SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0756 AM CDT Fri Jun 18 2021 Valid 181300Z - 191200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF ILLINOIS...INDIANA AND OHIO... ...SUMMARY... Scattered severe storms will pose a threat of damaging wind, large hail and perhaps a couple tornadoes this afternoon and tonight over parts of the Ohio Valley region and mid Mississippi Valley. ...Synopsis... In mid/upper levels, the northern-stream flow belt will be aligned generally from west-northwest to east-southeast over the northern CONUS. The highest-amplitude shortwave trough is apparent in moisture-channel imagery over southern SK, and forecast to move to WI by the end of the period. A more-subtle, but still apparent, perturbation over NE will move eastward toward the IL/IN border through 12Z tomorrow. Meanwhile, a weak, slowly retrograding trough over eastern/central TX and northeastern MX -- to the east of a major anticyclone over the Four Corners region -- will affect the motion of a tropical disturbance discussed below. At the surface, 11Z analysis showed a cold front from northern WI across western IA and south-central NE, to a low near GLD, then across central Co. The low is forecast to weaken and move roughly eastward to northeastern KS by 12Z. Another frontal-wave low may develop tonight over WI and move to Lake Michigan, ahead of the strongest mid/upper-level shortwave trough, with the front connecting those lows, and extending across southeastern CO by the end of the period. ...Midwest corridor... This outlook area is a spatial composite of two most-probable regimes that ultimately may merge overnight. In both, ultimate upscale growth will foster a dominant damaging-wind threat, with some significant (65+ kt) gusts possible. Enough low-level shear may develop to support some tornado potential (especially the eastern/first area). 1. Potential re-invigoration of convection is possible this afternoon associated with the UVV field now supporting the ongoing non-severe convection over the upper Great Lakes. This swath of ascent should encounter a diabatically destabilizing and advectively moistening plume of boundary-layer air over the OH/southeastern MI/northern WV region, with some backbuilding possible this evening westward across IN and perhaps eastern IL. A corridor of rich moisture now within reasonably short distance upstream (southeastern MO, southern IL, western KY) will spread up the Ohio Valley region, underlying sufficiently steep midlevel lapse rates to support a plume of 2500-3500 MLCAPE (locally/briefly higher). The same relatively veered surface winds that will enable that moist advection also will limit vertical shear, with forecast soundings reasonably showing effective-shear magnitudes of 25-40 kt. However, enough low-level speed shear will exist to enlarge hodographs and present effective SRH in the 150-250 J/kg range, supporting a blend of supercell and multicell structures, with bowing segments possible. Cold-pool aggregation will transition the threat to mainly wind with time, though the eastern and southern bounds of the threat will be regulated by decreasing inflow-layer instability. 2. Upshear, mostly later development this evening and tonight, across the southern IA/northern MO to central/eastern IL/western IN corridor. An initially blended hail/wind threat (some significant/ 2+ inch diameter hail possible with relatively discrete cells) will transition toward wind with time, as upscale growth occurs with east-southeastward extent. This activity will be supported by a combination of frontal forcing and isentropic lift to LFC, near the nose of a 40-50-kt southwesterly LLJ, with MLCAPE 2000-4000 J/kg and (despite modest near-surface flow) 35-45-kt effective-shear magnitudes. A conditional, very cap-dependent potential also exists for late-afternoon supercell formation along the boundary near the IA/MO border, in which case significant hail would be possible early in the convective cycle. Given these areas of hail potential, a corridor of 10% significant-hail threat has been introduced within (and not affecting) existing categorical bounds, which still appear valid for this outlook cycle. ...Central High Plains... Isolated to widely scattered thunderstorms should develop this afternoon and last into part of the evening, moving eastward across the outlook area. Isolated severe wind and hail will be the main concerns. After the frontal passage, northeasterly flow will veer gradually to easterly, imparting a substantial near-surface upslope component that will aid in enhancing deep shear, as well as in moist advection and storm-relative winds in the inflow layer. Associated lift, along with diabatic heating of elevated terrain, will promote deepening afternoon convection beneath steep midlevel lapse rates, with MLCAPE to near 1000 J/kg, atop a well-mixed and deep subcloud layer suitable for strong to locally severe gusts. 35-45-kt effective-shear vectors will support mostly multicells, but with at least transient supercell characteristics possible. ...Gulf Coast... A tropical storm is forecast to develop from an area of low pressure now over the west-central Gulf, its center making landfall on the south-central to southeastern LA coastline late tonight. [See the latest NHC advisories for specific details on forecast track and intensity, as well as all tropical-related watches and warnings.] Given: 1. The latest track forecast with eastward-expanded wind radii, 2. The strongly sheared and somewhat hybrid tropical/subtropical character of this system, and related highly asymmetric, eastward- skewed convective distribution expected (per NHC discussion)... The outlook is shifted eastward, but with similar shape, maintaining marginal tornado potential generally north through east of center through the end of the period. Inland spread of the juxtaposition between favorably moist/unstable marine air and enlarged hodographs should be limited this afternoon and overnight, before expanding day-2 (per that outlook) with the added influences of diurnal heating and additional inland translation of the system. ..Edwards/Broyles.. 06/18/2021 Read more LIVE: spc.noaa.gov
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