SPC AC 251259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Fri May 25 2018

   Valid 251300Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   OKLAHOMA AND NORTH TEXAS...AND PARTS OF THE UPPER MISSISSIPPI VALLEY
   REGION...

   ...SUMMARY...
   Severe thunderstorms are possible later today into the evening over
   parts of the upper Mississippi Valley and the southern Plains.  A
   few strong to severe storms are also possible over the interior
   Northwest and northern Maine.

   ...Synopsis...
   In mid/upper levels, a rather high-amplitude, yet progressive,
   pattern will persist through the period.  From west to east:  a
   strong cyclone -- its center now approaching the MRY-SFO area on the
   CA coast -- is forecast to progress inland through the period.  The
   500-mb low should be located near RNO by 12Z, with trough
   south-southwestward off southern CA and west of northern Baja. 
   Downstream, a ridge now located over the Rockies (from AB to NM) and
   over Chihuahua will move eastward, extending from far west TX
   northward through the central and northern High Plains by the end of
   the period.

   A small, weak cyclone now over southern MB should move slowly
   eastward to southeastward, devolving to an open-wave trough near the
   MB/ON border overnight.  A few weak vorticity lobes, embedded within
   the difluent flow south and southeast of the low, will cross the
   upper Mississippi Valley and parts of the Lake Superior/Lake
   Michigan regions.  Farther south, a shortwave trough and embedded,
   convectively enhanced vorticity max over eastern KS will move slowly
   eastward, merging into a broader, north-south swath of height
   weakness aloft that will connect with the strengthening deep-layer
   disturbance over the southern Gulf/Yucatan region.  Please refer to
   NHC outlooks for more information on the Gulf perturbation.

   At the surface, 11Z analysis showed a trough and weak cold-frontal
   zone from northwestern MN across central SD to western NE.  This
   boundary should move eastward across MN today to parts of Lake
   Superior, WI and IA by the end of the period.  A dryline analyzed
   near a GLD-SPD-ROW-MRF line should mix eastward today, with a low
   likely developing along it by midday over the eastern OK Panhandle/
   northeastern TX Panhandle region.  A warm front over western to
   Downeast maine should move northward slightly then stall, some of it
   merging with another frontal zone initially over southern QC and
   northern Maine.

   ...Southern Plains...
   An ongoing band of nonsevere thunderstorms is evident over portions
   of central and north-central OK, behind its own outflow boundary. 
   As of 12Z, mesonet and conventional data showed the boundary still
   progressive over central OK, and arching from OKM to between
   ADH-CHK, to near CSM, northwestward across Lipscomb County TX to
   near LBL.  If this activity maintains or reconstitutes organization
   while moving into (and lifting) a diabatically destabilizing
   boundary layer with mid/upper 60s F surface dew points over
   southeastern OK, more than isolated/marginal severe concentration
   may result, and additional probabilities may be needed.  At this
   time, that potential still is too conditional to warrant a
   categorical outlook boost.

   Additional thunderstorms should develop this afternoon in the area
   of relatively maximized heating and low-level convergence east of
   the surface low, near the outflow boundary, and evolve upscale while
   offering large hail and damaging wind.  The preconvective/inflow air
   mass will be strongly unstable, with hot and well-mixed subcloud
   layers underlying steep midlevel lapse rates.  2500-3500 J/kg MLCAPE
   (locally higher) should develop, amidst strong directional shear but
   modest low/middle-level flow.  Still, 30-40 kt effective-shear
   magnitudes should develop, suggesting potential for at least
   transient supercellular character to some storms (especially along
   the outflow boundary) amidst dominant multicellular modes.  Activity
   should develop upscale and move southeastward roughly astride the
   instability gradient represented by the outflow boundary, perhaps
   crossing the Red River into North Texas before weakening this
   evening and overnight.  Some leeway will be left on what now is the
   cool side of the outflow boundary to allow for later diabatic
   destabilization, hail from elevated convection, as well as some
   potential northeastward retreat of the feature before it is
   overtaken by the main complex.

   ...Upper Mississippi Valley region...
   As with the southern Plains region, an ongoing, nonsevere clusters
   of thunderstorms is producing outflow that will leave a focusing
   boundary behind.  The boundary should settle into south-central/
   southwestern WI and southern MN, with some northeastward retreat and
   cool-side diabatic destabilization possible by mid-afternoon.  As
   such, some leeway will be left poleward of the boundary for
   sufficiently favorable subsequent destabilization.  

   Near and south of the boundary, and along/ahead of the front,  steep
   midlevel lapse rates near the northeastern rim of the EML, with some
   DCVA possible from one of the vorticity lobes pivoting through
   cyclonic flow aloft.  That should overlie a strongly heated near-
   surface air mass with 60s F dew points, fostering preconvective
   MLCAPE of 2000-3000 J/kg, locally/briefly higher.  Scattered
   thunderstorms should develop this afternoon over western parts of
   the outlook area, shifting southeastward while offering the risk of
   sporadic large hail and damaging gusts.  Lack of more robust
   low-level and deep shear suggest predominantly multicellular
   character, with some organization into lines before encountering too
   much nocturnal stabilization and weakening this evening.

   ...Interior Northwest...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon and move generally northward to northeastward in a
   regime of difluent and mostly strengthening upper winds, overlying
   favorably heated/mixed boundary layers.  Isolated damaging gusts and
   severe hail are possible.  Sufficient moisture will remain during
   the boundary-layer heating/mixing process to support a corridor of
   MLCAPE 500-1000 J/kg atop the well-mixed subcloud layer, though
   modest low/middle-level flow will temper vertical shear.

   ...Northern Maine...
   Widely scattered to scattered thunderstorms are expected to develop
   over parts of northern ME and adjoining southern QC, moving east-
   southeastward to southeastward, as the cold front impinges on a
   boundary layer diabatically destabilized following morning cloud
   cover.  That, along with surface dew points increasing into the 50s
   F through the day, should offset modest midlevel lapse rates enough
   to contribute to MLCAPE generally in the 500-1000 J/kg range for a
   few hours mid/late afternoon, atop reasonably well-mixed subcloud
   layers.  That will be juxtaposed with strong speed shear amidst
   nearly unidirectional vertical wind profiles, yielding effective-
   shear magnitudes in the 35-45-kt range, suggesting the potential for
   a few organized cells with hail and gusts near severe limits
   possible.

   ..Edwards/Peters.. 05/25/2018

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

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