Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0259 PM CDT Wed Apr 05 2017 Valid 052000Z - 061200Z ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST GA INTO CENTRAL SC... ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS SURROUNDING THE HIGH FROM PORTIONS OF SC TO CENTRAL KY... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SURROUNDING THE MODERATE FROM PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC TO PORTIONS OF THE OH VALLEY... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MIDWEST SOUTHWARD TO THE GULF AND SOUTHEAST ATLANTIC COASTS... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE EASTERN STATES... ...SUMMARY... An outbreak of severe thunderstorms is underway from portions of the Southeast States to the Ohio Valley region and eastward to the Mid-Atlantic. Significant tornadoes will be possible, especially from parts of southeastern Georgia into South Carolina, and also from parts of eastern Alabama into south-central Kentucky. In addition, very large hail, and damaging wind gusts are expected. ...Discussion on Outlook Adjustments... The follow changes are made to the ongoing outlook: 1) The high risk (driven by 30-percent tornado probability) has been focused along/east of ongoing linear segments with embedded supercellular structures. Discrete cells ahead of this convection, as well as the aforementioned embedded cells, will continue to pose a risk for tornadoes, a few of which could be strong, considering a warm/moist boundary-layer environment characterized by ample effective storm-relative helicity. Indeed, as of 1945Z, a tornadic debris signature has been observed in Laurens County, GA. 10-percent tornado probabilities have been contracted to points near/south of the Ohio River. To the north, sufficient low-level moisture return appears unlikely, suggesting a lower tornado potential. 2) 45-percent wind probabilities are introduced across portions of the Ohio Valley southward to far northern Tennessee. Surface dew points have steadily risen into the 50s, with temperatures climbing into the mid 70s to near 80. Convection should increase in coverage this afternoon, with an initial supercellular mode eventually transitioning to a more linear mode by late afternoon/early evening. Strengthening low/mid-level southwesterly flow and steep 0-3km lapse rates should favor several swaths of strong/damaging winds across the upgraded area this evening. 3) The 45-percent hail/sig hail delineation has been removed from portions of southern Alabama/Georgia and adjacent portions of the extreme northern part of the Florida Panhandle. Ongoing convection and cloud cover over this region cast doubt upon the potential for vigorous convective re-development in its wake. Additionally, veering low-level flow and weaker forcing for ascent (than points farther north) also yield high uncertainty regarding the potential for this re-development. As such, the associated moderate risk has been removed from this area. ..Picca.. 04/05/2017 .PREV DISCUSSION... /ISSUED 1129 AM CDT Wed Apr 05 2017/ ...Portions of the Southeast States to the Ohio Valley region and the Mid-Atlantic... Ongoing semi-discrete supercells and supercell clusters are developing northeastward from parts of the central/eastern Gulf Coast into southern GA. This activity resides well ahead of a shortwave trough across the South-Central States and is evolving within a broad, moistening open warm sector. With observational data suggesting dewpoints in the lower 70s developing northward ahead of this activity, supporting MLCAPE around 2000-3000 J/kg aided by warm-sector insolation steepening low-level lapse rates beneath a residual EML plume, and given the ongoing semi-discrete nature to rotating updrafts developing as far south as the central Gulf Coast vicinity, there is increasing confidence that long-track supercells will be likely. Furthermore, with maturing midlevel mesocyclones already evident, and low-level SRH around 200-300 m2/s2 aiding the development of low-level mesocyclones amid the increasing low-level theta-e, confidence has increased in higher coverage of tornado potential -- including significant tornado potential -- across the now-upgraded High Risk area. This activity will spread across the High Risk area into the evening hours, as vertical wind profiles further strengthen with the approaching midlevel trough and 700-mb flow increasing over 50 kt. Observational trends and previous model guidance are the primary supporters of this High Risk upgrade, as opposed to the most recent model guidance which suggests a dry bias in thermodynamic profiles -- Reference Mesoscale Discussion 440. Outflow from ongoing convection from north GA to western SC serves as a northern bound to the greatest severe potential. Furthermore, confidence has increased that substantial severe risk including tornado potential will develop through parts of the Mid-Atlantic region into the overnight hours amid strong low-level and deep shear, and a moistening boundary layer. As a result, severe probabilities have been increased across parts of the Mid-Atlantic. Also, severe storms are expected to spread across parts of the Gulf Coast vicinity into the evening/overnight hours -- affecting parts of north/central FL with tornado potential. Farther to the west, a somewhat separate area of severe storm development will be likely from parts of the Ohio Valley region to the Tennessee Valley and vicinity in association with the primary midlevel vorticity maximum and related low-level baroclinic zone this afternoon. Strong low-level SRH in the destabilizing warm sector -- enhanced near the surface low tracking from parts of IL into OH -- will support organized, rotating updrafts. All severe hazards -- including significant hail and tornadoes -- will be possible from this afternoon into the evening.Read more
from SPC Forecast Products http://ift.tt/u32qoI
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