be the primary hazard with the ongoing, elevated storms.
Comparatively better wind potential exists with storms that can
become surface based across northern IL/IN where the steep lapse
rates and stronger low-level flow coincide.
..Mead.. 04/21/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...IWX...GRR...LOT...MKX...
LAT...LON 41968946 42508910 42798814 42848655 42598553 42158528
41408538 41028589 40808680 40838779 41148858 41488902
41968946
SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1051 AM CDT Tue Apr 21 2026
Valid 211630Z - 221200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
OVER THE CA CENTRAL VALLEY...AND OVER PORTIONS OF NORTHERN IL/IN...
...SUMMARY...
Isolated strong thunderstorms are possible in the California Central
Valley, capable of gusty winds and perhaps a tornado or two. Other
strong storms are possible in parts of northern IN/IL where gusty
winds may occur.
...Central CA...
Morning water vapor loop shows a well-defined shortwave trough
approaching the coast of central CA. An associated mid/upper level
jet will nose into CA, with forcing for ascent in the left-front
quadrant of the jet aiding in the development of afternoon/evening
thunderstorms over parts of the Central Valley. Forecast soundings
show steep low and mid level lapse rates and sufficient CAPE for a
few robust updrafts. Vertical shear will be strong enough for
supercell structures, including a risk of a tornado or two, small
hail, and gusty winds.
...Northern IN/IL...
A fast-moving shortwave trough currently over IA/MN will track
quickly southeastward into WI/IL/IN this afternoon. Visible
satellite and radar imagery show a line of high-based thunderstorms
developing ahead of the trough over southern WI. As these storms
track into northern IL/IN, continued heating/weak destabilization
will increase the risk of gusty/locally damaging wind gusts and
small hail in the strongest cores this afternoon.
..Hart/Flournoy.. 04/21/2026
SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1224 PM CDT Tue Apr 21 2026
Valid 221200Z - 231200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PARTS 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 broad trough within parts of the Northwest and Great Basin will
progress eastward through the day Wednesday. The mid-level jet will
round the base of this trough and impact the northern High Plains
during the evening overnight. A secondary mid-level jet will dig
into the Southwest by early Thursday morning. The upper-level ridge
across the Plains will begin to break down and shift eastward. At
the surface, a deep surface low will track along the Montana/Canada
border. Strengthening flow across the Rockies will promote a
moderately strong lee trough. Surface moisture will continue to
advect north and west during the period. Richer moisture will
generally remain within the southern Plains and adjacent High Plains
with less certain moisture quality farther north.
...Western Nebraska...western South Dakota...
With greater large-scale ascent and closer proximity to the surface
low, confidence in storm development is higher in these areas. The
main question will be the degree of moisture return that can occur
through the day. Model guidance still varies widely in this regard.
Dewpoints near 50 F are possible into southwest Nebraska, but this
may occur towards 00Z or later. Farther north, dewpoints in the 40s
F may be the ceiling for late afternoon/early evening. A probable
scenario is that high-based convection develops within the terrain
of eastern Wyoming/western South Dakota and within the surface
trough. Some intensification is possible as this activity encounters
greater surface moisture to the east. Severe wind gusts will be the
main hazard given the steep low-level lapse rates and inverted-v
thermodynamic profiles. Linear structures would be favored in South
Dakota given the deep-layer flow orientation. A supercell or two
could occur in western Nebraska, should storms form.
...Southern High Plains into Western Kansas...
With the synoptic trough lagging to the west, mid-level height falls
will be very weak during the afternoon. The dryline/surface trough
will be the focus for storm development. Temperatures nearing 90 F
behind the dryline appear sufficient for an isolated storm or two to
develop. That said, some guidance does not develop any convection.
Deep-layer shear will be roughly perpendicular to the surface trough
and mid-level lapse rates will be steep. Storms that can develop
will be supercellular. Large hail would be the main concern with
this activity.
...Eastern Montana...
High-based convection is possible in association with the deepening
surface low. Dewpoints will not likely reach 40 F. Gusty outflow
winds are possible, but potential for severe gusts is low.
..Wendt.. 04/21/2026
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