SPC AC 180556

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1256 AM CDT Fri May 18 2018

   Valid 181200Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
   HIGH PLAINS INTO THE SOUTHERN PLAINS...

   ...SUMMARY...
   Severe thunderstorms are possible near the Front Range of the
   Rockies eastward into parts of the central and southern Plains this
   afternoon and tonight.

   ...Synopsis...
   Persistent upper troughing, which has been in place across the
   western CONUS for the past several days, will lose amplitude as the
   primary upper low embedded in the trough continues to fill and a
   shortwave trough moves through the southern periphery of upper
   trough into the Plains. Farther north, a more well-defined shortwave
   trough will move through the Canadian Prairie provinces into
   northwestern Ontario. Weak upper low initially over the middle MS
   Valley will weaken as it gradually drifts northeastward. 

   At the surface, a cold front will move southeastward from the
   northern Plains into the central Plains, likely extending from a low
   over norther MN southwestward into southeast WY by 00Z Saturday. Lee
   low currently centered over southeast CO will deepen throughout much
   of the day before shifting eastward/southeastward overnight along
   the advancing frontal boundary. Strong to severe thunderstorms are
   anticipated from the southern High Plains through the central High
   Plains and eastward into the southern/central Plains as the
   approaching frontal boundary and shortwave trough interact with the
   moist and unstable airmass over the region.

   ...Southern/central High Plains into the southern/central Plains...
   Outflow along the southern flank of the ongoing MCS across NE and SD
   will likely provide the northern extent of the more probable severe
   threat today given the expectation that the airmass across much of
   the NE and SD will have difficultly recovering enough by the
   afternoon to support robust convection. Some storms are possible as
   a result of airmass destabilization and warm-air advection into the
   remnant outflow boundary. Given the likely elevated nature of these
   storms and generally weak flow, primary severe threat with these
   storms is expected to be isolated hail.

   Higher severe probability exists farther south from the southern NE
   Panhandle into the eastern TX Panhandle/western OK. In this area,
   favorable low-level moisture, steep mid-level lapse rates, and
   strong daytime heating will support airmass destabilization ahead of
   the approaching shortwave trough. Forcing for ascent associated with
   this shortwave trough, currently moving into AZ, is expected to
   result in convective initiation across the central and southern High
   Plains during the late afternoon. The higher terrain of the central
   Rockies will likely result in initiation earlier than areas farther
   south where a combination of modest synoptic forcing for ascent and
   convergence along the dryline will provide the impetus for
   convective initiation.

   Given the relatively modest mid-level flow, storms will likely
   quickly transition to a predominantly linear/cluster mode. Steep
   mid-level lapse rates support a hail threat, even if storms do trend
   linear quickly.  Strong wind gusts also likely, particularly within
   the more organized bowing segments. Better linear organization is
   possible today versus Thursday given the anticipated presence of a
   stronger low-level jet. If a well-organized MCS does develop, a
   corridor of more concentrated strong wind gusts is possible.
   Additionally, the strengthening low-level jet will lengthen
   hodographs and provide low-level shear profiles that are a bit more
   conducive to tornadoes. Linear convective mode will likely limit
   tornado probability but some low-end threat will exist along the
   southern flank of any convective lines as well as with any discrete
   convection that persists.  

   ...Much of the Southeast and Mid-Atlantic States...
   Widespread convection is anticipated during the afternoon as the
   moist airmass over the region destabilizes amidst daytime heating.
   Weak vertical shear across the area suggests that the only severe
   threat would come from water-loaded downbursts which result from
   updraft strengthening due to cell mergers. Mesoscale nature of this
   phenomena and anticipated isolated coverage preclude the need for
   any severe probabilities with this outlook.

   ..Mosier/Cook/Wendt.. 05/18/2018

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