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    Day 2 Outlook >
Aug 16, 2018 1300 UTC Day 1 Convective Outlook
Updated: Thu Aug 16 12:57:40 UTC 2018 (Print Version | 20180816 1300Z Day 1 shapefile | 20180816 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 161257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Thu Aug 16 2018

   Valid 161300Z - 171200Z


   The greatest concentration of severe-storm potential appears to be
   this afternoon and evening over portions of Oklahoma, eastern Kansas
   and western Missouri.

   In mid/upper levels, a high is forecast to build back over the
   southern Rockies through the period and onward, with ridge
   amplification northward across WY/MT, while the main belt of
   westerlies remains to the north of the Canadian border.  West of the
   ridge, a broad shortwave trough now located offshore of the Pacific
   Northwest and northern CA will move ashore WA/OR/northwestern CA
   this evening and tonight.  East of the ridging, a small mid/upper-
   level cyclone -- initially centered over the YKN area based on
   latest moisture-channel imagery -- is forecast to devolve to an
   open-wave trough.  By 00Z the trough should reach southern MN, IA
   and northern MO.  By 12Z it should reach northern IN and central/
   western IL.  A separate/small southern perturbation, enhanced by an
   MCV over the TX Panhandle, should move eastward to east-
   southeastward across OK through the period.  This feature should
   reach western AR, southeastern OK and north-central TX by 12Z,
   loosely phased with the system now over the north-central Plains.

   At the surface, 11Z analysis shows a wavy frontal zone,
   quasistationary from southern ME to southern WI, then a wavy cold
   front from a weak low over the LSE area southwestward through a
   couple more weak lows to southeastern CO.  As the Midwestern
   perturbation aloft advances, a frontal-wave low should form over
   Lower MI and move to southeastern ON by the end of the period, with
   the cold front southwestward across southern IL, southwestern MO,
   northwestern OK, and extreme northeastern NM.  At 11Z, a secondary
   warm-frontal zone was evident from the northern TX Panhandle across
   northern OK to southwestern MO, and should drift northward through
   the period before being overtaken by the larger synoptic cold front

   ...South-central Plains to Ohio Valley...
   The 15%/slight-risk outlook area represents a spatial amalgam of
   initially separate regimes offering hail and damaging wind -- one
   today into this evening over parts of OK and perhaps extreme
   southern KS, and another this evening and overnight from central KS
   to western MO mainly offering hail with isolated damaging wind.

   Widely scattered diurnal thunderstorms are possible across much of
   the combined marginal/slight categorical outlook areas -- with some
   relative clustering possible in the northern parts over IA/IL ahead
   of the mid/upper circulation, the eastern parts over the Ohio/
   Tennessee Valleys, and the southwestern rim over the southern Plains
   ahead of the MCV and related perturbation aloft.  Additional
   convection should develop this evening in a zone of low-level lift,
   near and north of the blending frontal zones over central/eastern

   Midlevel lapse rates are generally expected to diminish with
   eastward extent, thereby resulting in a west-to-east decrease in
   CAPE, given less of a change in low-level moisture/theta-e.  The
   strongest winds aloft, south of the weakening mid/upper trough and
   north of the southern/secondary warm front -- will be displaced from
   the largest low-level theta-e values along and south of the warm
   front.  Forecast soundings suggest preconvective MLCAPE may reach
   2500-3500 J/kg in central and northern OK, decreasing to the
   1000-2000 J/kg range over central/eastern KS.  Deep-layer forcing
   ahead of the MCV should encourage activity forming to its east over
   western/northern OK to move eastward into that favorable
   instability, with multicellular, clustered to linear modes
   predominant amidst the modest deep shear.  Slightly stronger mid/
   upper winds and upper-level lapse rates should exist over KS this
   evening, with semi-discrete to clustered modes possible, though weak
   low-level flow will still modulate overall organization.

   ...Interior OR...
   Isolated to scattered, high-based thunderstorms are expected this
   afternoon east of the Cascade crest, offering isolated severe gusts.
   Activity should form as MLCINH weakens and the boundary layer deeply
   mixes, amidst marginal low-level moisture.  Steepening of deep-layer
   lapse rates will be related to both subtle large-scale ascent
   increasing ahead of the offshore trough, and favorable surface
   heating.  Strengthening mid/upper winds will encourage northeastward
   storm motions, with some downward momentum transfer and evaporative
   acceleration through the mixed layer to the surface. Lack of greater
   moisture and related weakness of buoyancy will be the main factors
   mitigating greater severe potential, with MLCAPE generally remaining
   under 500 J/kg.

   ..Edwards/Peters.. 08/16/2018



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