SPC AC 141938

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0238 PM CDT Thu Sep 14 2017

   Valid 142000Z - 151200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE FOUR CORNERS STATES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   FOUR CORNERS SLIGHT RISK AREA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE NORTH-CENTRAL U.S....

   ...SUMMARY...
   Strong to severe storms may occur this afternoon and early evening
   across the greater Four Corners region with locally damaging wind
   and large hail the main threats. A few strong to severe storms with
   hail and strong wind gusts are also possible from the eastern
   Dakotas into northern Minnesota this evening and tonight.

   ...Discussion...
   Latest model guidance and observations, as well as evolution of
   ongoing convection, suggest that forecast reasoning from the prior
   outlook remains valid.  As such, excepting minor line adjustments,
   no changes to the ongoing outlook appear necessary at this time.
   Thunderstorms across the northern Arizona/southern Utah and into the
   Four Corners region will continue to pose some risk for local severe
   weather, while limited severe risk may possibly evolve over portions
   of the north-central U.S. later tonight.

   ..Goss.. 09/14/2017

   .PREV DISCUSSION... /ISSUED 1128 AM CDT Thu Sep 14 2017/

   ...Southwest US...
   Water vapor loop shows an upper low tracking eastward across
   southern CA.  Large-scale forcing ahead of the low will continue to
   spread eastward, promoting the development of scattered showers and
   thunderstorms across parts of AZ/UT and eventually into western
   CO/NM.  Visible imagery shows that strong heating will occur in most
   of this region, leading to steep low-level lapse rates this
   afternoon. Surface dewpoints are rather low, but a pocket of
   mid-level cooling associated with the upper low will help to yield
   afternoon MUCAPE values of 500-1500 J/kg.  Vertical shear is also
   forecast to become favorable for organized or occasionally rotating
   cells capable of hail and/or gusty winds.  Please refer to Mesoscale
   Discussion #1644 for further short-term details.

   ...MN...
   A weak surface cold front remains across northern MN today. 
   Afternoon heating along the boundary coupled with dewpoints in the
   60s will yield moderate CAPE values later today.  Most 12z CAM
   guidance shows multiple rounds of convection along and north of the
   boundary this evening and tonight.  A few of the more persistent
   storms may produce hail and gusty winds.  There was some
   consideration for an upgrade to SLGT risk.  However, it appears the
   higher convective coverage will come after midnight, when
   thermodynamic fields will be weakening.  Therefore have chosen to
   maintain the MRGL risk area.

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