Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Sat Jul 22 2017 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER OHIO VALLEY TO PARTS OF THE MID-ATLANTIC... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE UPPER MIDWEST/UPPER GREAT LAKES REGION... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISKS AND EXTENDING ACROSS THE MIDDLE MISSISSIPPI/LOWER MISSOURI VALLEYS TO CENTRAL PLAINS... ...SUMMARY... Scattered severe storms capable of hail or damaging wind are possible from parts of the Ohio Valley eastward into the Mid-Atlantic, and across parts of the upper Mississippi Valley and Upper Great Lakes today. More isolated and marginal severe storms are expected across the central Plains to mid Mississippi Valley. ...Synopsis... The upper-air pattern continues to feature a large area of high heights covering most of the southern 2/3 of the conterminous U.S., while a belt of enhanced west-northwesterly mean flow extends from the Canadian Rockies across the Upper Midwest and Great Lakes to New England. As ridging builds over the northern Rockies, a strong shortwave trough -- now evident in moisture-channel imagery over southern MB -- is forecast to move east-southeastward. By 00Z, this perturbation should include a compact 500-mb cyclone centered over ON, just north of the MN border, with trough south-southwestward to eastern NE. By 12Z, the mid/upper low should reach central/southern Lake Superior, with trough to northern MO. Downstream, an MCV related to ongoing convection is evident in composite-reflectivity and satellite imagery over northern OH. The related perturbation should reach southern/southeastern PA and northern VA by 00Z. At the surface, 11Z analysis showed a cold front from the ACK/Long Island area to northern VA, arching northwestward across western PA and northeastern OH as a warm front. This boundary should stall over the Mid-Atlantic then move northward before being overtaken by convection from the west. Meanwhile, a surface low related to the MB shortwave trough was analyzed over southeastern MB, with an occluded front extending to a weak triple-point low over southern MN. From there an ill-defined warm front was drawn east-southeastward over southern WI, and is forecast to move northward across WI today through an air mass partially modified by convective outflows over the last few days. A cold front, initially arching southwestward from the triple point across western IA and northwestern KS, should move to eastern WI, central MO and southern KS by the end of the period. ...Ohio Valley to Mid-Atlantic... A complex and somewhat conditional scenario exists for severe potential in this region today. As such, we will maintain a broad brushed corridor of 15%/slight-risk unconditional probabilities away from areas too stabilized by earlier/ongoing storms -- acknowledging that denser, localized/mesobeta-scale concentrations of severe (especially wind) are possible within. Two substantial areas of thunderstorms and associated clouds/precip are ongoing that will influence severe threats in and near this corridor: 1. A complex of strong thunderstorms over eastern OH that has produced isolated strong/damaging gusts overnight, and the northern OH MCV. Isolated severe potential exists over the next couple hours; see SPC mesoscale discussion 1393 for near-term details. This convection primarily is located along and north of an outflow boundary from earlier convection that extends east-southeastward into central WV. As the MCV and trailing trough proceed east-southeastward, either ongoing activity or newly generated storms will encounter a destabilizing boundary layer across the central Appalachians and across the adjoining Piedmont and Fall Line, to the lowlands of NJ and the Delmarva Peninsula. Pockets of strong surface heating are probable through breaks in cloud cover, contributing to minimal MLCINH and well-mixed subcloud layers supporting wind-gust potential. Forecast soundings suggest modest midlevel lapse rates will be offset by sufficient low-level moisture/destabilization to yield 800-1500 J/kg MLCAPE. Deep shear will diminish southward, with buoyancy weakening northward past the front -- the optimal overlaps between supportive westerlies aloft and instability being in the outlook area. Some subset of this swath may need 30%/enhanced wind probabilities as mesoscale trends/details become better focused. 2. A trailing area of initially non-severe thunderstorms across portions of northern IL and northeastern MO that mostly resides along and north of the outflow boundary from the leading MCS. This activity is being supported by a sheet of elevated inflow associated with the 30-40-kt westerly to west-southwesterly remnants of the prior nocturnal LLJ. As the LLJ continues to veer and weaken, this activity should diminish gradually and weaken as it moves across IL, IN and perhaps OH, substantially limiting/delaying strong insolation and related destabilization over those areas. Some guidance indicates widely scattered to isolated thunderstorms developing near the associated outflow/differential-heating boundary that should be retreating northward across IL/IN/OH in its wake. Any such activity may offer isolated severe hail and damaging gusts; however, the threat appears too low-end and conditional for more than marginal probabilities at this time. ...Upper Midwest... Scattered to widely scattered thunderstorms are expected to form this afternoon and evening, primarily in one or two arcs ahead of the compact upper wave/low over the northern MN/western Lake Superior area, with more isolated to widely scattered activity farther southeastward across WI near and south of the warm front. Sporadic damaging gusts and severe hail will be the primary concerns. Air-mass recovery and destabilization from both local diabatic heating and low-level WAA are expected to occur across this region today, behind the aforementioned IL activity, and around the remains of a smaller, ongoing convective area over central MN. This will occur in tandem with a marked increase in large-scale forcing for ascent aloft preceding the strong shortwave trough. Moisture and low-level theta-e generally will increase southward, while cooling/DCVA aloft will be greater in the north, with at least marginally suitable deep shear over the whole area. Forecast soundings suggest MLCAPE peaking in the 1000-1500 J/kg range away from the shallow/cold marine layer of Lake Superior, amidst 30-40 kt effective-shear magnitudes. Multicells and at least transient supercell structures are possible. The severe threat should diminish mid/late evening into the early overnight hours. ..Edwards/Thompson.. 07/22/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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