SPC AC 150559

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Thu Mar 15 2018

   Valid 151200Z - 161200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   There is a potential for thunderstorms from western Oregon, northern
   California, across the Four Corners States, and the Texas South
   Plains during the afternoon to early evening.  Then, thunderstorms
   will develop tonight across the central Plains to lower Missouri
   Valley, and across southeast Texas into the lower Mississippi
   Valley.  Severe storms are not expected.

   ...Synopsis...
   A large closed cyclone is expected to evolve from off the Pacific
   Northwest Coast into the northern Great Basin and southwest Canada
   this forecast period.  This system will become cutoff from the
   large-scale wind pattern, as a split-flow regime develops from the
   Pacific into the western half of the United States.  Within the
   southern stream, a shortwave trough moving through southern CA
   around 12Z today should become negatively tilted as it tracks
   through the Four Corners States into the central Plains.  The track
   of this trough and a couple of weaker impulses in the lower
   latitudes are expected to result in the breaking down of an upper
   ridge over the Plains to the northwest Gulf Coast region.

   ...Four Corners States...
   Height falls (60-100-meter per 12-hours at 500 mb) and cooling
   midlevel temperatures will steepen lapse rates as profiles moisten
   within the exit region of a midlevel jet accompanying the southern
   CA trough.  Despite scant precipitable water, buoyancy should
   increase for convection to deepen from the eastern Great Basin into
   the central Rockies through the afternoon.  Given 500-mb
   temperatures of -22 to -28 C, the strongest updrafts are forecast to
   reach heights deep enough for lightning discharge.  Although bulk
   shear will be too weak to support organized storms, the cold
   temperatures aloft and MUCAPE of a few hundred J/kg suggest small
   hail could occur.  

   ...Western OR and northern CA...
   The coldest 500-mb temperatures (less than -30 C) attendant to the
   aforementioned large closed cyclone will spread south-southeast
   across western OR into northern CA.  Forcing for ascent attendant to
   height falls and midlevel impulses rotating through the base of the
   cyclone combined with weak instability should allow for thunderstorm
   development through the afternoon.

   ...TX South Plains into southeast NM...
   Deep boundary-layer mixing is expected across this region through
   the afternoon.  Weak height falls within the southern extension of
   the southern CA trough moving across this region by late afternoon
   and low-level confluent flow in vicinity of the dryline should allow
   for a few high-based thunderstorms to form.  This activity should
   weaken this evening. 

   ...Central Plains to lower Missouri Valley into northern AR...
   In response to height falls attendant to the approaching southern CA
   shortwave trough, a lee surface low is expected to develop over
   eastern CO today, and move into northwest to north-central KS
   tonight.  A warm front will develop eastward from this low through
   central or northern KS into southern MO and far northern AR, with
   this boundary generally being the southern limit for elevated storms
   after dark to early Friday morning.  Exit region of an 80-kt 500-mb
   speed max should aid convective development across the High Plains
   of northeast CO into parts of central KS by early evening.  This
   activity will develop and spread eastward through tonight, expanding
   in coverage as a warm advection increases across the central Plains
   and MO within the nose of a broad, strengthening southerly low-level
   jet.  Precipitable water generally less than 1 inch and the
   combination of weak elevated instability and shear should preclude a
   severe threat across this region.

   ...Southeast TX into lower Mississippi Valley...
   A weak midlevel impulse moving into northern Mexico at this time is
   expected to track eastward today, reaching eastern TX into the lower
   Mississippi Valley tonight.  The southern branch of a strengthening
   low-level jet is expected to focus across the upper TX Coast this
   evening before shifting into LA after midnight.  A substantial
   moisture surge (precipitable water 1-1.5 inches) will be associated
   with this jet.  This combined with warm advection and weak forcing
   for ascent attendant to the northern Mexico impulse is expected to
   aid elevated thunderstorms primarily during the latter half of the
   period.  Weak buoyancy is expected to preclude a severe-weather
   threat.

   ..Peters/Cook.. 03/15/2018

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