SPC AC 021950

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0250 PM CDT Sat Sep 02 2017

   Valid 022000Z - 031200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF THE TEXAS BIG COUNTRY...

   ...SUMMARY...
   Thunderstorm development across parts of the Texas Big Country late
   this afternoon and evening will be accompanied by some risk for
   strong wind gusts.

   ...20Z Outlook Update...
   The lower/mid tropospheric remnants of Harvey are becoming
   increasingly sheared/deformed across the Ohio Valley, in advance of
   a vigorous short wave trough within the mid-latitude westerlies, now
   digging into the upper Great Lakes region.  An associated plume of
   tropical/seasonably high moisture content air is in the process of
   slowly advecting east of the Atlantic Seaboard.  While it has not
   yet cleared immediate southern Mid Atlantic coastal areas, it
   appears that deep layer wind fields/shear and instability are
   becoming too marginal to justify the continuation of even low
   tornado/severe wind probabilities.  

   Elsewhere, the categorical marginal risk/5 percent severe wind
   probabilities across the Texas Big Country region have been adjusted
   slightly to account for current destabilization trends evident in
   observational data.  These trends also provide the basis for
   adjustments made to the categorical/10 percent thunder line.

   ..Kerr.. 09/02/2017

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Sat Sep 02 2017/

   ...NC/VA through this evening...
   The remnant circulation of Harvey will eject northeastward today
   through tonight to the Mid Atlantic and southern New England, in
   advance of an amplifying shortwave trough now over the upper Great
   Lakes.  A weak surface low will develop north-northeastward in
   association with the ejecting midlevel trough, along a stalled front
   from eastern NC into the VA Tidewater area.  Widespread clouds and
   rain this morning will slow surface heating along and east of the
   front, with substantial cloud breaks not expected until mid-late
   afternoon spreading northeastward across NC.

   Given poor midlevel lapse rates and the morning clouds,
   surface-based buoyancy will remain marginal through much of the day.
   There may be a narrow window of opportunity for storm formation this
   afternoon/evening along the front from northeastern NC into
   southeastern VA, on the northeast edge of any cloud breaks. 
   Deep-layer vertical shear will be sufficient for some storm
   organization, and a low-end threat for damaging gusts and perhaps a
   brief/weak tornado. 

   ...West central TX this afternoon/evening...
   A midlevel shortwave trough is moving south-southeastward from the
   TX Panhandle, along with a diffuse surface front/trough.  It appears
   there will be sufficient daytime heating, moisture, and ascent with
   the wave to support scattered thunderstorm development this
   afternoon across the Low Rolling Plains after about 21z, and storms
   will subsequently move southward through late afternoon/evening. 
   Inverted-v profiles will support a low-end threat for damaging
   outflow winds with these storms.

   ...MN Arrowhead/northern WI/western Upper MI this afternoon...
   In the wake of morning elevated convection, some clearing is
   expected from MN into WI and western Upper MI this afternoon. 
   Isolated thunderstorm development will be possible along a weak
   surface cold front, though the front will lag the primary belt of
   ascent preceding the associated midlevel trough.  The 12z INL
   sounding revealed steep lapse rates and the potential for low-topped
   thunderstorms as the sampled environment spreads southeastward this
   afternoon.  While an isolated strong storm will be possible, the
   threat for hail or wind gusts above severe thresholds appears too
   marginal to warrant the addition of any probabilities.

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