NWS Storm Prediction Center - Day 2 Convective Outlook

May 25, 2018 1730 UTC Day 2 Convective Outlook
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SPC Day 2 1730Z Outlook Categorical SPC Day 2 1730Z Outlook Categorical Legend

 Forecast Discussion - Day 2 Convective Outlook

   SPC AC 251721

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1221 PM CDT Fri May 25 2018

   Valid 261200Z - 271200Z


   Isolated severe storms with hail and strong wind gusts are possible
   on Saturday across Montana into the western Dakotas.  A marginal
   severe threat may also develop across parts of eastern Wisconsin
   into upper Michigan. A tornado or two may also occur over parts of
   the southern and western Florida peninsula.

   ...Northern Rockies/Northern High Plains...
   An upper-level ridge will move slowly northeastward across the
   northern Plains on Saturday as an upper-level low remains over the
   Intermountain West. At the surface, a trough will deepen in eastern
   Montana on Saturday as a cold front advances southeastward into
   north-central parts of the state. Surface dewpoints may reach the
   mid 50s F along the surface trough by afternoon. As surface
   temperatures warm during the day, a narrow corridor of moderate
   instability should develop from northwest South Dakota extending
   northwestward into east-central Montana. A capping inversion is
   forecast to weaken by mid afternoon, allowing for the initiation of
   isolated convection. Convective coverage may increase during the
   evening mainly southwest of the upper-level ridge where some
   large-scale ascent may be present.

   NAM and GFS forecast soundings at 00Z/Sunday across eastern Montana
   disagree on how much instability will develop across eastern
   Montana. The NAM is more aggressive and has moderate instability in
   place by afternoon. In addition, 0-6 km shear is forecast to be in
   the 30 to 40 kt range with low to mid-level lapse rates around 8.0
   C/km. This would support an isolated severe threat with potential
   for hail and strong gusty winds. However, any severe threat will be
   dependent upon how much instability can develop. Due to model
   differences in instability and concerns about effects from the
   upper-level ridge, will maintain a marginal risk across the northern
   Rockies and northern High Plains for Saturday.

   ...Great Lakes...
   A shortwave trough is forecast to move into the western Great Lakes
   region on Saturday as a cold front moves across the Upper
   Mississippi Valley. Ahead of the front, surface dewpoints in
   northeast Wisconsin and Upper Michigan are forecast to be in the
   lower 60s F. In response to surface heating, a small pocket of
   moderate instability may be in place by afternoon. Both the NAM and
   GFS suggest that thunderstorm development may occur during the mid
   to late afternoon ahead of the front near the Wisconsin-Upper
   Michigan state line. MLCAPE values above 1000 J/kg along with 0-6 km
   shear near 30 kt would be enough for a marginal severe threat.
   Unidirectional winds and steep low-level lapse rates would favor
   short line segments with strong gusty winds. Hail could also occur
   in areas that destabilize the most.

   The National Hurricane Center is forecasting Tropical Storm Alberto
   to move north-northwestward into the eastern Gulf of Mexico on
   Saturday. Rainbands are forecast to extend eastward to the Florida
   Peninsula where thunderstorm development may take place during the
   day on Saturday. NAM forecast soundings in western Florida Saturday
   afternoon gradually increase low-level shear. By 00Z/Sunday, 0-1 km
   shear is forecast to be 20 to 25 kt in western Florida which may be
   enough for isolated storm rotation and a marginal tornado threat.

   Tornado:   2%     - Marginal
   Wind:      5%     - Marginal
   Hail:      5%     - Marginal

   ..Broyles.. 05/25/2018



Data courtesy the Storm Prediction Center
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