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Day 1 Outlook Convective Tornado
Hail Wind
Categorical Day1 1300Z Outlook
  
Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion

  
678 
ACUS01 KWNS 251247
SWODY1
SPC AC 251246

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0746 AM CDT Sun Jul 25 2021

Valid 251300Z - 261200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS
AFTERNOON/EVENING IN A CORRIDOR FROM SOUTHEAST NY TO NORTHERN VA AND
KY...ACROSS MUCH OF KS AND SD...AND NORTHWEST AZ/EXTREME SOUTHEAST
NV/CA...

...SUMMARY...
Isolated damaging winds are possible this afternoon/evening from
parts of the Northeast to the Ohio Valley, across Kansas and South
Dakota, and across northeast Arizona and extreme southeast
Nevada/California.

...Parts of the OH Valley into New England today...
A midlevel trough with embedded perturbations will move over the
Northeast today, as an associated/weak surface cold front moves
slowly southeastward across NY/PA and the OH Valley.  In the wake of
morning clouds/weakening convection, additional thunderstorm
development will be possible this afternoon along the cold front. 
Moderate buoyancy (MLCAPE near 1500 J/kg) is expected across
southeast NY and eastern PA, with larger buoyancy (MLCAPE of
2000-2500 J/kg) along and ahead of the front across the OH Valley. 
Effective bulk shear will be a little stronger (25-35 kt) across
NY/PA in closer proximity to the midlevel trough, and weaker toward
the OH Valley.  Given rather modest midlevel lapse rates and at best
weak forcing for ascent, storm coverage/severity both appear
somewhat limited.  Still, the strongest storms could produce
isolated strong downburst winds and some wind damage this afternoon
into this evening.

...SD late this afternoon into this evening...
A baroclinic zone will develop northward across SD today, in
response to lee cyclogenesis across the northern High Plains.  The
richer low-level moisture will be confined to areas farther south,
with strong heating/deep mixing expected along the boundary this
afternoon.  With surface temperatures of 98-100 F, mixing could be
deep enough to remove convective inhibition and allow the
development of a few high-based storms capable of producing isolated
severe outflow winds.  The convection will tend to develop eastward
during the evening, and will likely be maintained by increasing
low-level warm advection on the nose of a 40 kt low-level jet. 
Though the storms will likely be elevated tonight, warm/unsaturated
profiles in the low levels still suggest some potential for
downdrafts to reach the surface with strong winds (i.e., DCAPE
1200-1400 J/kg).  Marginally severe hail may also occur with the
strongest storms given MUCAPE near or above 1000 J/kg and effective
bulk shear of 35-45 kt.

...KS area today through late evening...
Boundary-layer dewpoints in the low 70s have spread northward into
much of KS, along and south of outflow-reinforced frontal zone. 
Outflow with the ongoing convection near the KS/NE border may help
storms persist through the day while spreading southward, and
additional storms will form along the front/outflow this afternoon. 
Despite fairly weak vertical shear, multicell clusters will be
capable of producing downbursts with isolated outflow winds
approaching 60 mph, given large precipitation loading (based on
MLCAPE of 2500-3500 J/kg and PW above 1.75").

...Northwest AZ and southeast NV/CA this afternoon/evening...
Around the northwest flank of the midlevel low over AZ, a belt of
25-30 kt northeasterly flow will persist through this evening.  At
the surface, a moist air mass with dewpoints in the low-mid 60s
extends into southeast NV.  The moisture and surface heating will
contribute to moderate buoyancy this afternoon, and the midlevel
flow will steer storm clusters southwestward from the higher
terrain, with the potential for isolated outflow gusts approaching
60 mph.

..Thompson/Gleason.. 07/25/2021

$$