SPC AC 251306

   Day 1 Convective Outlook CORR 1
   NWS Storm Prediction Center Norman OK
   0806 AM CDT Tue Jul 25 2017

   Valid 251300Z - 261200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EASTERN SOUTH DAKOTA AND SOUTHWESTERN MINNESOTA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   SOUTHERN SOUTH DAKOTA TO THE SOUTHWESTERN LAKE SUPERIOR AREA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AND EXTENDING TO MUCH OF THE CENTRAL HIGH PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE WESTERN
   GREAT BASIN REGION...

   CORRECTED FOR WIND GRAPHIC

   ...SUMMARY...
   Severe thunderstorms with damaging winds, large hail and the
   possibility of tornadoes are forecast across portions of the
   northern Plains and the Upper Mississippi Valley this afternoon and
   evening.  Isolated severe thunderstorm gusts also are possible over
   parts of the Great Basin.

   ...Synopsis...
   The northern-stream pattern aloft will become more zonal through the
   period, as troughing exits the Northeast and weakens across the
   downstream North Atlantic.  A strong shortwave trough -- accompanied
   by a compact mid/upper-level cyclone now located over the MB/SK
   border -- is forecast to move eastward across much of northwestern
   ON by the end of the period, with at least weak height falls
   extending across much of MN/WI and Upper MI.  A broad, messy area of
   convectively generated/enhanced vorticity -- now evident in
   moisture-channel imagery over parts of UT, western CO and eastern NV
   -- will eject northeastward, evolving into a more distinct shortwave
   trough over eastern SD and central NE by 12Z.  Meanwhile, a
   mid/upper low currently stationary over the northern CA coast will
   remain so today, then begin to move eastward across northern CA late
   tonight.

   At the surface, a wavy frontal zone was analyzed from a low offshore
   from the Mid-Atlantic across southern VA, KY, southern MO, and
   northwestward generally up the Missouri Valley to another low in
   central SD.  Under weak flow aloft, disorganized general
   thunderstorms will be possible near and south of the frontal zone
   over the Southeast today.  The western segment of the boundary will
   move northeastward as a warm front through southern/central MN.  A
   weak/slow-moving cold front -- now extending from the SD low to
   south-central WY -- will be overtaken from the northwest by a
   stronger front initially positioned from northern ND to
   southeastern/ south-central MT.  By 12Z, the combined boundary
   should extend from western Lake Superior across northern/western IA
   to a frontal-wave low over south-central NE or north-central KS,
   then southwestward to southeastern CO.

   ...Northern Plains to western Lake Superior Region...
   Scattered thunderstorms are expected to develop late this afternoon
   near the frontal zones, initially across southern and eastern SD,
   then into southwestern/central/south-central MN, as a combination of
   strong warm-sector heating and frontal convergence overcomes the
   cap.  Large hail is probable, along with at least isolated damaging
   gusts.  The cape/shear parameter space also supports a supercellular
   tornado risk, though longevity of favorable storm mode is uncertain.

   The 12Z ABR sounding sampled the base of a strong elevated mixed
   layer, with robust basal inversion between 750-850 mb.  Another
   sounding is planned there at 18Z. This is partly to evaluate the
   change in low-level thermodynamics through midday, including cap
   strength and the progress of boundary-layer theta-e advection under
   that cap, in the northern fringe of most focused severe
   probabilities.  Pending that and forthcoming 12Z model guidance, and
   evaluating current diagnostics and prior/overnight progs, no major
   changes to the prior outlook in this region are justified.  Some
   southward nudge of slight/enhanced probabilities is being done in
   deference to the most probable corridor of afternoon storm
   initiation near and east of the probable late-afternoon surface-low
   position.

   The air mass along and south of the frontal zones will destabilize
   substantially from both insolation and low-level warm/moist
   advection throughout the afternoon.  Erosion of CINH should occur
   preferentially over SD area where stronger/deeper mixing is likely. 
   Evapotranspirative augmentation of the moist-advection regime should
   lead to a plume of upper 60s/low 70s F surface dew points east of
   the low, combining with steep lapse rates aloft to support peak
   preconvective MLCAPE 3000-4000 J/kg.  Favorable low-level shear is
   forecast, with SRH maximized near the warm front in eastern SD and
   MN.  Weaknesses in the midlevel hodograph may contribute to messy
   storm modes after initial discrete to semi-discrete character.  The
   severe threat may persist northeastward tonight across parts of the
   eastern MN/western WI/southwestern Lake Superior region in a regime
   of favorable low-level warm advection near and north of the warm
   front.

   ...Western Great Basin...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon into early evening over higher terrain from the
   eastern Sierras across the western Great Basin.  Activity should
   move mostly northward to northeastward over lower intermountain
   terrain, where pockets of clearing permit enough insolation to
   develop a well-mixed subcloud layer, in turn supporting a hybrid
   wet/dry downburst thermodynamic configuration.  While mid/upper-
   level lapse rates are not forecast to be especially large,
   sufficient low-level moisture in place to support weakly inhibited
   MLCAPE in the 500-1500 J/kg range.  A large area of difluent,
   southwesterly to south-southwesterly flow will persist over the
   region east of the slow-moving mid/upper-level low/trough.  This
   will occur beneath a belt of relatively maximized 250-mb to anvil-
   level flow, contributing to both convective ventilation and
   cloud-layer shear, though weak low-level winds are forecast.  Lack
   of both buoyancy and greater low/middle-level flow/shear preclude a
   better-organized severe threat.

   ..Edwards.. 07/25/2017

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