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    Day 2 Outlook >
Apr 27, 2017 1300 UTC Day 1 Convective Outlook
Updated: Thu Apr 27 12:55:41 UTC 2017 (Print Version | 20170427 1300Z Day 1 shapefile | 20170427 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 271255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Thu Apr 27 2017

   Valid 271300Z - 281200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   ALABAMA AND GEORGIA TO PARTS OF THE GREAT LAKES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
   SOUTHEASTERN COLORADO ACROSS PARTS OF KANSAS...NORTHWESTERN
   OKLAHOMA...AND PARTS OF THE OKLAHOMA AND TEXAS PANHANDLES...

   ...SUMMARY...
   Isolated severe wind gusts may accompany thunderstorms from portions
   of the Great Lakes region to the Southeast States today into this
   evening, and across portions of the central and southern Great
   Plains late this afternoon into this evening.

   ...Synopsis...
   In mid/upper levels, mean troughing will persist across the
   west-central U.S., as one leading perturbation ejects away, and
   others reinforce the associated cyclonic flow and height deficit.
   That leading perturbation -- currently located from a vorticity max
   over northern IL southward across MS -- will move to WV, OH and
   eastern lower MI by 00Z, phasing with a 500-mb low now over the
   Northwest Angle region of MN.  Thereafter, it will eject
   northeastward into Canada, as synoptic-scale amplification occurs to
   the troughing over the Rockies and High Plains. 

   At the surface, the 11Z analysis showed a low over the western shore
   of Lake Michigan between MKE-GRB, with cold front to southern IN,
   middle TN, southeastern MS, and the northwestern Gulf.  The low will
   eject northeastward across James Bay through tonight.  By 00Z the
   cold front is expected to reach western NY, WV, the TN/NC border
   region, and southern AL, becoming diffuse and quasistationary over
   the northwestern Gulf.  By 12Z the cold front should extend from the
   Hudson Valley region across western NC, losing definition farther
   southwest.  Meanwhile, cyclogenesis will occur today over the
   southeastern CO and northern TX/OK Panhandles region, at the
   intersection of a frontogenetic zone and a lee trough that extends
   from there northwestward to central/southwestern MT.  The resulting
   frontal-wave low should migrate to the GUY-HHF corridor by 00Z, then
   east-northeastward to south-central/southeastern KS overnight, with
   a cold front southwestward across the TX South Plains region by 12Z.
    
   ...Great Lakes to GA/AL...
   Widely scattered thunderstorms in broken bands and small clusters
   are expected to develop through afternoon along and ahead of the
   surface cold front, across the western rim of the outlook area,
   before moving eastward to northeastward.  The main concern will be
   isolated damaging gusts.  

   An ongoing, broken/ragged band of non-severe convection and precip
   from eastern KY across eastern TN, northern/western GA and parts of
   southern AL still may be capable of an isolated damaging gust --
   especially along the southern end where rich low-level moisture
   still characterizes the low-level inflow region.  Clouds and precip
   associated with this activity will hinder destabilization across
   affected areas and downstream in mid/upper levels, into the central
   Appalachians and upper Ohio Valley.  Still, a combination of patchy
   surface heating and low-level warm/moist advection will yield areas
   of at least marginally favorable buoyancy in the prefrontal boundary
   layer.  

   Meanwhile, large-scale ascent/cooling preceding the ejecting
   shortwave trough will aid in destabilization over northern parts of
   the outlook area, generally from the central Appalachians to the
   Ohio Valley and Great Lakes, partly offsetting the weaker low-level
   theta-e expected there.  Meanwhile, though the large-scale upper
   support is lifting away from the area, favorable low-level theta-e
   will persist across the Carolinas/GA portion of the outlook, along
   with favorable deep/speed shear to support a few organized
   multicells, line segments, and small bows, with transient supercell
   structures also possible.  Isolated severe gusts of 50+ kt may occur
   with convection in this corridor, primarily from midday through
   afternoon, along with other convective gusts that are subsevere but
   capable of damage to trees and utilities. 

   The severe threat will not be uniform across this corridor.  For
   now, smaller-scale uncertainties within the broad marginal-risk area
   preclude a more concentrated area of greater unconditional severe
   probabilities; however, a 15%/slight-risk equivalent area may be
   added during the day as mesoscale trends warrant. 

   ...South-central Plains region...
   Aforementioned amplification of the synoptic trough, and the passage
   of several associated lower-amplitude shortwaves/vorticity maxima,
   will lead to a combination of strengthening deep-layer ascent and
   cooling aloft through several related processes.  Those include: 
   low-level warm advection, low/middle-level frontogenetic/
   cyclogenetic forcing, lift beneath the left-exit region of the
   110-130-kt upper-jet core, and passing shots of DCVA.  In sum, these
   should act on residual (but still sufficient) moisture to lead to
   afternoon, surface-based, high-based thunderstorm development over
   the High Plains of CO into parts of the TX/OK Panhandles and
   southwestern KS.  Activity should move rapidly eastward across this
   region through early evening, when it will encounter progressively
   more stable near-surface inflow air and weaken.  

   Forecast soundings suggest steep low/middle-level lapse rates and
   weak CINH, even for temps in the 60s F and dew points in the 30s,
   with MLCAPE reaching the 300-500 J/kg range.  Though lack of
   moisture precludes a more-substantial, better-organized severe
   threat, the presence of strong flow and well-mixed subcloud layers
   supports the potential for at least isolated severe gusts from any
   sustained convection that develops.

   ..Edwards/Broyles.. 04/27/2017

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z

        
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