SPC Oct 20, 2017 0600 UTC Day 2 Convective Outlook

By Newsroom America Feeds at 20 Oct 2017

SPC 0600Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1155 PM CDT Thu Oct 19 2017 Valid 211200Z - 221200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE SATURDAY AFTERNOON AND EVENING ACROSS MUCH OF CENTRAL AND EASTERN OKLAHOMA AND ADJACENT PORTIONS OF SOUTHEASTERN KANSAS AND NORTHERN TEXAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE PLAINS...NORTHWARD THROUGH THE LOWER MISSOURI AND UPPER MISSISSIPPI VALLEYS... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AREA... ...SUMMARY... Widespread thunderstorm development is expected late Saturday afternoon into Saturday night, from portions of the upper Mississippi Valley into the southern Plains. Some of this activity, particularly across the Plains, will be accompanied by a risk for potentially damaging wind gusts, some hail and perhaps a couple of tornadoes. ...Synopsis... Models indicate that large-scale mid/upper troughing, currently advancing inland of the Pacific Coast, will remain progressive through this period, likely reaching Manitoba, northwest Ontario, the upper Mississippi Valley and southern U.S. Plains by 12Z Sunday. Within this regime, the corridor of strongest mid-level height falls are forecast to spread east/northeast of the Canadian Prairies and northern U.S. Plains Saturday/Saturday night, associated with a couple of significant embedded short wave impulses. But another digging impulse may contribute to mid-level height falls across the southern Plains Red River Valley region Saturday night, while beginning to split away from the base of larger-scale troughing to the north. In lower levels, a deep associated surface cyclone is forecast to migrate from southern Manitoba into Hudson Bay, while a modest trailing cold front advances eastward/southeastward through much of the U.S. Plains and mid/upper Mississippi Valley region by the end of the Period. ...Plains/Mississippi Valley... Models generally indicate that the mid-level cold core and strongest mid-level forcing for ascent may tend to lag to the west of the cold front through the period. And severe weather potential, in general, may hinge on how fast the eastward and southeastward advancing cold front tends to undercut the pre-frontal initiating convective development, which remains unclear at this time. However, it appears that there will be at least a window of opportunity for substantive pre-frontal thunderstorm activity, accompanied by a risk for severe weather. At least a narrow plume of seasonably moist air (characterized by mid/upper 60s surface dew points) appears likely to precede the front in a corridor from the southern Plains into the upper Mississippi Valley. Models suggest that low-level moistening will occur beneath a plume of modestly steep lapse rates associated with elevated mixed layer air, which may contribute to CAPE up to 1000+ J/kg, and 2000 J/kg across north central Texas into Oklahoma. Although deep layer wind fields are not expected to be exceptionally strong, 30-50 kts in lower/mid-levels (somewhat stronger across parts of the lower Missouri Valley into upper Mississippi Valley), should be more than sufficient to support organized severe weather potential, given the instability. Guidance appears suggestive that vigorous storm development may initiate first across parts of the east central Plains and middle Missouri Valley late Saturday afternoon, before intensifying while increasing and spreading northeastward toward the upper Mississippi Valley through Saturday evening. This could include isolated discrete supercells early, before evolving into a broken squall line, with damaging wind gusts becoming the primary severe threat. Farther south, stronger instability, coupled with increasing forcing for ascent (enhanced by increasingly divergent high-level flow) may support storm initiation across parts of western/northern Oklahoma and adjacent portions of the Plains by early Saturday evening. This may include discrete supercells initially, in a corridor ahead of the cold front, and near its intersection with the dry line. However, fairly rapid and considerable upscale convective growth appears probable, with potentially damaging wind gusts becoming the primary concern by mid to late evening, before tending to become undercut by the southward advancing cold front. ..Kerr.. 10/20/2017 Read more



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