SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1200 AM CST Thu Dec 31 2020 Valid 311200Z - 011200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST TEXAS INTO SOUTHERN LOUISIANA AND SOUTHWEST MISSISSIPPI... ...SUMMARY... Severe thunderstorms -- including potential for tornadoes -- are possible today through tonight from southeast Texas into the lower Mississippi Valley and central Gulf Coast States. ...Southeast Texas through the lower Mississippi Valley... Cold front will stall from the TX coastal area through northern LA and central MS. This boundary might advance slightly northward as a warm front or remain stationary through a portion of southeast TX and the lower MS Valley today into the evening. Cyclogenesis will commence along the front near the central TX coast later this morning. This cyclone will subsequently develop northward through east TX within corridor of deep forcing for ascent associated with a northward-ejecting shortwave trough. A moist warm sector will reside south of the front with mid to upper 60s F dewpoints from southeast TX into southern LA and MS. However, widespread clouds and weak lapse rates will support only a modest thermodynamic environment with MLCAPE from 500-1000 J/kg. Deeper forcing for ascent will begin to overtake the western portion of the warm sector across southeast TX later this morning. This will result in more robust thunderstorm development in vicinity and east of the developing surface low near the central TX coast and southward along the cold front into the western Gulf. Activity will subsequently spread into southeast TX and southwest and central LA during the day before reaching southwest LA by early evening. Vertical wind profiles will strengthen as a coupled jet structure overspreads the warm sector in association with the ejecting shortwave trough. Mixed storm modes are likely. Large low-level hodographs with 300-500 m2/s2 0-1 km storm relative helicity and 50+ kt effective bulk shear will support organized storms, including bowing line segments and supercells with low-level mesocyclones capable of producing tornadoes and damaging wind. ...Central Gulf Coast states... A subtle warm front will move onto the central Gulf coast later today. The boundary layer will moisten with dewpoints increasing to the mid to upper 60s F. Showers and thunderstorms are expected to gradually increase and develop northward through the evolving warm advection regime later today into the overnight. Primary severe threat is expected during the evening and tonight as low-mid level winds increase downstream from the shortwave trough that will remain well west of this region. Hodograph size will increase substantially overnight, along with effective bulk shear supporting potential for a few supercells. The deeper forcing for ascent will remain west of this region, and most storms might remain slightly elevated given the most favorable wind profiles will evolve overnight. Nevertheless, the environment will support a supercell/tornado threat, assuming updrafts can root in the boundary layer. ..Dial/Wendt.. 12/31/2020
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