lundi 4 mars 2019
dimanche 3 mars 2019
Mesoscale Discussion 0142 NWS Storm Prediction Center Norman OK 1035 AM CST Sun Mar 03 2019 Areas affected...south-central and southeast LA Concerning...Severe potential...Watch likely Valid 031635Z - 031700Z Probability of Watch Issuance...80 percent SUMMARY...The severe risk is increasing over south-central and southeast LA. A tornado watch may be needed soon for parts of southeast LA (near and north of Lake Pontchartrain). DISCUSSION...The 9am CST raob from Breaux Bridge, LA (Vortex2SE special raob) showed little remaining convective inhibition and a profile exhibiting 1100 J/kg MLCAPE. Low-level shear is adequate but a tendency for veering low-level flow with time as the surface cyclone develops east across central AL will tend to limit hodograph size with time. Nonetheless, a severe risk mainly in the form of strong to locally severe gusts and perhaps a tornado may accompany the more intense storms late this morning through the afternoon. ..Smith/Hart.. 03/03/2019
Mesoscale Discussion 141
Mesoscale Discussion 0141 NWS Storm Prediction Center Norman OK 0959 AM CST Sun Mar 03 2019 Areas affected...southern and central AL...western FL Panhandle...southwest and west-central GA Concerning...Severe potential...Tornado Watch likely Valid 031559Z - 031800Z Probability of Watch Issuance...95 percent SUMMARY...The initial signs of discrete convective development are occurring late this morning in the warm sector. Rapid environmental changes are forecast to occur between 10am CST/11am EST and the mid afternoon. DISCUSSION...Visible satellite imagery shows widespread cloud cover across the destabilizing warm sector to the south of the primary frontal zone where the surface low is forecast to develop eastward across central AL into north-central GA later today. Surface dewpoints over the FL Panhandle and far southern AL have risen around 3 degrees F in the past hour and are indicative of strong poleward moisture advection occurring as the surface cyclone develops. The warming/moistening are contributing to MLCAPE increasing from near 0 J/kg to upwards of 1000-1500 J/kg by early-mid afternoon. Late morning VAD data show around 35-45 kt southwesterly 700mb flow from KMOB/KBMX/KMXX/KEOX in central and southern AL but stronger flow (50-55 kt) is now being observed farther west in Jackson, MS (KDGX) and Slidell, LA (KLIX). Models show the flow intensifying further over AL and GA this afternoon (55-60 kt 700mb). The end result is a hodograph exhibiting little weakness (no veer-back-veer tendency or a weak layer of winds). In summary, as moderate buoyancy and strong/veering flow through the profile combine with moist low levels, the threat for strong low-level mesocyclones associated with the discrete storms will increase, along with a corresponding risk for tornadoes of which a few may be strong.
Southern Mississippi Valley Sector
Areas affected...southern MS into southwest AL Concerning...Severe potential...Watch possible Valid 031513Z - 031615Z Probability of Watch Issuance...40 percent SUMMARY...A locally damaging wind threat may develop this morning. Convective trends will be monitored in the short term as to whether a tornado watch will be needed prior to an expected tornado watch issuance time by the 11am-12pm period. DISCUSSION...Radar imagery shows an intensifying band of thunderstorms over southern MS from near the surface low southwestward along the cold front. The developing band of storms is expected to move eastward near the primary frontal zone extending eastward from the surface low across south-central AL. The maritime warm front near the I-10 corridor is becoming more diffuse with time but the airmass along and south of the front is where appreciable surface-based buoyancy resides. As such, only weak instability is located currently over east-central MS to the east of Jackson. Nonetheless, as additional boundary layer warming/moistening occurs, the combination of a destabilizing boundary layer and the fast eastward motion of the squall line (40-45 kt) may facilitate an increased risk for horizontal momentum transport in the form of strong to locally severe gusts in the next 1-2 hours. If this appears imminent, a tornado watch may be needed sooner than a currently anticipated tornado watch issuance time by 11am-12pm CST.
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