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    Day 2 Outlook >
Jul 22, 2018 1300 UTC Day 1 Convective Outlook
Updated: Sun Jul 22 12:59:21 UTC 2018 (Print Version | 20180722 1300Z Day 1 shapefile | 20180722 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 221259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Sun Jul 22 2018

   Valid 221300Z - 231200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE SOUTHEAST AND NORTHERN/CENTRAL PLAINS...

   ...SUMMARY...
   Severe storms, primarily capable of damaging winds and large hail,
   will be possible across parts of the Southeast and northern/central
   Plains this afternoon and evening.

   ...Synopsis...
   In mid/upper levels, the pattern will remain dominated by three
   synoptic- to meso-alpha-scale features:
   1.  A large, stacked cyclone -- now centered over OH -- forecast to
   fill slowly but also expand southward over the Southeast through the
   period.  This will occur as a series of mesoscale vorticity lobes
   orbit the southern semicircle, leading to net height falls across
   most of GA, SC, AL, and FL.
   2.  A strong northern-stream trough -- now evident in
   moisture-channel imagery from a 500-mb low over northwestern SK
   across north-central/southwestern MT.  As the northern SK cyclone
   moves slowly eastward, the basal trough will change from positively
   to negatively tilted and decrease in amplitude, ejecting
   northeastward across MT to central ND by 00Z.
   3.  Persistent ridging over the south-central and southwestern
   CONUS, rooted in a strong high that will retrograde slowly across
   NM.

   At the surface, a low analyzed at 11Z near IPT will eject further
   inland and weaken today across the lower Great Lakes and southern
   ON, in tandem with a mid/upper-level vorticity lobe.  The associated
   warm front is forecast to move northward over eastern NY and parts
   of southern/western New England, generally becoming more ill-defined
   with time.  Farther west, a cold front was drawn initially from a
   low between GGW-GDV, southwestward cross northern WY.  By 00Z, this
   front should reach eastern ND, south-central SD, north-central NE,
   and eastern WY, with a prefrontal trough from central SD to eastern
   CO.  By 12Z the front should advance to north-central MN,
   northeastern NE and southeastern CO. 

   ...Southeast...
   The main severe threat will be associated with scattered to numerous
   thunderstorms that should develop quickly from midday through early
   afternoon over northern FL, and later in the afternoon near
   sea-breeze and confluence boundaries across the FL
   Panhandle/southern AL region.  Damaging gusts and large hail are
   expected, and a tornado cannot be ruled out.

   Favorable low/middle-level lapse rates will overlie strong surface
   heating, 70s to near 80 F surface dew points, PW to near 2 inches,
   and mean mixing ratios 19-20 g/kg.  Forecast soundings in the
   preconvective environment accordingly show strong to extreme
   buoyancy, with MLCAPE 3500-4500 J/kg, locally near 5000 J/kg.  These
   buoyant profiles will be deep also, with equilibrium levels near 50
   kft and max parcel levels potentially topping 60 kft.  As such, both
   water-loaded severe pulses and upscale cold-pool organization appear
   possible in support of the convective-wind hazard.  Directional
   shear should be small, given the westerly boundary-layer wind
   component.  However, deep-layer speed shear will be favorable for
   organized multicells and at lest transient/heavy-precip supercells,
   with effective-shear magnitudes ranging from 30-35 kt over the
   northern FL Peninsula to 40-45 kt across the western Panhandle and
   adjoining parts of AL.  As such, a large-hail risk exists also. 
   Given the lack of larger ambient hodographs, tornado potential is
   more conditional on storm-scale processes and to mesobeta-scale
   convection/boundary interactions.

   Evolution of one or two primary complexes is possible from initial
   convection over northern FL and/or activity moving southeastward out
   of the western activity into northwestern FL later.  Damaging wind
   will become the main concern as any such upscale organization
   proceeds.

   ...Northern/central Plains...
   Scattered thunderstorms are forecast to develop along/ahead of the
   front and near a prefrontal trough this afternoon.  First activity
   should be over parts of ND where large-scale/deep-layer ascent will
   arrive sooner and stronger ahead of the ejecting mid/upper
   perturbation -- then over SD and NE where heating, low-level
   instability and theta-e will be greater.  Large hail and severe
   gusts are possible.

   A well-defined axis of low-level moisture extends from northeastern
   OK north-northwestward across the central Plains to western SD and
   central/northwestern ND, per surface analysis.  Dew points should
   remain commonly in the 60s F under modest capping through much of
   the preconvective time period, in a narrow corridor near the trough,
   and ahead of the front.  Satellite imagery indicates cloud debris
   from earlier convection is breaking up over NE and SD, with a swath
   of clearing northward into ND,  This should allow sustained surface
   heating through afternoon, helping to boost MLCAPE into the
   1500-2500 J/kg range in a narrow, north/south-elongated
   preconvective corridor over the outlook area.  Deep shear will be
   marginal over most of this region, with effective-shear magnitudes
   generally 30-40 kt, and a blend of multicells and short-lived
   supercell structures possible.  The severe threat (especially hail,
   later wind) should wane with eastward extent as modes get messier
   and inflow-layer theta-e diminishes this evening.

   ..Edwards/Peters.. 07/22/2018

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

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

        
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