Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0759 AM CDT Mon Oct 02 2017 Valid 021300Z - 031200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF WESTERN KANSAS AND SOUTH-CENTRAL NEBRASKA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM THE NORTHERN TEXAS PANHANDLE TO PARTS OF NORTHEASTERN COLORADO AND THE UPPER MISSISSIPPI VALLEY... ...SUMMARY... Several episodes of strong to severe thunderstorms are expected from afternoon at least into late evening, in a corridor from the central Great Plains to the upper Mississippi Valley. At this time, the greatest potential for organized severe weather appears to be over parts of western Kansas into south-central Nebraska. ...Synopsis... A high-amplitude large-scale pattern, characterized by eastern ridging and western troughing, will dominate the continental U.S. this period. Associated cyclonic flow will continue to cover much of the West; however, the trough will reorient toward a positive tilt, in response to the behavior of several shortwaves embedded in that flow field. A leading/closed 500-mb low will meander over eastern MT the whole period. A trailing shortwave trough -- now evident in moisture-channel imagery from southwestern UT to near the southern tip of NV, will eject northeastward. By 00Z, this perturbation should reach the UT/CO border region. The shortwave trough and its vorticity field then should accelerate northeastward, weaken and become deformed amidst confluent flow rimming the MT cyclone. Meanwhile, another shortwave trough -- now extending from the inland Northwest across Vancouver Island, should pivot southward and strengthen into a closed cyclone over northern CA by 12Z. At the surface, 11Z analysis showed an occluded front from southern MB southward across western MN. The occlusion triple point over the Siouxland region was ill-defined amidst stabilized air from precip, but the associated quasistationary front became better-defined southwestward across eastern NE, northern/western KS and northern NM. The High Plains portion of the front may retreat northward for several hours this afternoon into this evening, beneath neutral to weakly positive height-change fields aloft. A dryline, drawn initially from northeastern NM southwestward between ELP-LRU, should mix northeastward across the western Panhandles and into southwestern KS today south of the front. ...Central Plains to Upper Mississippi Valley... Multiple episodes or loci of strong to severe convective development are possi ble within this corridor. Some spatial overlap and/or mergers between them comprise the outlook areas as a whole. The most probable initial regimes are: * Dryline to vicinity of cold-frontal intersection: Isolated to widely scattered, surface-based thunderstorm development now appears increasingly probable mid/late afternoon, near the dryline, along and south of its intersection with the cold front. A growing consensus of convection-allowing guidance support this scenario, which appears reasonable given the heating expected just south of the front along the dryline, along with related removal of MLCINH in forecast soundings, in conjunction with convergence along the front itself. Planar progs and forecast soundings suggest the parameter space into which this activity would move will favor supercell structures with large hail and isolated damaging gusts possible. Surface dew points low-mid 60s F and diabatic heating along and just ahead of the dryline should yield MLCAPE exceeding 2000 J/kg in a narrow corridor, juxtaposed with 45-55 kt effective-shear magnitudes and 200-300 J/kg effective SRH. Hail-model application to forecast soundings suggests that at least isolated hail near 2 inches in diameter may occur, especially with any sustained/discrete supercell. A sig-hail probability line may be introduced in a later outlook if confidence increases in coverage/duration of supportive storm mode. A tornado cannot be ruled out, especially with any discrete or semi-discrete storm that remains surface based while interacting with the baroclinic zone. Northern members of this convective plume eventually may merge with the southern part of the post-frontal band described below. * Post-frontal, High Plains initiation: As large-scale lift aloft related to the Great Basin shortwave trough approaches late this afternoon, widely scattered to scattered thunderstorms should develop near the Foothills and eastward across the High Plains of northeastern CO. This process will be supported further by a substantial easterly/upslope component to the low-level flow. Initially dependent mainly on elevated inflow parcels, this activity should encounter an increasingly unstable layer above the surface, resulting from low-level theta-e advection, and may produce isolated large hail. Potential exists for upscale growth into a wind-favoring bowing segment or cluster, whose downward momentum transfer may become strong enough to penetrate the near-surface/post-frontal stable layer with near-severe gusts in a few locales. This convection may last long enough to merge with the southwestern parts of the next regime. * Post-frontal band, central Plains northeastward: Convective coverage should increase from late afternoon through evening behind the surface front, from the central Plains into parts of the upper Mississippi Valley region, offering the risk of at least isolated large hail. The main limiting factor for hail coverage/intensity may be the presence of too many storms and related modal interference, though this also may support greater than currently depicted potential hail coverage early in the cycle of convective development, prior to excessive mergers/interference. Elevated MUCAPE 1000-2000 J/kg and favorable deep shear are expected over this region. ..Edwards/Dial.. 10/02/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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