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    Day 2 Outlook >
Jun 26, 2017 1300 UTC Day 1 Convective Outlook
Updated: Mon Jun 26 12:53:43 UTC 2017 (Print Version | 20170626 1300Z Day 1 shapefile | 20170626 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 261253

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0753 AM CDT Mon Jun 26 2017

   Valid 261300Z - 271200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE INTERIOR NORTHWEST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   THE CENTRAL PLAINS ACROSS MUCH OF NEW MEXICO AND THE SOUTHERN HIGH
   PLAINS...

   ...SUMMARY...
   Thunderstorms with marginally severe hail and strong wind gusts are
   possible late this afternoon into early evening across parts of the
   Pacific Northwest and from the southern and central Rockies eastward
   into the High Plains.

   ...Synopsis...
   In mid/upper levels, strong ridging that has dominated the western
   U.S. will begin to break down and, in the case of the northern
   branch of the ridge, shift eastward through the period.  This will
   occur as two initially separate Pacific perturbations phase into a 
   single synoptic-scale trough today:
   1.  A large, intense, northern-stream shortwave trough moving over
   the southern AK Panhandle and just offshore southwestern BC,
   anchored by a low/vorticity max now entering northwestern BC.  The
   low will proceed eastward across northern BC to northwestern AB by
   12Z, with troughing southward to the ID Panhandle.
   2.  A less-intense but still well-defined perturbation in
   moisture-channel imagery, moving northeastward toward the northern
   CA and OR coasts.  This trough will weaken but still link with the
   northern-stream feature as it reaches western ID and northern NV
   late in the period.

   Meanwhile, as the downstream northern branch of the ridge moves
   eastward across the northern/central Rockies, difluent northwest
   flow aloft will persist over the central and southern Great Plains,
   but generally not as strong as yesterday due to a lessening of the
   height gradient aloft.  The latter is related to the eastward shift
   of troughing and related cyclonic flow across the Great lakes and
   Ohio Valley.  Associated cold air aloft and residual low-level
   moisture, beneath strong mid/upper winds, may support small hail and
   strong gusts from a few thunderstorms over parts of eastern/northern
   NY and VT, but severe potential looks too low/conditional for a
   categorical outlook at this time.

   At the surface, the 11Z analysis showed a cold front from offshore
   of the NC coast across southeastern GA, extreme southeastern LA, and
   southeast TX, becoming quasistationary northwestward over
   central/northwest TX, before being overtaken by an expanding
   outflow/cold pol from a dissipating MCS.  This front will become
   quasistationary along most of its length and slowly weaken through
   the period.  Another frontal zone, evident from a weak low near PLN
   across eastern WI, northern IA, and northern NE, is forecast to move
   southward over the mid/upper Mississippi Valley region and central
   Plains through the day, stalling then returning northward late
   tonight over the central High Plains.

   ...Interior Northwest...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon east of the Cascade crest, moving northeastward
   across the outlook area into early evening, with some increase in
   coverage possible due to clustering and cold-pool effects.  The
   potential exists for isolated severe gusts and large hail.  Strong
   diabatic surface heating and adequate low-level moisture will
   underlie strengthening deep-layer winds and large-scale ascent aloft
   in advance of the Pacific perturbation.  Moisture should remain
   relatively maximized in the preconvective environment from northern
   ID southwestward across south-central WA and central/south-central
   OR, despite effects of strong vertical mixing as afternoon heating
   persists.  This will support MLCAPE 500-1000 J/kg, locally higher. 
   To the south and north of the outlook area, despite strong surface
   heating and steep low/middle-level lapse rates, moisture and
   therefore CAPE will decrease.

   Some of this activity also will pose a risk for dry-thunderstorm
   fire ignitions, especially over eastern OR and southwestern ID;
   refer to the SPC day-1 fire-weather outlook for more details.

   ...Central Plains...
   Widely scattered thunderstorms should develop near the frontal zone
   this afternoon and move southward to southeastward across this lobe
   of the outlook area, offering isolated hail and gusts near severe
   levels.  With augmentation from evapotranspiration helping to offset
   vertical mixing of the initially very shallow moist layer near
   surface, dew points now mostly in the 50s F should increase somewhat
   by mid afternoon, generally reaching the upper 50s to low 60s F. 
   Shallow but well-mixed subcloud layers should develop with diurnal
   heating beneath 500-1000 J/kg MLCAPE in a generally east-west
   corridor, based on modified 12Z RAOBs and forecast soundings from
   several models.  

   Despite the eastward motion of the Great Lakes troughing, cyclonic
   flow aloft with favorable deep/speed shear will remain over the
   region, yielding 40-50 kt northwesterly effective-shear vectors,
   aligned with a substantial component orthogonal to the frontal zone.
    This will support a mix of discrete and clustered modes, and some
   supercellular characteristics may be noted in the more persistent
   cells.  The lack of both more robust moisture/instability and
   stronger low-level lift precludes a better-organized or denser
   severe threat.  Convection should diminish after dark while
   encountering a progressively more-stable boundary layer.

   ...South-central High Plains, CO, NM, west TX...
   A relative gap may exist between this regime and the central Plains
   one described above, across parts of southwestern KS/southeastern
   CO/northeastern NM/western OK Panhandle region.  Intense convective
   overturning and stabilization has been performed by last night's
   MCS, the remnants of which now are moving south-southeastward and
   weakening across a large swath of west TX, and this potential gap
   area will be the slowest to recover.  However, spatial precision of
   such a gap, along with the tendency of the Raton Mesa and Sangre de
   Cristos to initiate convection in even more marginal thermodynamic
   environments than evident today, still leaves too much uncertainty
   to carve those parts of the outlook area away at this time.

   Farther southwest over the bulk of NM, and west TX, recycled MCS air
   in the north, transitioning to a mix of MCS-processed and
   unprocessed trajectories in the south (but in weaker shear/flow
   aloft), will be available for afternoon development on the higher
   terrain.  Widely scattered to locally scattered thunderstorms are
   expected to form this afternoon as diabatic heating and areas of
   weak upslope flow reduce MLCINH.  MLCAPE 1000-2000 J/kg should
   develop atop well-mixed subcloud layers, amidst effective-shear
   magnitudes ranging from around 35 kt over southeastern CO to less
   than 20 kt in southern NM and far west TX.  An MCV and
   outflow/differential-heating boundaries related to the prior MCS
   also may focus storm development/propagation over parts of southwest
   and far west TX.  This environment will support primarily a patchy,
   slow-moving multicellular storm mode during afternoon and early
   evening with isolated severe gusts and hail.

   ..Edwards/Picca.. 06/26/2017

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

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

        
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