SPC Mar 8, 2018 0830 UTC Day 3 Severe Thunderstorm Outlook

By Newsroom America Feeds at 8 Mar 03:20

SPC 0830Z Day 3 Outlook
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0218 AM CST Thu Mar 08 2018 Valid 101200Z - 111200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SATURDAY AFTERNOON AND EVENING ACROSS PARTS OF SOUTHERN ARKANSAS...NORTHERN LOUISIANA AND ADJACENT PORTIONS OF NORTHEAST TEXAS AND WEST CENTRAL MISSISSIPPI... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING AREAS OF THE ARK-LA-TEX INTO LOWER MISSISSIPPI VALLEY... ...SUMMARY... A few severe storms appear possible across parts of the Ark-La-Tex region into the lower Mississippi Valley Saturday afternoon and early evening. ...Discussion... Models indicate that mid/upper flow may begin to amplify across the eastern Pacific into western North America during this period, with large-scale ridging building within the mid-latitude westerlies inland of the Pacific coast through the northern U.S. Rockies, downstream of an evolving trough/low over the eastern Pacific. At the same time blocking appears likely to remain prominent downstream, centered over the eastern Canadian Provinces, with a broadly confluent northwesterly flow prevailing to its southwest, east of the Rockies through the southern/mid Atlantic Seaboard. Within the northwesterly flow, models indicate several short wave perturbations will continue to dig across the Plains, lower Mississippi Valley, and northeastern Gulf of Mexico. However, rather large spread persists among the various models concerning these features and associated surface developments. Based on the evolving larger-scale upper pattern, lower amplitude surface frontal wave development east of the southern Plains Red River Valley through the Gulf States still appears the more likely outcome. The deterministic NAM appears an outlier with its depiction of a more amplified surface wave developing with convection over the Gulf of Mexico and approaching the Tampa FL area toward 12Z Sunday. However, there have been occasional indications of this within preceding day's medium range model output. ...Southeastern Plains into Gulf Coast... Considerable convective development may be ongoing or in the process of developing by 12Z Saturday across parts of the central and northwestern Gulf of Mexico and the lower Mississippi Valley into the Ozark Plateau. Models suggest that this activity may persist, with a gradual eastward development (particularly across the Gulf of Mexico). It appears that this will impede the eastward advection of a plume of higher low-level moisture content spreading into eastern Texas by 12Z Saturday. And a frontal zone enhanced by rain-cooled air seems likely to form across the Ark-La-Tex southeastward into the lower Mississippi Valley. This appears at least one point of general consensus among the models. To the west of this front, surface dew points may increase through the mid 60s along and to the west of the Sabine Valley through the day. Beneath the eastern/southeastern periphery of rather steep mid-level lapse rates supported by differential thermal advection, models indicate that this may contribute to peak afternoon mixed-layer CAPE in excess of 1000 J/kg, with similar instability perhaps developing as far east as the immediate cool side of the frontal zone. As a result, continuing warm advection along and to the immediate cool side of the front may contribute to vigorous elevated convection capable of producing severe hail, as strengthening northwesterly mid-level flow associated with the digging short waves increases convective layer shear. While it still appears that stronger large-scale ascent will remain to the cool side of the effective front through much of the period, outflow generated by the elevated convection could initiate storms rooted closer to the boundary layer along the front near/after peak heating. If this occurs, the upscale growth of a small organizing thunderstorm cluster appears possible. This may be accompanied by a risk for potential damaging wind gusts, particularly across and southeast of the Arkansas/Louisiana border before weakening in stable/rain-cooled air across southeastern Louisiana/southern Mississippi Saturday evening. ..Kerr.. 03/08/2018 Read more



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