re: Winter thread: 3.5" officially in BR on the 8th day of meteorological winter!!

Area Forecast Discussion
National Weather Service
New Orleans LA

745 PM CST Thu Dec 7 2017

.SOUNDING DISCUSSION... Still an over-running pattern sounding but with some significant features that are lending clues to what may unfold. Temperature 41F at the surface only cools to 38F at 1200 ft, then remains at 38F to 2700ft, which is sub-cloud dry air. Saturated inversion layer from 850mb/4300ft, 31F at base warms to 39F at 720mb/9000ft. Then moist adiabatic lapse rate to tropopause found at 100mb with a temperature of -70C. Winds NE 20-30kt surface to 4900ft, SW 10-108 kt above 4900ft. Peak wind 226/108kt at 40.0kft.


Some winter micro-meteorology...a top down approach for precipitation determination shows snow formation from 20kft falling into a dry layer between 14kft and 11.5kft as snow virga that melts in the warm layer from 9kft down. A 7 degree dewpoint depression at 2600ft would yield a wet-bulb around 2C/35F, which means a cold rain but still too warm to support sleet or snow. How tough is the call right now? Currently 3 freezing levels noted, the primary one at 12.9kft where the bright band melting level is noted on radar, then a lower level shallow freezing layer between 4999ft and 4852ft - a mere 147 ft of depth for any warm raindrop to transition into an ice pellet. As more rain aloft falls into the column, it will slowly deepen the depth of the freeze zone to support better sleet production later tonight. It will still take more cold air advection in the low and sub-cloud layer to support snow production, which may still arrive by daybreak Friday.

Surface analysis shows 40s surface temps for much of LA and south MS at this time. Dual Pol radar hydrometeor classification loop does show a transition zone aloft aligned from roughly McComb to Lafayette, LA with dendritic snow north of this zone and liquid rain south of the zone. This is the more favored corridor for any wintry precipitation in time, but cold air advection will have to become more pronounced and agressive to cool the lower part of the sounding. Did receive a report of rain mixing with sleet near Easleyville in St Helena Parish and this does match well with the dual pol data.