SPC AC 131959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0259 PM CDT Sun May 13 2018

   Valid 132000Z - 141200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
   APPALACHIANS TO THE MID-ATLANTIC...ACROSS PARTS OF NORTHERN
   IL/IN...AND ACROSS PARTS OF THE SOUTHERN/CENTRAL PLAINS...

   ...SUMMARY...
   Scattered severe storms remain possible across parts of the
   Appalachians to Mid-Atlantic and southern/central Plains late this
   afternoon and evening, as well as portions of northern Illinois and
   Indiana late tonight.

   ...Appalachians to Mid-Atlantic...
   Have made only minor adjustments to damaging wind probabilities,
   primarily to account for some southwestward advance of the front
   across Maryland and northern Virginia. Also expanded the marginal
   risk (for hail/wind) slightly northward over western Pennsylvania,
   where satellite and meso-analysis trends suggest some ongoing
   heating/modest destabilization ahead of developing convection.
   Farther southwest, convection also appears to be deepening within a
   corridor of locally enhanced low-level confluence over eastern Ohio.
   These cells should gradually spread east/southeast as they mature,
   with an attendant threat for damaging winds and large hail.
   Therefore, the slight risk remains unchanged.

   ...Illinois into Indiana...
   Sizable uncertainty remains with the potential for severe weather
   late tonight. Ongoing convection over southeastern Iowa remains
   north of the surface front and elevated atop a stable layer.
   Moreover, visible satellite trends suggest it is unlikely that this
   complex's trajectory is favorable for eventually ingesting
   surface-based parcels. In turn, these cells may weaken with eastward
   extent, as elevated storm-relative inflow becomes less favorable for
   convective maintenance.

   The primary severe threat should thus remain with initially elevated
   convection developing over Illinois late tonight. Nonetheless,
   uncertainty persists with regards to an eventual convective complex
   becoming at least partially surface based through the overnight as
   it moves into Indiana. Assuming it gains some southward component of
   propagation and becomes rooted near the surface, a damaging wind
   threat would evolve late tonight. Therefore, no adjustments are made
   to the ongoing slight risk, but may be needed with the 01Z update.

   ...Central Plains...
   The marginal risk was expanded westward over parts of southern
   Nebraska, where the terminus/focus of a low-level jet should combine
   with steep mid-level lapse rates to foster some hail threat with
   elevated cells overnight.

   Additionally, the marginal risk in Colorado was expanded slightly
   southward to account for visible satellite trends and the potential
   for more eastward propagation with a discrete cell or two that may
   develop and acquire mid-level rotation.

   ..Picca.. 05/13/2018

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Sun May 13 2018/

   ...Central Appalachians to Mid-Atlantic States...
   Quasi-stationary front runs west/east across central OH into far
   southwest PA before arcing southeast across northern/eastern VA.
   Robust diabatic heating will occur in the warm sector within the
   eastern extent of the massive EML plume emanating from the central
   states. While a mid-level speed max is passing east of the region,
   sufficient deep-layer shear will persist for a few supercells amid
   MLCAPE around 600-1200 J/kg. Severe hail and damaging wind will be
   the primary hazards, but a tornado is possible with any supercell
   moving along the SRH-enriched front. Have upgraded to Slight Risk to
   account for this potential.

   ...TX/OK/KS...
   Influx of richer low-level moisture beneath very steep mid-level
   lapse rates associated with a massive EML should result in large
   warm-sector buoyancy to about 3000 J/kg to the east of the TX
   dryline and southeast of a quasi-stationary front in KS. Scattered
   storms (more numerous than previous days) should develop along the
   dryline in the late afternoon. Deep-layer shear will be modest
   (effective values around 20-30 kt) even with a veering wind profile
   with height. Even so, the impressive thermodynamic environment will
   favor large hail and severe wind gusts with multicells and a few
   embedded supercells. Convection should largely weaken as inhibition
   strengthens greatly east of the dryline after sunset. 

   ...IA/IL/IN...
   Low-level warm advection will again be centered on the IA to
   northern IL area tonight resulting in widespread elevated convection
   north of the quasi-stationary front expected to lie along the IA/MO
   border east through central IL/IN. While elevated buoyancy will be
   sufficient for a severe hail risk, rapid evolution to a
   west/east-oriented cluster should marginalize this threat. However,
   some CAMs suggest that this elevated MCS will organize into a
   forward-propagating bow across parts of northern IL/IN by 09-12Z
   Monday morning where the surface front becomes increasingly diffuse.
   A risk for strong to isolated severe wind gusts may develop in spite
   of large inhibition for surface-based parcels.

   ...CO/WY...
   Repeated initiation of widely scattered storms is anticipated during
   the late afternoon and evening within a persistent upslope flow
   regime. Winds aloft and vertical shear along the eastern periphery
   of the Great Basin upper low will be sufficient for a couple
   supercells developing off the Front Range. Severe hail should be the
   primary hazard. Given a pervasive stratus deck across much of the
   central High Plains, the tornado/wind risk will likely be confined
   to where stronger diabatic heating can occur. This appears most
   probable near/north of the Denver cyclone where at least pockets of
   greater heating should occur along the Palmer Divide.

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