SPC AC 191259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Wed Apr 19 2017

   Valid 191300Z - 201200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS FAR
   EASTERN NEBRASKA INTO MUCH OF IOWA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
   PLAINS TO THE UPPER MS RIVER VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   SOUTH-CENTRAL PLAINS TO THE SOUTHERN GREAT LAKES...

   ...SUMMARY...
   Severe thunderstorms capable of large hail, damaging winds, and a
   few tornadoes will be possible from the central Plains to the upper
   Mississippi Valley mainly from mid-afternoon into tonight.

   ...Central Plains/middle MO Valley to upper MS Valley...
   Embedded within a moderately strong belt of west-southwesterly
   mid-level winds, water vapor imagery features an eastward-moving
   shortwave trough from WY into the north-central High Plains. This
   impulse will continue east-northeastward and reach the Upper Midwest
   tonight. 

   To the north of a roughly west/east-oriented front, bands of
   convection (strong in some cases) are ongoing this morning across
   eastern NE into southern IA and northwest IL. A few of the stronger
   cores could produce hail this morning mainly from far eastern NE
   into western IA. On the southern fringes of this early-day
   convection and related cloud cover, the air mass is expected to
   become moderately unstable (1500-2000 J per kg MLCAPE) late this
   afternoon. This will especially be the case across southeast
   NE/northeast KS ahead of a southeastward-moving cold front, and
   across IA near and south of a northward-shifting warm front. Current
   thinking is that surface-based thunderstorms will initially develop
   by mid/late afternoon across east/southeast NE and northeast KS into
   far western IA. 

   Very steep mid-level lapse rates, moderate buoyancy and
   supercell-favorable wind profiles will support the possibility of
   large hail. A tornado risk will also exist late this afternoon into
   early/mid-evening, particularly across west/central IA in vicinity
   of the northward-spreading effective warm front, where near-50-kt
   winds around 1-2 km AGL will contribute to 0-1 km SRH on the order
   of 250-300 m2/s2.

   As forcing for ascent and low/mid-level winds increase, storms will
   expand east/northeastward during the evening and overnight into
   additional parts of IA, southern MN and the adjacent upper MS Valley
   vicinity, including both elevated storms capable of hail and
   near-surface-based storms capable of damaging winds/brief tornado
   risk near the north/northeastward-shifting warm front.

   Farther south, other severe thunderstorms capable of large hail and
   locally damaging winds will persist this evening into the overnight
   across KS into western/northern MO near and ahead of the
   southeastward-moving cold front. 

   ...Southern Great Lakes to upper OH Valley...
   Additional moistening will occur across the region today in advance
   of a slow-southeastward-moving front. A weak westerlies-embedded
   impulse, related to convection near the IA/IL Quad Cities this
   morning, will continue eastward today and have at least some
   influence on increasing diurnal convective development this
   afternoon from northeast IL into northern portions of IN and OH.
   Sufficient heating and destabilization in the presence of around 35
   kt of effective shear may support storms capable of isolated severe
   hail and locally damaging winds.

   ..Guyer/Jewell.. 04/19/2017

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