Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Fri Apr 21 2017 Valid 211300Z - 221200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF SOUTH-CENTRAL/SOUTHEASTERN OKLAHOMA AND NORTH-CENTRAL/NORTHEAST TEXAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM WESTERN OKLAHOMA TO NORTH TEXAS AND CENTRAL ARKANSAS... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK...AND EXTENDING ACROSS THE MID-SOUTH TO PARTS OF THE COASTAL MID-ATLANTIC... ...SUMMARY... Severe thunderstorms may offer damaging gusts, large hail and a few tornadoes through tonight from parts of the southern Plains into the Ozark Plateau. Isolated severe thunderstorms may also impact portions of the Mid-South into the Mid-Atlantic with wind and hail. ...Synopsis... The mid/upper levels will be characterized by a complex pattern, involving split flow downstream from a negatively tilted trough now located over the Rockies. A leading/northern-stream perturbation -- now evident in moisture-channel imagery over the upper Great Lakes and northern ON -- is forecast to migrate eastward to QC through the period, with a trailing trough back southwestward over OH at 12Z. Upstream shortwave ridging over the northern Plains will form a temporarily closed 500-mb high over the Dakotas, as a segment of the Rockies trough undercuts it and moves across the central Plains. Late in the period, that high will devolve back to a shortwave ridge and begin to phase with progressive ridging over the Great Basin and northern Rockies. Meanwhile another vorticity max over the northern Rockies should dig southeastward. The net result of all this height and vorticity rearrangement, by the end of the period, should be a small mid/upper-level cyclone moving from southern KS to southern/western MO, with a trough west-northwestward across KS to a secondary low/vorticity max over northern CO and southern WY. The nose of a cyclonically curved, 110-120-kt 250-mb jet streak will shift eastward across the southern High Plains and western OK by 00Z. By 12Z, that jet streak will arch from the lower Ohio Valley across OK to CO and western WY. At the surface, a cold front associated with the Great Lakes trough extended from northern/western PA southwestward across eastern KY and western TN, becoming a stationary to slowly moving warm front fro central AR across extreme southern OK, through a weak frontal-wave low near FDR, and another low near the southeastern corner of NM. This front will be reinforced by outflow from convection that will sweep east-southeastward across OK this morning into afternoon, as discussed below. Late this afternoon, a frontal-wave low should form over eastern OK, moving to parts of central/northeastern AR by 12Z with the front trailing across east TX, south-central TX, and the Big Bend region. The front will intersect a dryline this afternoon over the Big Country region of west-central TX, then overtake the dryline as it plunges southward through the Edwards Plateau overnight. ...Southern Plains to AR... An organizing, initially elevated MCS -- moving east-southeastward from the TX Panhandle over western OK -- will present a risk for large hail this morning. Also, an increasing threat for severe gusts is expected as the forward-propagational cold pool gets better organized and approaches progressively higher-theta-e, less-stable air nearer to the front. See SPC watch 153 and related mesoscale discussions for near-term details. Also, clusters of thunderstorms with isolated, embedded severe hail cores and gusts approaching severe limits are ongoing over parts of central OK and western AR, and will continue to offer a threat for sporadic severe while shifting eastward through the remainder of the morning. With time, the MCS will progress further across central and eastern OK, becoming closer to surface-based as it encounters destabilizing air near the surface. Additional thunderstorms are expected to develop this afternoon over southern OK and north-central TX near the Red River, along the effective, cold-pool-augmented frontal surge extending southwest from the complex. The environment into which this activity will move will be favorable for all severe threats from supercells, bows and clustered modes. The environment along and south of the frontal zone will be characterized by strengthening vertical shear and large-scale ascent, as well as pockets of diabatic heating amidst surface dew points in the 60s F. This will contribute to a very favorable parameter space for supercells and at least a few tornadoes, with large low-level hodographs, effective SRH 200-500 J/kg, effective-shear magnitudes 60-65 kt, and MLCAPE around 2000 J/kg. Any relatively discrete and sustained supercell impinging on that sort of parameter space could produce a significant tornado as well as very large/damaging hail. However, the significant-tornado risk still remains too conditional and uncertain for an areal sig-tornado outlook due to concerns over: 1. The surging cold front/outflow and its potential interaction with/undercutting of storms, and 2. Potential for convective mode to get rather clustered and messy near the front, even without direct undercutting of updrafts by the cold surge itself. The severe potential should wane after about 06Z as the regime becomes more anafrontal and the remaining convection encounters a stabilizing boundary layer. ...Mid-South to Mid-Atlantic... Widely scattered to scattered thunderstorms are expected to develop this afternoon near the front, with more isolated coverage in the warm sector. Activity should develop and intensify in a modestly capped, destabilizing air mass characterized by surface dew points generally in the mid 50s to low 60s (locally higher), offsetting weak mid/upper-level lapse rates enough for 500-1500 J/kg MLCAPE. Pockets of relatively sustained surface diabatic heating and well-mixed subcloud layers will favor strong/isolated damaging downdrafts. Weak near-surface winds with a substantial westerly component will restrict low-level shear; however, sufficient flow aloft should exist for around 30-40 kt effective-shear magnitudes in some areas, in support of multicellular to marginally/briefly supercellular organization. As such, a few severe hail reports also cannot be ruled out. ..Edwards/Dean.. 04/21/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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