Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0759 AM CDT Sat May 27 2017 Valid 271300Z - 281200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PARTS OF SOUTHEASTERN KANSAS...NORTHEASTERN OKLAHOMA...NORTHERN ARKANSAS...CENTRAL/SOUTHERN MISSOURI...SOUTHERN ILLINOIS...SOUTHWESTERN INDIANA...WESTERN KENTUCKY...AND WESTERN TENNESSEE... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM EASTERN KANSAS AND CENTRAL OKLAHOMA TO EASTERN TENNESSEE/KENTUCKY... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK AND EXTENDING INTO NORTH TEXAS AND PARTS OF VIRGINIA AND NORTH CAROLINA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AND EXTENDING TO PARTS OF SOUTHERN COLORADO AND SOUTHWEST TEXAS... ...SUMMARY... Widespread severe wind gusts are forecast from the Ozark region to the lower Ohio Valley today. Additionally, very large to giant hail and tornadoes will be possible from the Red River Valley northeastward to the Ozark Plateau. Damaging winds, large hail, and a couple tornadoes will also be possible across the Tennessee Valley, middle Ohio Valley, and portions of the Mid-Atlantic. ...Synopsis... The upper-air pattern features a broad fetch of zonal to west-southwesterly flow over much of the central/eastern U.S., downstream from a positively tilted trough. That trough -- now evident in moisture-channel imagery from a low over northern MB across ND, WY, UT and southern NV -- should extend from MN to northern NM by the end of the period. At the surface, 11Z analysis showed a wavy, generally quasistationary frontal zone from VA to OH then west-southwestward across southern IL and central MO. A cold front was evident from northwestern IA across eastern/south-central NE to northeastern NM. The front has been diffused temporarily behind an outflow boundary related to ongoing convection over northeastern KS, but will catch up to that boundary and thus the warm sector with time. A wavy dryline was drawn from the eastern TX Panhandle to the Big bend region. The dryline will move eastward today to central/southern OK and west-central TX. The cold front will overtake the dryline and the outflow boundary from northwest-southeast through the overnight hours, reaching eastern MO and northeast through southwest TX by 12Z. ...Lower Missouri Valley, Ozarks to Ohio Valley and Red River... The ongoing convection over northeastern KS was forecast by most of the synoptic and high-res progs initializing from 00-09Z to weaken and shrink much sooner than evident in current coverage/intensity. This casts some uncertainty on the northern and western parts of the outlook area, in two ways: 1. Whether this initial convection and related isallobaric perturbation will evolve into or directly influence the eventual/expected major wind-producing MCS as it encounters diurnally destabilizing air over eastern KS and western MO, and/or 2. The influence of the trailing outflow boundary on subsequent thunderstorm potential over south-central/southeastern KS. Thunderstorms either developing along this boundary, or crossing it from behind, should gain access to surface-based inflow and expand/intensify rapidly. In either event, the downshear air mass will be very favorable for upscale organization of a forward-propagating, potentially derecho-producing MCS. Significant-severe/hurricane-force thunderstorm gusts and numerous damage reports are possible in the moderate-risk area. An upgrade may be warranted later today. However, uncertainties related to influence of the ongoing activity and timing of greatest storm organization preclude offering any more-focused unconditional probabilities at this time. In addition to wind, line-embedded/tornadic circulations are possible. Embedded supercells may occur, especially in any frontal backbuilding that may occur toward northeastern/central OK late afternoon and evening. Very steep midlevel lapse rates will overlie a richly moist and strongly heated boundary layer containing upper 60s to mid 70s F surface dew points and 15-18 g/kg mean mixing ratios. This yields MLCAPE locally exceeding 6000 J/kg in forecast soundings southeast of the cold front and outflows, and east of the dryline, from the western Ozarks across eastern OK to the Red River, amidst effective-shear magnitudes of 45-55 kt. Such extreme instability is near record levels, based on historic soundings. The CAPE-shear parameter space appears comparable to a fast-developing, dryline-initiated supercell/tornado event on 26 May 1997 in eastern OK, but with the added risk this time of a derecho on the northeast side. Such extreme instability will support explosive growth of thunderstorms that do break the cap -- whether through forced ascent on a cold pool or front, or in any discrete to semi-discrete cells that can move off the dryline. Dryline-storm risk is conditional and more isolated due to the strong EML and related CINH, but also potentially dangerous with giant hail of 3-5 inches in diameter and significant tornadoes possible. By late evening, a swath of convection should extend from the Ohio Valley into southern OK and perhaps north TX, with the overall severe threat diminishing overnight. ...Mid South, Tennessee Valley to VA/NC... Ongoing clusters of thunderstorms over the western parts of this area will continue to pose a risk of severe gusts and large hail through at least mid/late morning, with additional development possible farther south over parts of northeastern AR and western TN. Refer to WW 274 and related mesoscale discussions for latest near-term guidance. Embedded within the zonal-flow fetch, a southern-stream perturbation is apparent over the eastern Ozarks region to southern IL, associated with the ongoing convection. This feature will proceed eastward across the southern Appalachians today. Associated large-scale ascent/destabilization aloft, overlying diurnal boundary-layer destabilization and related erosion of CINH, will support of further convective development eastward toward the southern Appalachians. The ongoing complex, or a separate area of convection progged by some models to develop over southern KY and northern TN, would move east-southeastward toward the southern Appalachians given an upscale-growing, forward-propagational forcing from an aggregating cold pool. Guidance is inconsistent in this regard. The potential for a more concentrated area of damaging wind (separate from and ahead of the potential derecho discussed above) currently is too conditional for a more concentrated zone of wind probabilities, but one may need to be introduced in a later outlook as mesoscale uncertainties are better-resolved. The northern/glancing influence of the aforementioned perturbation aloft also may support thunderstorm formation this afternoon into early evening over VA near a surface frontal zone. South and southwest of the front, seasonally steep lapse rates aloft, overlying surface dew points ranging from the upper 50s to mid 60s F, will support areas of 1500-2500 J/kg MLCAPE. Supercell potential may be greatest near the front, which will serve as a source of low-level vorticity and backed flow; however, at least transient storm rotation will be possible anywhere within this swath, given favorable deep shear. ...South-central High Plains... Isolated to widely scattered thunderstorms should form behind the surface cold front this afternoon, as surface flow veers to more of an easterly upslope component and related forcing overcomes CINH. Isolated severe hail or gusts are possible, and a supercell or two cannot be ruled out. Deep shear will be favorable for supercells, but modest low-level moisture and theta-e should preclude a greater severe threat. ..Edwards/Goss.. 05/27/2017Read more
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