SPC AC 271255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Wed Sep 27 2017

   Valid 271300Z - 281200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NORTHERN NEW ENGLAND...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EASTERN ARIZONA TO SOUTHWESTERN NEW MEXICO...

   ...SUMMARY...
   Marginal risks for severe thunderstorms exist this afternoon into
   early evening, over parts of northern New England, eastern Arizona
   and southwestern New Mexico.

   ...Synopsis...
   In mid/upper levels, troughing will persist over the western U.S.,
   but as part of a height-pattern evolution toward a Rex configuration
   from the end of this period into day 2.  By 12Z, the northeastern
   part of a high-amplitude northeastern Pacific ridge should break
   off, forming a closed, roughly 583-decameter 500-mb high over the
   northern Rockies.  Meanwhile, broadly cyclonic flow will continue
   over most of CA, the Great Basin, southern Intermountain region and
   southern Rockies.  The related mid/upper-level low over the lower
   Colorado River Valley area should weaken somewhat and eject
   northeastward toward the Four Corners or southern UT.

   Meanwhile, synoptic-scale, northern-stream troughing will amplify
   somewhat and shift eastward over the Great Lakes and eastern Canada.
   A strong associated shortwave trough -- now evident in moisture-
   channel imagery over northern ON -- will move eastward across
   central/southern QC through the period, deamplifying somewhat as a
   trailing perturbation moves over northern ON. 

   At the surface, the cold front related to the lead northern-stream
   shortwave was analyzed at 11Z from Georgian Bay southwestward across
   northwestern OH, southwestern IN, central AR, and into a precip
   plume between SAT-DRT.  By 00Z, the front should extend from eastern
   QC across northern NY, the upper Ohio Valley, to the upper TX
   coastal plain and Coahuila, arching northwestward over the
   Chihuahuan Desert to south-central NM. By 12Z, the front should
   extend from Downeast Maine through the northwestern quadrant of the
   T.S. Maria vortex over the Atlantic, to the north-central/
   northwestern Gulf and north-central Mexico. 

   ...Northern New England...
   One or two broken bands of thunderstorms should develop mainly over
   southern QC and organize as they move east this afternoon and early
   evening.  Activity should form along and ahead of the cold front,
   aided by a combination of low-level theta-e advection and related
   moistening, diabatic warming at the surface, frontal lift, and
   large-scale DCVA/ascent aloft ahead of the lead perturbation. 
   Modified model soundings yield MLCAPE generally 500-1000 J/kg,
   limited from being much stronger by the presence of weak midlevel
   lapse rates.  Still, favorable deep shear (e.g., effective-shear
   magnitudes 40-50 kt) is expected as the mid/upper trough approaches
   and winds aloft strengthen.  This should support the development of
   a mixed-mode scenario with multicells, small bows and supercells all
   possible.  

   ...Eastern AZ to southwestern NM... 
   Widely scattered thunderstorms are expected to form this afternoon
   from the eastern part of the Mogollon Rim eastward and southeastward
   across nearby portions of NM west of the Rio Grande, with isolated
   severe gusts/hail possible. 

   Low-level moisture continues to advect into this region to the
   northwest of the cold front, transported amidst post-frontal net
   easterly components of orographically influenced strength and
   duration.  This process should continue through the afternoon.  That
   process will combine with pockets of sustained diabatic surface
   heating to boost buoyancy, resulting in MLCAPE generally 500-1000
   J/kg over persistent cloud-free areas that also support subcloud
   mixing.  The theta-e increase, along with localized terrain lift,
   will reduce MLCINH and support convective development.  A 40-50-kt
   500-mb jetlet shifting around the southeastern quadrant of the
   mid/upper-level cyclone, combined with those low-level easterly
   components, will contribute to favorable deep shear for a few
   supercells.  Forecast soundings suggest effective-shear magnitudes
   around 50 kt, with elongated 0-3-km hodographs permitting SRH
   150-200 J/kg.

   ..Edwards/Jewell.. 09/27/2017

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z