Day 8 of New Orleans, LA (KMSY)

Low-level clouds and southeasterly winds prevented it from cooling below 64F last night. There was enough sun and warm advection for a high of 77F today, though it apparently didn’t warm up after noon perhaps due to some cooling from the shelf waters of the Gulf of Mexico and some clouds. The sunshine apparently mixed down strong winds down from aloft with a maximum of 23 kt, higher than USL, which is uncommon. A relatively weak cold front with some showers and possibly thunderstorms will arrive late tonight into tomorrow morning, though the parent storm is way off to the north and not strengthening, limiting the dynamics, and we lose the daytime instability with the late night timing of the cold front, which is why all models have only a relatively weak line of showers and thunderstorms with generally <0.2” of total rainfall. Still, sometimes even weak convection in warm, moist low-levels can be efficient rain producers, leading to a tricky rainfall forecast. The lack of a strong squall line and nocturnal timing limiting instability also means that the cold front will accompanied by only modest winds of <20 kt. However, there could be stronger winds in the southerly flow ahead of the cold front tonight (40 kt winds aloft but with lack of vertical mixing at night) or with the westerly flow that is more well mixed due to sunshine allowing for mixing of winds down to the surface tomorrow afternoon (though with winds aloft weaker than tonight). 

 

Although there will be a temperature drop with the cold front, the high will actually be behind the cold front tomorrow as winds turn westerly, not advecting cold air rapidly, but the cool advection favoring descent and clearing skies just in time for afternoon high temperature, and models are usually too cool for highs in this setup, showing the cold air moving in too fast (especially HRRR). The westerly winds also mean no cooling influence from Lake Pontchartrain. The low will be at the end (Saturday 06z) with cold advection with westerly winds, though the air mass behind the cold front is still rather mild, and there looks to be just enough wind to prevent ideal radiational cooling even with clearing skies, keeping temperatures still on the warm side of models.

 

Source: PivotalWeather