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Tuesday, May 18, 2021

SPC May 18, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0254 PM CDT Tue May 18 2021 Valid 182000Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PORTIONS OF EASTERN TEXAS INTO THE ARKLATEX REGION... ...SUMMARY... Severe thunderstorms, including the risk for large hail, damaging winds, and a couple of tornadoes remain a concern across south-central and east Texas toward the ArkLaTex region this afternoon through evening. ...20Z Update... Much of the previous forecast remains on track. A gradual increase in convective coverage and intensity is still expected ahead of an outflow boundary across portions of south-central and eastern TX into the Arklatex this afternoon into this evening. 1500+ J/kg MLCAPE, 25+ kts of deep-layer shear, and the presence of 20+ kt 925-850 mb flow suggest that organized strong to severe storms, including a few supercells, are likely this afternoon. Large hail, damaging gusts, and a couple of tornadoes all appear possible. Please see MCDs #671 and #673 for more details. Organized strong thunderstorms, with a Marginal severe risk, remain likely across the portions of the southern High Plains ahead of a surface low and associated dryline. Storms have been developing over the past couple of hours, with a gradual increase in intensity noted. 1000+ J/kg MLCAPE is present ahead of the dryline, but with deep-layer flow and associated shear profiles, suggesting that any severe threat should remain quite isolated through the rest of the afternoon into the evening. More favorable shear profiles may be realized locally in the immediate vicinity of outflow boundaries, where brief storm organization may promote short-lived instances of damaging gusts, large hail. A brief tornado also cannot be ruled out. Please refer to MCD #672 for more information. An MCV continues to slowly meander eastward along the central Gulf Coast Shoreline, preceded by modest low-level flow. Boundary-layer-based instability remains relatively marginal (i.e. 500 J/kg MLCAPE per 19Z Mesoanalysis). However, adequate surface heating has resulted in over 100 J/kg 0-3km CAPE, which partially overlaps a rich low-level vertically oriented vorticity environment in proximity to the MS Panhandle. The 1848Z LIX VWP also shows the presence of over 150 m2/s2 0-3km SRH. As such, brief tornado cannot be ruled out for the remainder of the afternoon. ..Squitieri/Goss.. 05/18/2021 .PREV DISCUSSION... /ISSUED 1130 AM CDT Tue May 18 2021/ ...Central/east Texas to ArkLaTex region... An upper low centered over New Mexico/southern Colorado will continue to shift slowly east-southeastward toward the southern High Plains. A lingering storm cluster and related outflow persists across the Edwards Plateau late this morning, with the outflow boundary extending roughly west/east near and south of I-10 to the south of the Sonora/Junction areas, and then northeastward toward the Hill Country and I-35 corridor of central/north-central Texas. With considerable convective overturning across much of west-central/northwest into western north Texas last evening/night, low-level moisture is maximized at midday across South Texas as well as the eastern third of Texas into Louisiana/Arkansas. The air mass to the south/east of this modifying boundary will remain moist (generally mid 60s F surface dewpoints) and is expected to become at least moderately unstable with negligible CINH by mid-afternoon. Late-morning visible satellite imagery already shows ample cloud breaks to the south/east of the composite outflow. Upwards of 2000-3000 J/kg MLCAPE is plausible across south-central/east-central/northeast Texas by peak heating. Low-level/deep-layer shear will be maximized in proximity to the effective boundary and support a mixed mode of supercells and organized linear bands/bowing segments capable of damaging winds, some hail, as well as a tornado risk. With aid of a strengthening low-level jet/warm conveyor toward/after sunset, one or more east/northeastward-propagating linear clusters of storms may evolve by early evening across east/southeast Texas and the ArkLaTex vicinity with a damaging wind and some tornado risk continuing through late evening, and possibly part of the overnight. ...West/northwest Texas and eastern New Mexico... The low-level moist axis, characterized by moderate moisture, persists east of the dryline, particularly in closer proximity to the Texas portion of the Rio Grande (sampled by 12Z Del Rio observed sounding). While low-level moisture and buoyancy will be more modest with northward extent (1000-1500 J/kg MLCAPE at most), some stronger/locally severe semi-low-topped thunderstorms are expected to develop near a surface trough from far eastern New Mexico into the Texas/Oklahoma Panhandles and Texas South Plains. Isolated large hail should be the most probable severe risk, although some severe-caliber winds could occur. A brief tornado (probably via non-supercell processes) could occur in vicinity of the surface boundary across the northeast New Mexico and west/northwest Texas border vicinity. ...Gulf Coast including parts of LA/MS/AL... Within a moist environment, enhanced low-level flow appears to persist via an MCV near/east of a decaying cluster of storms. This scenario may support a brief tornado risk this afternoon through early evening. Read more LIVE: spc.noaa.gov