Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0255 PM CDT Fri Apr 21 2017 Valid 212000Z - 221200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF SOUTHEASTERN OKLAHOMA...NORTH CENTRAL AND NORTHEASTERN TEXAS...AND SOUTHWESTERN ARKANSAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE SOUTHERN PLAINS AND OZARK PLATEAU... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IN A CORRIDOR SURROUNDING THE SLIGHT RISK AREA EAST NORTHEASTWARD THROUGH THE TENNESSEE VALLY AND MID ATLANTIC COAST REGION... ...SUMMARY... Severe thunderstorms capable of producing damaging gusts, large hail and a few tornadoes are expected through tonight from parts of the southern Plains into the Ozark Plateau. Isolated severe thunderstorms may also impact portions of the Mid-South into the Mid-Atlantic with wind and hail. ...20Z Outlook Update... Adjustments have been made to severe probabilistic and categorical outlook lines, mostly in an attempt to account for the latest convective trends, and the progression of the surface cold front and preceding convective outflows. ...Southern Plains into Ozark Plateau... An initial organized convective system, which evolved across central/eastern Oklahoma earlier today, now appears in the process of weakening across parts of northwestern Arkansas. New convection is in the process of developing in its wake, near/east of the Interstate 35 corridor of Oklahoma, as a cyclonic 50-70 kt 500 mb jet streak noses east southeast of the Rockies, through the Texas/Oklahoma Panhandle region. Subsequent evolution of this convection remains at least somewhat unclear, but due to the frontal progression and the influence of the initial convective cluster, strongest new convective development probably will eventually become focused near the Red River by late this afternoon. This is within the thermal gradient along the northern periphery of a plume of warmer and capping elevated mixed layer air, where forcing for ascent will be aided lower/mid tropospheric warm advection. The upscale growth of another mesoscale convective system accompanied a risk for potentially damaging wind gusts still appears possible. Prior to this, early boundary layer based development may include a couple of supercells, particularly (roughly) near the Sherman TX/Durant OK area, where the environment could become supportive of an isolated strong tornado or two. ...Tennessee Valley into the Mid Atlantic region... Considerable thunderstorm activity is now underway across the Tennessee Valley through, and to the lee of the central/southern Appalachians, in the presence of weak to modest instability and shear. The environment appears conducive to storms capable of producing hail and strong surface gusts, but any activity able to reach severe levels still seems likely to remain fairly sparse in coverage and relatively short-lived. ..Kerr.. 04/21/2017 .PREV DISCUSSION... /ISSUED 1140 AM CDT Fri Apr 21 2017/ ...Synopsis... While an upper-level trough moves across Ontario/Quebec and the adjacent northeast U.S. and a second over the northeast Pacific reaches the West Coast late, the main trough associated with the primary convective/severe risk today is currently shifting into the central U.S. An initial perturbation within this trough will shift across Kansas/Oklahoma with time, reaching the Ozarks overnight. Meanwhile, a strong northwesterly mid- and upper-level jet on the back side of the trough will maintain broader cyclonic flow over the central U.S. through the period, in the wake of the passage of the initial/aforementioned smaller-scale perturbation. At the surface, a cold front currently extends from NY/PA southwestward across the mid-South to Arkansas, and then westward roughly along the Red River Valley. Here, convectively-induced low moving along the front will allow some brief retreat of the boundary northward across the southeast Oklahoma/southern Arkansas vicinity as the low moves quickly eastward through the afternoon. In the wake of the low, a southward surge of the convectively reinforced front is expected across the southern Plains through the end of the period. ...Southern Plains eastward across the Ozarks/mid MS Valley... A fast-moving/slightly elevated MCS continues moving across Oklahoma at this time, just north of the surface front now draped across southern Oklahoma. This early/well-organized cluster of storms complicates the forecast substantially, as current model guidance -- notably CAM runs and in particular the latest HRR -- have failed to depict this MCS almost entirely. Convective outflow on the southern fringe of this line of storms is shifting quickly southeast across southwest and south central Oklahoma, where it has overtaken the synoptic front and resulted in a convectively reinforced frontal surge which should continue into north Texas with time. Ahead of this fast-moving system -- extrapolated to reach western Arkansas and southeast Oklahoma by 18Z, a moist/slowly destabilizing airmass is in place near and south of the synoptic front. Slight northward advance of the front will occur ahead of the MCS, but eventual southward suppression is expected as the MCS passes and convective outflow interacts with the front. The eventual location of the outflow/front conglomerate will be significant, as it will serve as a northern fringe of later-to-be-realized surface-based severe threat expected to evolve across the Arklatex into north Texas. Redevelopment of convection south of the ongoing MCS -- near and ahead of the advancing front/outflow is expected, and with moderate instability in conjunction with a favorably sheared environment, supercells are anticipated along with attendant risks for very large hail, locally damaging winds, and a few tornadoes. Risk should spread east across the Arklatex and southeast across north Texas this evening, before diminishing in conjunction with diurnal cooling/stabilization. Given potential for greatest severe risk to extend farther south and east than prior outlooks indicated due to ongoing convective evolution, the ENH risk area is being adjusted southward a bit toward the DFW metroplex, and eastward into the Arklatex. Farther north, the ongoing MCS should continue across Arkansas today, possibly continuing across the mid-Mississippi Valley later this afternoon and evening. With some severe risk -- including potential for locally damaging winds and hail -- extending east of the river, either with this MCS or else with later/convective redevelopment, will extend SLGT risk east into parts of KY and TN. ...Mid-Atlantic region westward into the TN Valley... Scattered showers and a few thunderstorms are ongoing at this time along the cold front, which currently extends from central NY/western PA southwestward across eastern Kentucky/middle Tennessee. As this front advances very slowly southeastward today, heating through scattered to broken cloud cover will support continued/modest destabilization, with mixed-layer CAPE 500 to 1000 J/kg expected in the pre-frontal warm sector this afternoon. Flat to weakly anticyclonic flow aloft from roughly the mountains east through evening suggests isolated coverage of stronger storms, though 30 to 40 kt mid-level west-southwest flow will contribute to shear sufficient for organized -- possibly weakly rotating -- updrafts. As such, risk for hail and/or locally damaging winds will be possible with a few sustained storms within the MRGL risk area.Read more
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