SPC AC 042003

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0303 PM CDT Fri Sep 04 2026

   Valid 042000Z - 051200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN
   GREAT LAKES INTO THE MID-ATLANTIC STATES...

   ...SUMMARY...
   A couple of organized thunderstorm clusters are expected to develop
   across the southern Great Lakes and mid/upper Ohio Valley through
   tonight, accompanied by the risk for damaging wind gusts. More
   widely scattered strong thunderstorms east of the Blue Ridge may
   also produce damaging wind gusts this evening.

   ...20Z Update...

   The northern portion of the Marginal Risk was expanded westward into
   portions of northeastern Iowa to account for recent convective
   development associated with a mid-level perturbation visible on
   water vapor satellite imagery. An unstable airmass and modest
   deep-layer shear will support loosely organized convection, capable
   of producing isolated occurrences of 60 MPH winds and 1-inch hail. 

   Elsewhere, the western portion of the Slight Risk area was expanded
   across Illinois, Indiana, and portions of Ohio, where thunderstorms
   have developed along a surface boundary and may pose an increased
   threat for 55-70 MPH wind gusts and 1-inch hail. See MCDs #2234 and
   #2235 for more detail. 

   Further downstream across North Carolina, the severe wind
   probabilities were extended southward to account for recent
   convective trends. A corridor of higher wind probabilities was
   considered across portions of Virginia, but uncertainty in the
   longevity of the convection moving across the Appalachians and
   overall convective organization precluded adding additional
   highlights at this time.

   The 30% wind threat area is expected to see convective development
   later this evening, as storms move from the Great Lakes into the
   region. There is some uncertainty in the northward extent of the
   area owing to the drop off in buoyancy, but continued spread in the
   high-resolution forecast guidance regarding the timing and placement
   of convection precludes additional adjustments at this time. 

   Convection is also anticipated to develop across portions of
   north-central Nebraska into South Dakota during the early morning
   hours. These storms could produce isolated occurrences of hail,
   though uncertainty in the timing of convective initiation and
   maturation suggests this will primarily pose a threat in the Day 2
   time frame.

   ..Halbert/Weinman.. 09/04/2026

   .PREV DISCUSSION... /ISSUED 1146 AM CDT Fri Sep 04 2026/

   ...Midwest/southern Great Lakes to Mid-Atlantic States...
   As large-scale pattern amplification occurs, a belt of moderately
   strong northwesterly flow aloft will tend to overlie a frontal zone
   that extends from southern portions of Minnesota/Wisconsin
   east-southeastward to Pennsylvania/New Jersey. That said, ongoing
   convection has effectively modified/southward-shunted the effective
   front in several areas, and this will be the primary focus for storm
   intensification/redevelopment this afternoon, as well into the
   evening/late night.

   The steeper mid-level lapse rates will remain displaced west of the
   region, but ample low-level moisture (i.e. upper 60s/low 70s
   dewpoints) coupled with afternoon temperatures from the mid 80s to
   mid 90s will still support moderate to strong buoyancy near/south of
   the effective boundary. General storm re-intensification and/or new
   development is anticipated this afternoon, particularly south/ahead
   of currently ongoing clusters. Bowing line segments will be the
   predominant storm mode, with damaging winds as the most common
   hazard. 

   The threat for damaging wind gusts will extend southeastward into
   the Mid-Atlantic this afternoon and evening amid a very warm and
   moist airmass. Vertical shear will be weaker with southward extent,
   but ample moisture (i.e. PW values from 1.8" to 2"), strong
   buoyancy, and steep low-level lapse rates will support the potential
   for strong outflow capable of wind damage. Bowing line
   segments will be the primary storm mode here as well.

   ...Southern Wisconsin/southern Lake Michigan vicinity...
   The same general meteorological thematic scenario as described in
   the prior section applies to this region, and severe probabilities
   have been extended westward regionally, albeit for a somewhat
   conditional/uncertain scenario. A robust reservoir of
   moisture/locally extreme buoyancy will exist particularly to the
   south of ongoing midday convection. The region will be influenced by
   the amplifying upper ridge, but there is at least some possibility
   that storm redevelopment/intensification could occur near/east of
   the surface triple point and/or near outflow, with some related
   severe potential. Development could also occur toward the southern
   Lake Michigan vicinity later tonight as isentropic ascent increases
   near/northeast of the surface boundary.

   ...North-central Plains/Dakotas...
   While perceived severe probabilities remain below 5%, some
   strong/sporadic severe potential may exist also South Dakota, far
   northern Nebraska and/or southeast North Dakota later today through
   late tonight, although confidence in specific meteorological
   scenarios remains low.

   Weak lee troughing will contribute to modest low-level convergence
   from western Kansas across western/central Nebraska into southwest
   South Dakota. Dewpoints will likely be in the low to mid 60s F
   within this convergence zone, and steep mid-level lapse rates will
   contribute to moderate/strong buoyancy. Even so, warm mid-levels may
   prevent the airmass from completely destabilizing. The remaining
   convective inhibition, coupled with the presence of large-scale
   height rises across the region, casts doubt to whether successful
   convective initiation will occur. Any mature updrafts could result
   in strong downdraft, but uncertainty remains high. Other storm
   development could occur tonight, potentially as a byproduct of a
   subtle mid-level wave/ongoing convection ongoing across South Dakota
   this morning.

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