Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Tue May 16 2017 Valid 161300Z - 171200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF WEST TEXAS TO PARTS OF IOWA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK AND EXTENDING TO PARTS OF THE CENTRAL HIGH PLAINS AND UPPER MISSISSIPPI VALLEY... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK... ...SUMMARY... Episodes of severe thunderstorms are expected today and tonight within a swath from west Texas across the central Plains to the western Corn Belt and mid Missouri Valley. Tornadoes, severe gusts, and large, damaging hail are expected within this corridor. ...Synopsis... Progressive synoptic-scale troughing will dominate the upper-air pattern today over the western/central U.S., with two primary/embedded perturbations influencing low-level mass fields and convective potential: 1. A strong, compact, mid/upper-level cyclone -- now apparent in moisture-channel imagery over Vancouver Island, forecast to move southeastward to southern ID by the end of the period. As heights fall across the northwestern U.S. and Great Basin ahead of that feature... 2. A strong shortwave trough, with embedded 500-mb low over southern NV at this time, will pivot eastward then east-northeastward across the Four Corners and northwestern MX. By 12Z, the trough should extend from eastern CO across eastern NM and far west TX. The 11Z surface analysis showed a surface low over northeastern SD, warm front from there across northeastern IA and southern Lake Michigan, and cold front southwestward across central NE to northeastern Co, and a reinforcing cold front over central SD and south-central WY. A wavy dryline was drawn from near IML southward across the KS/CO border area to the western TX Panhandle, southeastern NM and far west TX. The SD low should move northeastward across MN to northern ON through the period, while a second low deepens by 12Z along the combined frontal zone over southern NE. The dryline will mix eastward across western KS, the TX/OK Panhandles and South Plains today, and off the Caprock by mid/late afternoon. Cold frontogenesis is expected over eastern NM and west TX this evening and overnight as the shortwave trough ejects, with the resulting baroclinic zone overtaking the dryline from the west tonight. Though the combined "slight" and "enhanced" outlook area appears as one continuous swath on the map, it mostly represents the spatial overlap of three rather distinct severe-storm regimes. We'll emphasize those below, in rough start-time order. ...Early afternoon through evening: northern TX/OK Panhandles, Western/central KS to IA... Widely scattered to scattered thunderstorms should form during early-mid afternoon near the dryline where the EML and MLCINH weaken soonest. This activity initially will risk large/damaging hail, severe gusts and a few tornadoes. Within the first few hours after initiation, however, a preponderance of guidance from synoptic down to high-res/convection-allowing progs indicate aggregation of convection into a forward-propagating MCS, which may sweep northeastward across KS and the Missouri Valley at least across parts of IA through this evening and tonight, before encountering enough stable air to weaken. This convection-evolution scenario appears plausible given: 1. The geometry of the deep-layer flow, 2. Sufficiently weak afternoon/early-evening CINH to be prone to forced slab ascent by aggregating cold pools, 3. A moist axis already in place to support a corridor of maximized buoyancy later today from central KS to IA, southeast of residual outflow pools from prior overnight convection, and 4. Well-mixed subcloud layers in forecast soundings that support an increasing damaging-wind potential with northeastward extent, given such modal morphology. Activity will be supported by a southwest-northeast plume of preconvective MUCAPE ranging from 2000-3500 J/kg, weakening late tonight in the northeastern part as CINH increases, along with 35-45 kt effective-shear magnitude. Pockets of favorable low-level SRH may develop to support some tornado potential even in a QLCS mode, whether with embedded supercells or misovortices. ...Midafternoon into evening: Southern Plains dryline... In mid/late afternoon, several discrete to semi-discrete supercells are expected to develop and move northeastward at 25-35 kt off the dryline. As this activity crosses the eastern Panhandle region, northwest TX, and parts of western OK, it will offer tornadoes, very large/damaging hail and sporadic severe gusts. Some giant (3-inch or more) hail is possible, along with potential for a couple of strong, relatively long-lasting tornadoes, from the mature phases of at least one or two cyclic supercells within convective arc. The all-hazards threat from these storms should last at least an hour or two into early evening, when the low-level jet increases and enlarges hodographs, and before the boundary layer stabilizes too much to permit sustained surface-based inflow. Thereafter, these supercells should spin down over western OK and northwest TX, south of the relatively fresh/cold outflow from the KS complex. Te environment will be characterized by strong surface heating and mixing along the dryline, with forecast soundings suggesting: near-zero MLCINH as early as around 20Z, 60s F surface dew points beneath steep midlevel lapse rates, 2500-3500 J/kg MLCAPE in a narrow plume ahead of the dryline, 60-65 kt effective-shear magnitude, and afternoon effective SRH 150-250 J/kg increasing to near 400 J/kg along the western rim of the 40-55 kt evening LLJ. Winds aloft should be oriented with enough of an orthogonal component to the dryline to support some discrete storms, though a few forecast hodographs show evidence of slight veer-back-veer weaknesses in the 2-4-km AGL layer. CINH will strengthen considerably eastward toward the US-81 corridor ahead of this activity and ahead of the main plume of large-scale forcing arriving later overnight. This should lead to late-evening demise of most of these afternoon/dryline supercells. ...Overnight: Southern Plains front/dryline... A mixture of initially discrete supercells and a band of thunderstorms should develop along the dryline and the overtaking front this evening and tonight. Any supercells will pose the risk of large, damaging hail, given the available moisture, large lapse rates/buoyancy, and favorable deep shear. Frontogenetic forcing and large-scale DCVA/cooling aloft will impinge on the moist sector and lead to a mostly quasi-linear belt of convection overnight, with damaging wind becoming the dominant threat before the activity encounters increasing CINH and weakening LLJ-related storm-relative flow late overnight. A tornado or two cannot be ruled out throughout either phase as well. ..Edwards/Dial.. 05/16/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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