SPC AC 271940

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0240 PM CDT Wed Sep 27 2017

   Valid 272000Z - 281200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON OR EARLY EVENING ACROSS PARTS OF NORTHERN NEW ENGLAND...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHWESTERN NEW MEXICO INTO SOUTHEASTERN ARIZONA...

   ...SUMMARY...
   Marginal risks for severe thunderstorms exist this afternoon into
   early evening, over parts of northern New England, eastern Arizona
   and southwestern New Mexico.

   ...20Z Outlook Update...
   Adjustments to the categorical and probabilistic outlook lines have
   been made, mostly in an attempt to better account for ongoing trends
   evident in the latest observational data.  Perhaps the most notable
   change is the reduction in thunderstorm probabilities below the
   categorical (10 percent) threshold across much of Texas and adjacent
   areas of the south central United States.

   Cool/stable low-level  air has overspread most of the high Plains to
   the lee of the southern Rockies, where mid-tropospheric lapse rates
   are also generally weak and unlikely to destabilize appreciably
   through the remainder of this period.  Moist, potentially unstable
   boundary layer air is largely confined to Deep South Texas, where
   scattered thunderstorm activity is ongoing.  A categorical thunder
   outlook is also being maintained within a narrow corridor roughly
   along the I-35 corridor of central Texas.  Despite weak low-level
   convergence in the presence of general surface ridging, weak
   destabilization is evident within a zone of stronger differential
   surface heating across this region.  Ongoing convection near the
   Dallas/Forth Worth metroplex has recently support lightning, and
   this could continue into early evening.

   ..Kerr.. 09/27/2017

   .PREV DISCUSSION... /ISSUED 1114 AM CDT Wed Sep 27 2017/

   ...ME/NH/VT...
   Water vapor imagery shows a strong shortwave trough moving across
   Ontario.  Lift ahead of this system is expected to result in
   scattered afternoon/evening showers and thunderstorms moving from
   Quebec into northern New England.  Strong heating is occurring in
   this area, with afternoon MLCAPE values expected to exceed 500 J/kg.
   Forecast soundings show favorable deep layer vertical shear for
   rotating/bowing storm structures and the chance of damaging wind
   gusts in the stronger cells - mainly after dark.  The primary
   factors that will likely limit a more robust severe event are poor
   lapse rates and only marginal instability.  Given 12z CAM guidance,
   parts of far northern ME have the greatest risk of more organized
   severe storms.  However, will maintain the MRGL risk category due to
   the very limited area of concern.

   ...NM/AZ...
   An upper low continues to affect the southwest US, with deep
   southwesterly flow aloft across parts of NM/AZ.  The threat of
   isolated strong/severe will develop again this afternoon along a
   weak baroclinic zone that is slowly moving westward across the
   region.  Similar to the last several days, a storm or two may
   produce hail and/or gusty winds later today, but coverage should be
   sparse.

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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z