SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0655 AM CST Tue Feb 12 2019 Valid 121300Z - 131200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF SOUTHEASTERN MISSISSIPPI TO CENTRAL GEORGIA... ...SUMMARY... A few damaging wind gusts are possible from eastern Mississippi across portions of Alabama and Georgia today, and a tornado cannot be ruled out. ...Synopsis... In a relatively progressive upper-tropospheric pattern, the main perturbation affecting convective potential will be a synoptic trough now located from the Dakotas southeastward over the OMA/MKC areas, then southwestward over north-central/southwest TX. Within this trough, a strong northern-stream shortwave perturbation -- currently apparent in moisture-channel imagery over SD and eastern NE -- should wrap through the trough and evolve into a closed 500-mb cyclone over WI/IL by 00Z. Meanwhile the more positively tilted southern lobe will move eastward across the Mid-South and Arklatex regions, reaching eastern portions of KY/TN, northern/central AL, and southern MS. By 12Z, the cyclone center should cross Georgian Bay (Lake Huron), with the synoptic trough south-southwestward across the southern Appalachians to the central Gulf. At the surface, 11Z analysis showed a low over northeastern IL with cold front arching across western IN, southeastern MO, central AR, western LA, and Gulf waters off TX. A secondary/reinforcing boundary was apparent from southwestern AR across central TX. A warm front was drawn from northeastern AL across southwestern GA and northern FL. The cold front should proceed eastward across the Southeast through the period, preceded by a band of convection currently evident from eastern TN across northern AL to southeastern portions of MS/LA. ...Southeast... A band of convection, with isolated to widely scattered embedded, episodic thunderstorms, is forecast to become better-defined over the next few hours as it moves into a marginally destabilizing air mass across portions of MS/AL. For short-term potential, please refer to SPC mesoscale discussion 99. As the main convective band progresses eastward over more of the outlook area today, isolated damaging gusts, perhaps a severe gust, are possible, and a tornado cannot be ruled out despite the weak lapse rates and linear mode. The destabilization will occur from a mix of boundary-layer theta-e advection and subtle surface diabatic heating beneath broken to overcast cloud cover. MLCAPE near 500 J/kg should develop over parts of eastern/southeastern AL and western GA. The boundary-layer destabilization will be more prevalent with southeastward extent over AL/western GA away from the thickest clouds. However, as the main low-level cyclone and accompanying isallobaric forcing eject away from the area, low-level lift and shear are each expected to weaken with time over the regions experiencing greatest direct heating. Deep shear and low-level shear generally will remain favorable across the outlook area until the line passes, though some veering of near-surface winds will reduce bulk shear values with time ahead of the convection -- especially the southern portion closest to the Gulf. ..Edwards/Gleason.. 02/12/2019
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