SPC AC 180600

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0100 AM CDT Thu May 18 2017

   Valid 181200Z - 191200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHERN AND CENTRAL KANSAS INTO NORTHWEST OKLAHOMA...

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   HIGH RISK AREA ACROSS PARTS OF OKLAHOMA AND KANSAS...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   MODERATE RISK AREA ACROSS PARTS OF THE SOUTHERN AND CENTRAL
   PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   ENHANCED RISK AREA ACROSS PARTS OF THE SOUTHERN AND CENTRAL PLAINS
   INTO LOWER MISSOURI VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER GREAT LAKES REGION...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE SOUTHERN AND CENTRAL PLAINS NORTHEASTWARD INTO THE MID
   MISSISSIPPI VALLEY AND NORTHEASTERN STATES...

   ...SUMMARY...
   An outbreak of severe thunderstorms with tornadoes, very large hail
   and wind damage is expected to develop across parts of the southern
   and central Plains from this afternoon into this evening and
   overnight. Strong long-track tornadoes will be likely from parts of
   central and southern Kansas southward into western Oklahoma. Large
   hail and wind damage will also be possible outside of these areas
   across parts of the southern and central Plains. A severe threat is
   also expected across parts of the lower Great Lakes region late this
   afternoon and early this evening.

   ...DISCUSSION...

   ...Tornado Outbreak Likely Across Parts of the Southern and Central
   Plains Today and Tonight...

   ...Southern and Central Plains...
   An upper-level low will move eastward across the Four Corners region
   and central Rockies today as a lead shortwave rotates around the
   southeastern side of the system into the southern and central
   Plains. At the surface, a very moist airmass will advect northward
   across Oklahoma into southern and central Kansas with surface
   dewpoints reaching the mid 60s to lower 70s by afternoon. Strong
   instability should be in place to the east of a well-developed
   dryline situated from just east of Dodge City southward across far
   western Oklahoma by mid-afternoon. Thunderstorms are forecast to
   initiate rapidly during the mid to late afternoon along the dryline
   with numerous severe thunderstorms moving east-northeastward across
   the southern and central Plains during the late afternoon and early
   evening. A very moist airmass, strongly unstable warm sector,
   favorable deep-layer shear and impressive low-level shear appears
   likely for a tornado outbreak from central and southern Kansas
   southward across western Oklahoma.

   At this time, surface maps show a very moist airmass across the
   eastern third of Texas with lower to mid 70s F dewpoints at most
   observation sites.  This airmass is advecting northward quickly and
   appears certain to overspread the moderate and high risk areas by
   this afternoon. Deterministic and hi-res solutions suggest that a
   strongly unstable airmass will be in place by afternoon with MLCAPE
   values reaching the 3000 to 4000 J/kg range. In addition, the exit
   region of a 45 to 60 kt mid-level jet is forecast to be located
   across northwest Oklahoma and south-central Kansas by late afternoon
   which will create strong deep-layer shear profiles over the top of a
   pristine airmass. NAM forecast soundings late this afternoon show
   0-6 km shear in the 50 to 60 kt range to the east of the dryline
   with gradually veering winds with height in the low to mid-levels.
   This wind profile will support discrete supercell formation. Backed
   surface winds to the southeast, strong speed shear in the boundary
   layer and a strengthening 40 to 50 kt low-level jet will be very
   favorable for tornadoes. Low-level shear is forecast to increase
   during the early evening with 0-3 km storm-relative helicities
   reaching the 350 to 450 m2/s2 range across the moderate and high
   risk areas. This will be favorable for strong tornadoes with several
   long track tornadoes moving across the high risk and moderate risk
   areas. High-end tornadoes will also be possible across the high risk
   area. Forecast soundings also show very steep 700 to 500 mb lapse
   rates of 8.0 C/km. This combined with the strong deep-layer shear
   will make large hail likely with supercells. The more intense
   supercells will likely produce hailstones of greater than 2 inches
   in diameter. By late evening, MCS formation should take place as
   several severe storm clusters move eastward across northern Oklahoma
   and eastern Kansas. Some tornadoes along with large hail and wind
   damage should be associated with this MCS.

   Further south across west-central Texas, a dryline is forecast to be
   situated from near Vernon, Texas south-southwestward to just west of
   San Angelo, Texas. Widely spaced thunderstorms are forecast to
   initiate just to the east of the dryline along this corridor. Strong
   instability, moderate deep-layer shear and steep mid-level lapse
   rates evident on forecast soundings will support the formation of
   supercells. An isolated tornado threat along with a potential for
   very large hail will be possible with supercells that develop during
   the late afternoon and early evening.

   ...Lower Great Lakes Region/Mid Mississippi Valley...
   An upper-level trough will move eastward across the Great Lakes
   region today. At the surface, a cold front is forecast to advance
   southeastward into the lower Great Lakes region by late afternoon as
   moderate instability develops along to the south of the front. 
   Thunderstorms are expected to develop just ahead of the front from
   northern Indiana east-northeastward into northern Ohio and western
   New York late this afternoon. This convection will be supported by
   increasing low-level convergence along the front and be aided by
   large-scale ascent associated with the upper-level trough. NAM
   forecast soundings along this corridor at 00Z/Friday show MLCAPE
   vales in the 1000 to 1500 J/kg range along with 0-6km shear values
   of 35 to 45 kt. This environment may support isolated supercell
   development with large hail possible. However, multicell may be the
   preferred storm mode due to front being oriented parallel to the low
   to mid-level flow. A wind-damage threat will be most likely with
   bowing multicell structures.

   ..Broyles/Mosier.. 05/18/2017

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