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Wednesday, June 2, 2021

SPC Jun 2, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0743 AM CDT Wed Jun 02 2021 Valid 021300Z - 031200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN HIGH PLAINS TO PARTS OF SOUTH TEXAS...AND OVER PARTS OF THE MID-SOUTH TO LOWER OHIO VALLEY REGION... ...SUMMARY... Isolated severe hail and/or gusts will be possible in two areas: from the southern High Plains to parts of south Texas, and over parts of the Mid-South to lower Ohio Valley region. ...Synopsis... A slowly progressive upper-air pattern will feature a ridge from the Great Basin northward over western Canada, a cut-off low over northern/central Baja, and a synoptic-scale trough over the central CONUS. This trough -- initially evident in moisture-channel imagery from northern MN across portions of KS/NE to west TX -- will shift to the upper Great Lakes, IL, MO, eastern OK, and central TX by the end of the period. At the surface, 11Z analysis showed a weak frontal-wave low over northeastern AR, with cold front arching across southeastern AR to southeast TX, becoming quasistationary over parts of south-central/ southwest TX. The TX frontal segment should shift northward erratically today as a warm front, while the Mississippi Valley segment drifts eastward to the south of the slow-moving low. ...Southern High Plains to parts of south Texas... A small MCS -- trailing from the area of an MCV likely being reinforced by the current convection -- is moving southeastward over parts of northwest TX, and may offer isolated, marginally severe gusts that penetrate a near-surface stable layer. See SPC mesoscale discussion 836 for near-term details. The ongoing MCS also may influence other severe potential today, such as by laying down an outflow/differential-heating boundary for additional northwest-flow convection to form over parts of central TX this afternoon. The zone of greatest mid/upper-level cyclonic curvature and height gradient associated with the trough aloft will shift across portions of central/north TX and perhaps southwestern OK today. This will foster a channel of 35-45-kt flow at 500 mb and 75-85 kt at 250 mb. Associated enhancement to deep shear will, in turn, support supercell potential with any sustained, relatively discrete convection that can develop across this area, either in a zone of destabilization behind the MCS or on its outflow boundary, and move southeastward. The threat is uncertain, however, given both the subtlety of any potential foci for storm initiation outside the possible boundary, location/strength of the boundary itself, and destabilization concerns related to cloud cover and precip over north TX and southern OK from the MCS. Still, the Marginal line has been expanded northeastward and eastward some to give more room for these conditional potentials. Otherwise, isolated to widely scattered thunderstorms are expected to form this afternoon over diurnally heated higher terrain of central/southern NM and west TX, shifting eastward to southeastward across adjoining portions of the outlook area later in the afternoon/evening. Residual yet still sufficient/supportive boundary-layer moisture is expected, with dew points on the Plains and in valleys generally in the 50s to isolated lower 60s F. Above that, midlevel lapse rates will be steep enough to support 1000-2000 J/kg MLCAPE (locally higher, especially over west-central TX to the southern Llano Estacado). Though midlevel winds are not expected to be particularly strong with greater distance from the trough, strong directional veering with height will contribute to effective-shear magnitudes in the 25-40-kt range, suggesting a blend of multicell and sporadic/isolated supercell characteristics. To the extent mesoscale uncertainties become resolved today, one or two corridors of greater unconditional probabilities may be needed within this vast 5% hail/wind area. ...Mid-South to lower Ohio Valley... Isolated damaging to severe gusts are possible with thunderstorms over the outlook area today, along/ahead of the front and low. A tornado also may occur, conditional on aid to low-level/inflow-layer shear/vorticity from boundaries interacting with potential supercells. Isolated, marginally severe hail also is possible, predominantly over the Mid-South region where the buoyancy/shear parameter space will be relatively maximized. An extensive area of precip and isolated embedded thunderstorms is evident in composited radar imagery from southeastern LA to OH, including all but about the southwestern 1/5 of the outlook area. Slow diurnal and warm-advection-related destabilization within the AL/MS/TN part of the ongoing convective/precip swath may contribute to increasing coverage/intensity of activity through midday, while areas farther southwest and west diabatically destabilize more readily. The latter, along with rich low-level moisture with surface dew points commonly in the upper 60s to near 70 F, will support 1000-2000 J/kg MLCAPE and weak MLCINH, within about 100 nm east of the Mississippi River today, also east of the low and frontal zone. Buoyancy should diminish with eastward/northeastward extent into areas affected longer by current clouds/precip. Ahead of the mid/upper trough, sufficient gradient flow aloft will spread over the area to support 25-40-kt effective-shear magnitudes, with mainly multicell modes but transient supercells also possible. ..Edwards/Gleason.. 06/02/2021 Read more LIVE: spc.noaa.gov