SPC AC 041240

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0740 AM CDT Mon Sep 04 2017

   Valid 041300Z - 051200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   FAR EASTERN ILLINOIS...NORTH-CENTRAL INDIANA...NORTHWEST OHIO AND
   FAR SOUTHERN LOWER MICHIGAN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   ENHANCED RISK AREA FROM THE MID MISSISSIPPI VALLEY NORTHEASTWARD
   INTO THE LOWER GREAT LAKES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   SLIGHT RISK AREA FROM SOUTHEAST KANSAS AND FAR NORTHEAST OKLAHOMA
   NORTHEASTWARD INTO THE LOWER GREAT LAKES AND NORTHEAST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHERN AND CENTRAL ARIZONA...

   ...SUMMARY...
   Severe thunderstorms are likely across the lower Great Lakes and
   Ohio Valley region, perhaps as far southwest as southeast Kansas,
   this afternoon and evening. Additional strong storms may impact the
   higher terrain and lower deserts of central and southern Arizona.

   ...Lower Great Lakes/Ohio Valley/Mid Mississippi Valley/Central
   Plains...
   An upper-level trough will dig quickly southeastward into the Upper
   Mississippi Valley today. A broad belt of west to west-southwesterly
   cyclonic flow will be in place ahead of the trough across much of
   the Great Lakes and Ohio Valley. At the surface, a cold front will
   advance southeastward today across the central Great Lakes, upper to
   mid Mississippi Valley and central Plains. A few thunderstorms
   should be ongoing along the front this morning. Ahead of the front,
   surface dewpoints are forecast to increase into the mid to upper 60s
   F by this afternoon.  This combined with warming surface
   temperatures should result in the development of moderate
   instability. Moderate instability is expected to first develop in
   the mid Mississippi Valley by late this morning and then spread
   east-northeastward into the Ohio Valley and southern Great Lakes
   this afternoon. This will aid convective initiation of surface-based
   thunderstorms along and just ahead of the front during the 20Z to
   22Z timeframe beginning in southern Lower Michigan and far northern
   Indiana. Thunderstorm coverage should expand quickly
   west-southwestward along the front during the late afternoon. The
   development of a linear MCS will be possible by early evening from
   the Great Lakes west-southwestward into the mid Mississippi Valley.

   At mid-levels, a 70 to 80 kt jet max associated with the upper-level
   trough will translate eastward across the upper Mississippi Valley.
   This jet max is forecast to increase in strength as it approaches
   the Slight and Enhanced risk areas. 0-6 km shear is forecast to
   reach the 40 to 50 kt range along the front as convection organizes
   late this afternoon. As a result, the combination of instability and
   deep-layer shear will be favorable for severe thunderstorm
   development. Although supercells will be possible early in the
   event, the models suggest that linear development will be favored
   due to low-level convergence and forced ascent along the front. This
   will likely make wind damage the greatest severe weather hazard as a
   line of storms moves southeastward across the region. An enhanced
   risk for wind damage will be most likely from far eastern Illinois
   northeastward across north-central Indiana into northwest Ohio, near
   the southwestern section of a 40 to 50 kt low-level jet. Isolated
   large hail may also develop with the more discrete rotating storms
   that develop during the late afternoon, when instability is forecast
   to be maximized. A marginal threat for hail and strong wind gusts
   will be possible across southwestern Missouri and southeast Kansas
   late this afternoon into this early evening.

   ...Arizona...
   An area of high pressure will be in place today from the Four
   Corners region northwestward into the Intermountain West.
   Thunderstorms are forecast to initiate on the southern periphery of
   the high in southern Arizona as instability develops this afternoon.
   Forecast soundings at Phoenix for 00Z/Tuesday show instability
   mainly in the mid-levels with super-adiabatic lapse rates from the
   surface to 700 mb. This may be enough for a few strong wind gusts
   especially if a cluster of multicells can get going just after peak
   heating.

   ..Broyles/Guyer.. 09/04/2017

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