Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Wed Apr 26 2017 Valid 261300Z - 271200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE ARKLATEX REGION INTO CENTRAL AND SOUTHEASTERN ARKANSAS... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM PARTS OF EAST TEXAS TO SOUTHEASTERN MISSOURI AND NORTHERN MISSISSIPPI... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AND EXTENDING INTO NORTHERN PARTS OF EASTERN ILLINOIS AND WESTERN INDIANA... ...SUMMARY... Severe thunderstorms are expected across the Lower Mississippi Valley. Tornadoes, large hail, and damaging winds are most probable from northeast Texas across parts of Arkansas and northern Louisiana through afternoon. Isolated severe thunderstorms may extend as far north as Illinois and into the central Gulf States during the overnight hours. ...Synopsis... The mid/upper-level pattern will remain characterized by a synoptic-scale trough over portions of the Rockies and Great Plains regions. Associated cyclonic flow aloft will spread eastward overs the Mississippi Valley, upper Great Lakes and lower/middle Ohio Valery through the period, traversed by several shortwaves of variable amplitude. The primary/basal shortwave trough -- now evident in moisture-channel imagery over the southern High Plains to the Big Bend region of TX -- is forecast to pivot eastward across TX and OK through the day, reaching southeastern KS, eastern OK, and northeast through south-central TX by 00Z. Overnight, this feature will weaken and eject northeastward to IL and western TN. Meanwhile, a weaker perturbation approaching the Pacific Northwest coast will move southeastward and inland through the period, extending from the northern Rockies to near the Four Corners by 12Z. A 150-170-kt 250-mb jet will follow this perturbation inland. Ahead of that shortwave trough, a broad area of cold air aloft and weak low-level moisture will support a general thunderstorm potential over parts of the northern Rockies and Great Basin. At the surface, a cold front extended from a surface low near LSE across eastern IA, southwestward across MO, southeastern OK, extreme north-central TX, and west-central TX. The surface low should migrate erratically northeastward across WI and the northern Lake Michigan/eastern Upper Michigan area, reaching eastern Lake Superior or adjoining portions of ON by the end of the period. The cold front by 00Z should extend from southeastern WI across northern/ central IL, eastern AR, western LA, to the mid/upper TX coast. By 12Z, the front should extend from western Lower MI across IN, central KY, middle TN, northern AL, southeastern MS, southeastern LA, and the northeastern Gulf. ...East TX to Mid-South... A band of thunderstorms currently over parts of southeastern OK and western AR should encounter favorably moist low-level/warm-sector air that will destabilize through the morning, amidst strengthening deep shear. The main hazards through the rest of the morning will be hail and damaging gusts; however, a tornado or two cannot be ruled out. For more near-term information, see SPC watch 161 and related mesoscale discussions. The warm sector is forecast to destabilize through the afternoon, offering increasing buoyancy juxtaposed with strengthening deep shear and favorable low-level hodographs. Surface dew points commonly in the mid/upper 60s F and steep midlevel lapse rates will contribute to MLCAPE 1500-2500 J/kg, amidst 45-60 kt effective-shear magnitudes. Some uncertainty remains regarding density of convection within the line and its effect on severe type/coverage. A pearl-necklace configuration of embedded supercells/bows would support more of a tornado risk, as has been outlooked, whereas more solid coverage would transition the threat to mainly wind with embedded/QLCS mesovortices offering some tornado concern. With early development over the DFW Metroplex this morning indicating sufficient large- scale lift for prefrontal storms, and with the potential for line-embedded supercells moving into a favorable parameter space for a tornado threat, will maintain the moderate-risk and enhanced-risk equivalent tornado probabilities for now, but each shifted slightly northward in deference to two features: 1. Afternoon surface winds -- relatively backed (southerly to southeasterly) flow progged to occur north of I-20 and primarily into AR, with somewhat veered winds from I-20 southward in east TX and western LA, and 2. The outflow boundary from earlier convection draped across north-Central AR, which may act both as a focus and northern delimiter for substantial severe potential in that state as it slowly retreats northward ahead of the near-frontal storms. The severe threat with the main band of convection may extend as far east as central to eastern AL and middle TN overnight, given forced ascent on the convective boundary, intensifying gradient flow above the surface related to the ejecting mid/upper perturbation, and favorable low-level moisture. Weakening of both lapse rates and boundary-layer instability also is forecast with eastward extent across AL/TN/KY, accounting for the diminishing probabilities for very late in the period. ...Mid Mississippi Valley, IL, IN... The northern extension of the convective regime discussed above may produce damaging gusts over parts of this region today, with isolated hail and a conditional/marginal tornado probability as well. Confidence remains low, and uncertainty strong, regarding sufficient destabilization with northward extent across this area to support more than a slight to marginal categorical gradation in equivalent wind probabilities today, despite the strengthening deep-layer wind fields and infusion of at least somewhat supportive low-level moisture. The concerns mainly revolve around weak low/middle-level lapse rates and resultant weak buoyancy, including restrictions on diurnal destabilization imposed by 1. Direct effects of cloud cover streaming off convection farther southwest and 2. Low-level trajectories emanating at least partly from areas of convective outflow that have surged eastward over eastern/southern MO and northern AR. However, the potential for an organized band of wind-damage-producing convection to reach at least the lowest part of the Ohio Valley appears sufficient to include that area in a 15% outlook for now, which may need to be extended further as mesoscale/ destabilization trends warrant. ..Edwards/Mosier.. 04/26/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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