SPC May 18, 2017 1630 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 18 May 12:46

SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1139 AM CDT Thu May 18 2017 Valid 181630Z - 191200Z ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF SOUTHWESTERN AND CENTRAL KANSAS SOUTH INTO NORTHWEST OKLAHOMA... ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE SOUTHERN TWO-THIRDS OF KANSAS AND INTO WESTERN AND NORTHERN OKLAHOMA... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SURROUNDING THE HIGH AND MDT RISK AREAS...AND EXTENDING EAST TO FAR WESTERN MISSOURI AND SOUTH INTO CENTRAL TEXAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS A BROAD SWATH OF THE CENTRAL AND SOUTHERN PLAINS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE LOWER GREAT LAKES VICINITY EAST-NORTHEAST TO WESTERN NEW ENGLAND... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE CENTRAL AND SOUTHERN PLAINS SURROUNDING HIGHER-RISK AREAS...AND EAST-NORTHEAST ACROSS THE MIDWEST AND INTO NEW ENGLAND SURROUNDING HIGHER-RISK AREAS... ...SUMMARY... An outbreak of severe thunderstorms with tornadoes, very large hail and wind damage is expected to develop across parts of the southern and central Plains from this afternoon into this evening and overnight. Some strong, long-lived tornadoes are expected, along with hail to near 4 inches in diameter, over parts of Kansas and Oklahoma. Large hail, tornadoes and wind damage also will be possible outside of these areas across parts of the southern and central Plains. Locally damaging gusts and sporadic hail also are expected across parts of the lower Great Lakes region to western New England this afternoon and early evening. ...Synopsis... A large upper low centered over Utah this morning and comprised of two smaller centers of upper circulation -- one moving east-northeast over southwest Colorado and the other moving south-southwest across northeast Nevada and western Utah -- is forecast to weaken overall while making very slow eastward progress today. With large-scale/broad upper troughing surrounding this feature affecting roughly the western half of the country, dowstream ridging will largely prevail over the east. The exception will be across the Great Lakes and into New England, where low-amplitude northern-stream short-wave troughing will progress eastward with time, north of the southern-stream ridge. At the surface, a low over the upper Great Lakes this morning is forecast to move/develop eastward through southeast Canada -- crossing the Ottawa and St. Lawrence Valleys through tonight. A trailing cold front will become increasingly west-to-east oriented across the Midwest/Ohio Valley region, as the low moves quickly eastward, while some northward retreat of the front may occur into Kansas as a southern Plains lee low strengthens ahead of western U.S. upper troughing. Continued northward advection of low-level moisture across the southern Plains and into parts of Kansas will support development of widespread strong-to-severe storms near this front, and near and ahead of a dryline extending southward across Texas and Oklahoma. As a result, a broad, potentially high-end, all-hazards outbreak of severe weather is expected this afternoon. ...Central and southern Plains... A complex -- but likely higher-end -- severe weather outbreak is expected to evolve this afternoon and evening, near and south of a warm front moving slowly northward into Kansas and east of a dryline mixing slowly eastward across the southern High Plains. Very moist low-level air continues streaming northward ahead of the developing lee low over the High Plains, which combined with strong heating and very steep lapse rates aloft within the elevated mixed layer will result in strong destabilization, with mixed-layer CAPE into the 3000 to 4000 J/kg range expected by late afternoon. Along with initiation near the warm front and southward along the dryline mid afternoon, CAMs as well as to some degree operational model runs continue to strongly suggest a secondary area of convective development across parts of central Texas, ahead of the dryline. Cellular initial development in all areas will rapidly organize/become supercellular, with attendant risk for very large hail and locally damaging winds. In addition, with low-level flow likely to remain backed near the frontal zone and southward across western and central Oklahoma, a very favorable low-level wind field (0-1 km shear in excess of 40 kt and helicity in excess of 300 m2/s2) suggests that right-moving supercells will be favored, and capable of producing strong/damaging tornadoes. With time, guidance suggests upscale growth of convection -- both across Texas with storms spreading eastward across central and north Texas through the evening, and across the Kansas/Oklahoma area where one or more northeastward-surging, bowing bands of storms are indicated. Given the degree of CAPE, dry air aloft, and strong shear, an organized/widespread damaging wind risk is also apparent. Greatest overall combined risk appears to remain across Kansas and the northwest half of Oklahoma, though expansion of the ENH risk area (to include all severe hazards) is being introduced southward to central Texas at this time. Risk should continue through the evening and into the overnight hours, with bands of storms spreading east into parts of Missouri and Arkansas, but with likelihood for some continued redevelopment of storms across western portions of the risk area near the dryline, as large-scale ascent persists ahead of the slowly advancing western upper trough. ...Lower Great Lakes vicinity into portions of New England... Showers and scattered thunderstorms are forecast to increase this afternoon, from the Midwest/Ohio Valley area east-northeast across the lower Great Lakes and into parts of New England. A moist boundary-layer airmass in place (upper 50s to low- to mid-60s) will continue to heat/destabilize this afternoon, permitting storms to form both along the advancing cold front, as well as along pre-frontal confluence bands and lake-breeze boundaries. While somewhat modest west-southwesterly flow aloft is expected over much of the Ohio Valley area, limiting severe risk to some degree, stronger deep-layer west-southwesterlies centered in the vicinity of the international border suggest potential for greater storm organization and possibly upscale growth into one or more clusters/bands, which will be capable of producing locally damaging winds and hail. Coverage of storms -- and thus severe risk -- will diminish gradually through mid to late evening, as the boundary layer diurnally stabilizes. ..Goss.. 05/18/2017 Read more



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