SPC AC 090547

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1247 AM CDT Mon Jul 09 2018

   Valid 101200Z - 111200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR CENTRAL AND
   EASTERN MONTANA INTO WESTERN AND NORTHERN NORTH DAKOTA...

   ...SUMMARY...
   Severe thunderstorms are possible over central Montana Tuesday
   afternoon and evening and later into northeast Montana and North
   Dakota during the evening and overnight hours.

   ...Synopsis...
   A vigorous mid-level shortwave trough is forecast to move from the
   Pacific Northwest into northwestern MT by early evening and east
   into central Saskatchewan, while evolving into a closed low by early
   Wednesday morning.  A surface low will develop in lee of the Rocky
   Mountain Front over north-central MT and develop eastward into
   northeast MT by early evening before the primary low occludes over
   central Saskatchewan.  A cold front will push eastward through
   central and eastern MT late in the day and after dark as a plume of
   moisture extends west from ND into MT.  Farther east, a cold front
   will move southward over the OH Valley and Mid-Atlantic states in
   association with a mid-level disturbance over the Lower Great Lakes.

   ...MT into ND...
   Low-level moisture (50s degrees F boundary-layer dewpoints) within
   easterly flow beneath a shield of steep 700-500mb lapse rates (8
   degrees C/km) will contribute to moderate buoyancy over MT.  A belt
   of strong mid-level flow will overspread the warm sector as a cap
   weakens by mid afternoon.  Widely scattered thunderstorms are
   forecast to develop over the higher terrain near Yellowstone and
   southwest MT.  Effective shear magnitudes (40-50 kt) will support
   organized storms, including the possibility for supercells, with
   large to very large hail and isolated severe gusts possible. 
   Farther north, stronger deep-layer forcing for ascent will likely
   result in scattered thunderstorms developing over southwest
   Saskatchewan into perhaps far northern MT.  Upscale growth into a
   forward-propagating severe squall line is possible within this
   corridor over southern Saskatchewan and glancing northeastern
   MT/northwest ND within the moisture plume---potentially resulting in
   a derecho.  Considerable uncertainty remains regarding this
   convective-scale dependent scenario, but the possibility for the
   southern portion of the squall line moving through northeast MT and
   later into northwestern ND (northern half of the Bakken) would
   translate to severe gusts (50-75kt).  It is possible later outlooks
   include higher severe (wind) probabilities.

   ...OH Valley...
   A cold front will push southward during the day into a reservoir of
   lower 70s degrees F dewpoints.  A mid-level shortwave trough will
   move from the northern Great Lakes into the Northeast/central
   Appalachians during the Day 2 period.  Weak large-scale forcing for
   ascent associated with this feature and convergence near the front
   will combine with diurnal heating and lead to widely scattered
   thunderstorms developing by mid afternoon.  Isolated strong to
   locally severe gusts (50-60 mph) capable of wind damage are possible
   with the more intense thunderstorms.  This activity will weaken
   during the evening coincident with the loss of heating.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   2%     - Marginal
   Wind:     15% SIG - Slight
   Hail:     15% SIG - Slight

   ..Smith.. 07/09/2018

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   NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z