SPC AC 171253

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0753 AM CDT Thu May 17 2018

   Valid 171300Z - 181200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   HIGH PLAINS BETWEEN NORTH DAKOTA AND NORTHWEST TEXAS...

   ...SUMMARY...
   Severe storms are expected this afternoon and evening across
   portions of the High Plains.

   ...Synopsis...
   The upper-air pattern is characterized by troughing over the West,
   and also east of the Mississippi River, with weak ridging over the
   High Plains.  That ridging is forecast to shift eastward gradually
   through the period, as the western trough (with primary vorticity
   max and small 500-mb cyclone initially over western NV) moves
   eastward to UT and AZ.  Several lower-amplitude/precursory
   shortwaves will eject through the downstream southwesterly flow
   field, including a perturbation now apparent in moisture-channel
   imagery over eastern UT.  This feature should reach eastern WY by
   00Z, with considerable strengthening due to convective vorticity
   augmentation thereafter.  By 12Z, the trough should be located over
   the central Dakotas and northern NE, perhaps with one or two
   embedded MCVs.  Meanwhile, east of the ridge, a binary
   mid/upper-level cyclone is evident with centers near MKC and over
   northeastern AR.  The southern lobe -- primarily related to an MCV
   -- should move southeastward with gradual weakening, while the
   northern one moves slowly southeastward across MO.

   At the surface, 11Z analysis shows an Arctic cold front from
   northern New England across southern ON, southern Lake Michigan, and
   southern WI, becoming quasistationary west-northwestward to a low
   between 2WX and DIK, to another weak low near GGW, then extending
   over southern AB.  This front should proceed southward/southeastward
   across the remainder of New England and parts of the Mid-Atlantic
   and portions of IL/IN/IA through the period, while the area of low
   pressure shifts eastward slowly across southern ND.  Temporarily
   shunted southward into northwestern SD by ongoing convection, the
   baroclinic zone should retreat to near or just north of the ND/SD
   line by late afternoon.

   A dryline and nearly collocated lee trough should remain largely in
   place, save for minor/mesoscale oscillations due to mixing and
   outflows, from the northern Plains low-pressure area southward
   across eastern CO.  Some eastward motion is likely into the TX
   Panhandle and South Plains/Permian Basin regions, due to relatively
   intense heating/mixing.  A secondary area of low pressure should
   develop over eastern CO this afternoon.  Otherwise, a broad,
   diffuse, slowly weakening, quasistationary frontal zone was analyzed
   from central/southwest TX across TN to the Delmarva region.  Little
   movement is expected with this feature today.

   ...Great Plains...
   Scattered thunderstorms are expected to develop across the High
   Plains in generally north-south corridors, near the dryline/lee
   trough.  Greatest potential convective concentration still appears
   to be over parts of the southern High Plains, where strong heating
   and relatively maximized theta-e is expected, and from northeastern
   CO to the western Dakotas near and south of the Arctic front --
   though isolated activity is possible in between and on either side. 
   The main concerns will be damaging gusts and large hail, though a
   tornado or two cannot be ruled out where favorable storm/boundary
   interactions may occur.

   Buoyancy should increase throughout the day along and east of the
   dryline/lee trough, as a combination of strong diabatic surface
   heating and moisture advection/transport occur.  Surface dew points
   generally should be in the 50s F over the central/northern Plains
   moist sector, and in the 50s to low 60s over the southern Plains,
   where considerable variability may occur due to vertical mixing
   within residual pockets of outflow air.  Steep midlevel lapse rates,
   associated with a well-developed EML, will spread over that moist
   sector, leading to a narrow corridor of favorable buoyancy, peaking
   over the southern Plains slight-risk area where surface heating will
   be most intense.  In that area, 2000-3000 J/kg MLCAPE is possible. 
   Given its higher boundary-layer theta-e and the presence of some
   favorable cloud-layer shear beneath the northern rim of a
   subtropical jet, relatively organized multicells and perhaps
   transient supercells are possible with the southern Plains activity.
    However, weak midlevel winds are expected, and storms should
   aggregate upscale relatively quickly.

   Farther north, some at least short-lived supercell potential is
   possible over portions of northeastern CO/southern NE Panhandle
   region where backed near-surface flow north of the lee low enhances
   hodographs, and near an area of backed flow and vorticity
   enhancement related to the frontal zone and low-pressure area in ND.
   A plume of large-scale ascent preceding the ejecting shortwave
   trough may support some relatively dense convective coverage and
   cold-pool aggregation over the northern Plains outlook area as well,
   south of the frontal zone.

   ...Southeastern States...
   Scattered thunderstorms are expected to develop this afternoon over
   a large area from MO eastward to VA/MD (near and south of a
   residual/weak frontal zone), and southward to southeastern MS and
   portions of the FL Peninsula.  Activity will be aided by areas of
   strong diabatic heating of a generally richly moist but also
   well-mixed boundary layer, weak MLCINH, sea-breeze boundaries near
   the coasts, outflows across the region, and an area of ascent near
   and southeast of the Ozarks/Mid-South perturbation.  Some convection
   associated with the latter is evident over western MS into
   northeastern AR and southern MO at this time.

   Within this broad area, isolated wet downbursts approaching severe
   limits and capable of minor damage are possible, and a few clusters
   may aggregate with well-developed mesobeta-scale cold pools.  At
   this time, the risk appears too isolated and/or dependent on
   uncertain localized boundary evolutions for an unconditional/
   categorical severe area, though mesoscale trends will be monitored
   for potential concentrations of more than localized/brief wind
   events.

   ..Edwards/Goss.. 05/17/2018

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