Day 1 of New Orleans, LA (KMSY)

In an incredible coincidence, we start forecasting for a once-in-a-lifetime snowstorm for New Orleans. The high will occur at the beginning before the start, as cloud cover and wind prevent radiational cooling. Often, these will lead to more vertical mixing and warmer surface temperatures than modeled, especially at night when models often erroneously show some nocturnal inversion.

 

It will already be cold (by New Orleans standards) then, but the snow will cause evaporative and subliminational cooling in the dry low-level air, and temperatures will quickly drop below freezing. Snow will be heavy at times with stiff northerly winds. The northerly winds off Lake Pontchartrain (water temperature of ~52F) will lead to slight moderation of the cold, but it won’t be more than a couple of degrees given the heavy snow and widespread, deep cold air. The wind off the lake could also lead to lake-enhanced snow, something that almost never happens there. The onshore wind also will lead to vertical mixing of stronger winds aloft down to the surface due to the increased instability induced by the relatively warm lake water, but counteracted by heavy snow’s tendency to suppress winds a little. Such enhancement is often not handled by models well, though the low-level lapse rates might not be quite steep enough for much lake enhancement.

 

Snow will stop tomorrow evening, and then skies will clear out, leading to a cooldown to the low possibly at the end (Wednesday 06z), though the winds off the lake will remain too strong for good radiational cooling. It might be around the same temperature as during the heaviest snow tomorrow morning (likely upper 20s F) when dynamic cooling from strong ascent is the strongest.

 

Source: PivotalWeather

 

Source: PivotalWeather