Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1230 PM CDT Tue May 16 2017 Valid 171200Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY INTO WEDNESDAY NIGHT ACROSS PARTS OF IOWA AND NORTHERN MISSOURI INTO PORTIONS OF SOUTHEASTERN MINNESOTA...NORTHWESTERN ILLINOIS AND SOUTHERN/CENTRAL WISCONSIN... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE LOWER MISSOURI VALLEY INTO THE UPPER MIDWEST...SOUTHWARD INTO THE OZARK PLATEAU... ...SUMMARY... Severe thunderstorms are expected Wednesday into Wednesday night, particularly near and northeast of the lower Missouri Valley into portions of the Upper Midwest. Some of these will be accompanied by the risk for tornadoes, one or two of which could be strong. ...Synopsis... A belt of westerlies emanating from the mid-latitude Pacific appears likely to remain amplified across the U.S. through this period, with broad troughing inland of the Pacific coast into the Mississippi Valley, and ridging remaining prominent eastward across much of the Atlantic Seaboard. Within the upper troughing, one vigorous short wave impulse emerging from its base, now across the Four Corners region, is forecast to continue a northeastward acceleration to the east of the Rockies Wednesday/Wednesday night, as a more significatn upstream impulse digs southeast of the northern Intermountain region through the Great Basin. Models suggest that the lead impulse, accompanied by a deep embedded lower/mid tropospheric cyclone, will probably maintain considerable strength across the central Plains through portions of the mid/lower Missouri Valley during the day Wednesday. Thereafter, it may begin undergoing considerable deformation and weakening within an increasing confluent regime across the Upper Midwest into upper Great Lakes region, ahead of a short wave impulse pivoting around the southwestern periphery of a closed low to the northeast of Hudson Bay. Within the warm sector of the cyclone, ahead of a trailing cold front, and along a stalled frontal zone to the northeast, a continuing return flow of Gulf Moisture appears likely to support sufficient destabilization to support considerable thunderstorm activity Wednesday through Wednesday night. Some of this probably will be severe, including a risk of large hail, locally damaging wind gusts and few tornadoes. Additional scattered thunderstorm activity appears possible beneath the mid-level cold pool spreading across parts of the Great Basin and Rockies, and in association with low-level moistening and daytime heating beneath and near the crest of the eastern ridge, along the eastern slopes of the central Appalachians, and across parts of northern New England. Destabilization may also become sufficient to support a few thunderstorms across interior and coastal south Florida. ...Lower Missouri/Mississippi Valley region... Some uncertainties linger, primarily concerning the extent of warm sector boundary layer destabilization across parts of the lower Missouri Valley into Upper Midwest during the day Wednesday. Considerable modification of the broad plume of warm elevated mixed layer air, now present across much of the central/southern Plains into the lower half of the Mississippi Valley, appears probable by early Wednesday, with the axis of at least the northern half of the narrowing remnant plume tending to spread east of the mid/upper Mississippi Valley. And models suggest that remnants of prior convection may tend to inhibit or at least slow the development of a corridor of stronger heating ahead of the surface cold front wrapping into the surface low center. Despite these related issues, guidance suggests that cooling aloft, within the exit region of a 70-90 kt cyclonic 500 mb jet streak, above a moist boundary layer characterized by surface dew points in the lower/mid 60s, probably will contribute to modest warm sector destabilization (CAPE of 1000+ J/kg) by late afternoon. Aided by favorable lower/mid tropospheric forcing for ascent, this is expected to be sufficient to support increasing discrete storm development across parts of northwest Missouri/southwestern Iowa by mid to late afternoon, before spreading northeastward across Iowa and northern Missouri, toward the Upper Midwest through the remainder of the afternoon and evening. With 40-50 kt south to southwesterly 850 mb flow contributing to large clockwise curved low-level hodographs, the environment could become conducive to supercells capable of producing tornadoes, some strong, in addition to severe hail and locally damaging wind gusts. This threat may continue through Wednesday, before diurnal boundary layer stabilization and weakening of the low/mid-level synoptic system contribute to weakening trends. Other, more widely scattered strong/severe storm development appears possible southward, well ahead of the cold front, across parts of the middle Mississippi Valley and Ozark Plateau Wednesday afternoon and evening. ..Kerr.. 05/16/2017Read more
from SPC Forecast Products http://ift.tt/ws9HZn
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