SPC Mar 17, 2018 1300 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 17 Mar 2018

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0759 AM CDT Sat Mar 17 2018 Valid 171300Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL/NORTH-CENTRAL TEXAS TO PARTS OF EAST TEXAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF THE SOUTH-CENTRAL APPALACHIANS ACROSS NORTH CAROLINA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK IN THE APPALACHIAN REGION... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE TX SLIGHT RISK AND EXTENDING INTO THE ARKLATEX... ...SUMMARY... Severe thunderstorms are possible this afternoon into tonight from the south-central Appalachians to northern/eastern North Carolina, and over parts of central Texas into the Arklatex region, with hail and damaging gusts possible in each area. ...Synopsis... The mainland portion of the mid/upper-level pattern is expected to remain dominated on the synoptic to large scale by two complex cyclones, containing multiple embedded vortices: the first covering much of QC, New England and the Canadian Maritimes, and the second over the Pacific Northwest and southwestern Canada. A progressive shortwave field will exist around and between those gyres. Two leading, nearly phased, low-amplitude perturbations -- apparent in moisture-channel imagery over the CVG region (KY/IN/OH) and over central AL to western GA -- will move over the Atlantic east of present positions during 18-21Z. A compact, upstream cyclone -- currently over northern MO and central/southern IA -- is expected to devolve to an open-wave trough in the next several hours. This feature should accelerate southeastward across the Ohio Valley around 00Z and offshore from the VA/NC Tidewater region between 06-12Z. Meanwhile, a strong shortwave trough and accompanying speed max -- initially located west of northern CA -- will pivot through the base of the synoptic trough attendant to the Northwest cyclone. The shortwave trough will reach AZ by 12Z. As this occurs, northeastward ejection is forecast for a southern-stream/ subtropical-branch perturbation, currently apparent in moisture- channel loops around 127W, between 18-24N. As height falls occur to its north, this feature should eject rapidly northeastward to far west TX and Chihuahua by the end of the period. In turn, a weaker/preceding perturbation -- now located from southern NM southwestward across Sonora -- will move northeastward to parts of southwestern OK and est TX by 00Z, and weaken in the mean ridge position over AR and northeast TX overnight. At the surface, 11Z analysis showed an occluded low over northeastern MO, with an occluded/warn frontal zone extending east-southeastward across eastern KY and becoming diffuse in the adjoining mountains. A secondary warm-frontal zone was drawn from southern IL southeastward across northern/east-central AL. A cold front extended southwestward from near PAH over eastern AR, becoming wavy and quasistationary across the TXK area to a weak low between FST-MAF. A dryline intersected this front east of the low, near SEP. As low-level warm/moist advection occurs through the day, the warm-frontal zones should consolidate across a corridor from eastern KY over southwestern VA, with the combined warm front reaching north-central/eastern NC tonight. The TX surface low should shift eastward to at or near the dryline/front triple point in the northern Hill Country or just north of there near I-20 this afternoon. ...Central TX to Arklatex region... Severe thunderstorms are expected to develop this afternoon over the northern Hill Country, near the front/dryline triple point and perhaps adjoining parts of either boundary, moving northeastward then eastward as supercellular evolution takes place. Large, locally significant/damaging hail and strong-severe gusts are expected. Although the low-level winds are not ideal in terms of near-ground shear/SRH, some window may exist for tornado potential if relatively discrete supercell characteristics can be maintained into late-afternoon/early-evening period when hodographs enlarge somewhat and LCL lowers, before the near-surface air stabilizes diabatically too much. Forecast soundings suggest favorable surface heating (even under mid/upper-level cloud cover), steep low/middle-level lapse rates, 60s F surface dew points, weak MLCINH, and MLCAPE rising to near 2000 J/kg. Low-level shear and hodograph size may be modest for much of the afternoon; however, deep shear will be favorable with effective-shear vectors around 50 kt anticipated. Activity should progress eastward and become more clustered east of about I-45 and over parts of East TX and the Arklatex region, offering at least sporadic hail/gust potential into mid/late evening before substantial weakening occurs. ...South-central Appalachians across NC... An ongoing, broken swath of precip -- with widely scattered to scattered embedded thunderstorms in clusters -- was evident in reflectivity composites from parts of western SC, central GA and northern AL to portions of western WV, eastern KY and OH. This activity is related to the lead perturbation/vorticity lobe, and should proceed eastward through the remainder of the morning with general weakening probable. In its wake, the air mass over the western part of the outlook area -- generally west of the highest Appalachians - is expected to destabilize through the day from three main factors: 1. Diabatic heating and related steepening of boundary-layer lapse rates; 2. Low-level warm advection, further aiding air-mass recovery from the morning clouds/precip; 3. Large-scale DCVA/cooling aloft immediately ahead of what is now the IA/MO mid/upper cyclone. Early-stage activity will form this afternoon in the frontal zone, residual convective boundaries and perhaps areas of well-heated high terrain, in an environment of strong, supercell-favoring deep shear (effective-shear magnitudes 50-60 kt along and just south of the front). Forecast soundings suggest 50s F surface dew points and available heating will boost MLCAPE commonly into the 500-1500 J/kg range west of the highest terrain, with values locally near 1500 J/kg possible as midlevel lapse rates steepen. The potential for relatively discrete rotating modes to develop indicates severe hail, despite the lack of more robust buoyancy. It is still in clear whether some evolution of the afternoon storms move entirely over the mountains and strengthen again to their east, or additional development occurs this evening east of the Blue Ridge as the warm front and favorable boundary-layer theta-e impinge on the NC Piedmont area. Either way, a cluster or small MCS with damaging-wind potential is possible this evening into tonight, moving coastward in and near the frontal zone. ..Edwards/Mosier.. 03/17/2018 Read more



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