SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Wed Jun 21 2023 Valid 211300Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... ...SUMMARY... The greatest organized severe-thunderstorm potential will be over portions of the central and southern Plains, where severe gusts, very large hail, and a few tornadoes are possible. ...Synopsis... Only relatively minor adjustments are expected to the basic large- scale pattern over the CONUS, the upstream side of which will feature high-amplitude, slow-moving troughing across the Northwest and coastal northern CA, southwestward over the Pacific. A series of mostly low-amplitude shortwaves will traverse the foregoing/broad southwest-flow field extending from southern CA across the central/ northern Rockies to the adjacent High Plains. To the southeast, the eastern part of a largely along-flow, elongated banner of cyclonic vorticity -- initially located over parts of AZ and northwestern MX -- will move northeastward today across the Four Corner/San Juan Mountains region, reaching the central High Plains by 00Z. Farther northeast, a convectively enhanced shortwave trough/ vorticity lobe now over parts of KS/NE is penetrating the mean-ridge position from northern MX to the Upper Great Lakes. This feature should drift eastward to southeastward over KS, while the northern part of the ridge closes into an elongated anticyclone centered over northern ON. This will create a temporary Rex block configuration, in tandem with the broad, weak, persistent cyclone now centered over the GA/AL border area. Numerous small-scale vorticity lobes are embedded in the cyclone's orbit, including one now evident in moisture-channel imagery between CSG-MAI, moving southeastward. The cyclone as a whole should retrograde northwestward to westward through the period, ultimately capturing the KS perturbation by 12Z tomorrow. The surface analysis at 11Z showed a wavy, quasistationary frontal zone from just off the NC Coast through southern GA, a low over southern AL, near the mouth of the Mississippi River, south-central LA, northeast TX, northwestern OK, and western KS, where it is intercepted by an outflow boundary from ongoing strong-severe convection over south-central KS. This boundary will move little through this evening, except where shunted on the mesoscale by convective processes. A separate cold front ove the eastern ND/central SD/western NE/southeastern WY area will move slowly eastward/southeastward, probably stalling or even retreating slightly near the Cheyenne Ridge. A dryline now over southeastern CO, the western TX Panhandle, and the Permian Basin, will mix somewhat eastward today, narrowing the moist sector between it and the southern frontal zone. ...Central/southern Plains... The outlook across this region remains highly conditional in nature, with modulations still possible by the ongoing KS MCS and/or its outflow boundary, as well as uncertainties in timing/locating the point of no return for upscale-growth potential of afternoon/evening convection over the southern Plains. As such, little substantive change is being made to probabilities this outlook cycle. The most-confident scenario at this time remains two relatively maximized nodes of severe potential (including supercells) near an axis of seasonally very high moisture content: 1. Central High Plains just east of the Front and Laramie Ranges. A northwest/southeast corridor of very rich boundary-layer moisture for any given elevation will be maintained between the dryline/ mountains and the fronts, from northeastern CO/southeastern WY area southeastward to the TX Coast. This part of that plume will be characterized by surface dewpoints commonly in the upper 50s to upper 60s F in this region. RAP and HRRR may be exhibiting their usual over-mixing bias here and drying dewpoints too much away from the dryline. Modified forecast soundings show: strong veering with height area-wide, increasing deep shear northward to within the frontal zone and upslope region in WY, largest low-level hodographs and lowest LCL (hence greatest unconditional tornado probabilities) in the WY area, and steep deep-layer lapse rates. MLCAPE between 2500-3500 should be common near the moist axis by mid/late afternoon. Farther east, a more-uncertain scenario involves development on the east side of the moisture plume, represented by the outflow-reinforced baroclinic zone in KS. Potential would be greatest for a stalled boundary instead of one reinforced and kept progressive in westward motion by convection to its east continuing into (or rejuvenating during) the midday to afternoon hours. 2. Southern Plains ahead of the dryline. An ongoing small MCS that has produced a couple estimated severe gusts is expected to move southeastward for another few hours, mainly northeast of the surface front, but within a transitional zone of still somewhat-favorable buoyancy and lift to sustain the activity near severe levels. Risk areas have been adjusted to accommodate this activity. This afternoon into early evening, an area of scattered thunderstorms is expected to develop near and east of the dryline, and more conditionally on the front/outflow boundary, as strong diabatic heating and 70s F surface dewpoints signal enough low-level theta-e to overcomes MLCINH at the base of the EML. Strong veering of winds with height and around 35-45 kt effective-shear magnitudes indicate potential for supercells, amidst MUCAPE of 3500-5000 J/kg. Any relatively sustained supercells may produce significant (2+ inch diameter) hail, and the potential exists for giant hail greater than 4 inches locally. The wind/significant-wind threats are extended farther southeast toward central TX and perhaps may reach the coastal plain, depending on how much cold-pool aggregation and upscale/forward-propagational processes develop, the magnitude and timing of which remain quite variable in progs. However, confidence has increased in enough upscale MCS growth to warrant some southeastward extension of the wind/significant-wind areas. ...Southeast... Scattered to locally numerous thunderstorms in bands and clusters will affect the northeastern Gulf and adjoining parts of FL, potentially pivoting northeastward over southern GA in cyclonic flow around the basal vorticity lobe. Additional/afternoon development is likely over the peninsula as well, with strong diurnal heating mainly along and south of a cloud/precip boundary now over central FL. Damaging to isolated severe gusts will be the main concern. A deep troposphere and moisture-rich boundary layer will contribute to peak MLCAPE reaching the 1500-2500 J/kg range (locally higher over south FL), despite modest mid/upper-level lapse rates and flow. A relative minimum in potential may exist over portions of central/ north-central FL where destabilization timing/magnitude is uncertain in the wake of a large area of ongoing clouds/precip. However, some progs indicate recovery from late afternoon through tonight in support of some evening/overnight potential. As such, will refrain from carving a relative gape in unconditional probabilities, pending further mesoscale trends. ..Edwards/Jewell.. 06/21/2023
