In the southeast region, March 31st ended up being a day to stay inside as a low-pressure system moved through Mississippi, Alabama and eventually through Georgia explaining the storms that Atlanta experienced during the mid-afternoon. Moving into April 1st, the squall line moved east of Atlanta as some storms were still being experienced around 0Z but by 12Z April 1st, the precipitation had moved off the coast of the Carolinas, shown in Figure 1. April 1st, although a sunnier day for the southeast, brought severe weather in the northeast region with a low-pressure system moving through until it moved off the coast of Rhode Island by 12Z. This system brought snow (shown in blue) on the backside of the storm to Pennsylvania and New York with the addition of some lake effect snow in the trough moving through Ohio and Illinois.
Figure 1: Surface map showing pressure contours and precipitation type at 18Z March 31st (top left), 00Z April 1st (top right) and 12Z April 1st (bottom) to show how the low-pressure system moves east of the southeast region and the trough settles over the southeast before also moving east of it.
After the squall line moved through the southeast region, behind it came a cold front that stretched down through Louisiana and eventually connected with the low-pressure system in the northeast bringing cooler temperatures to not only the southeast but the Carolinas, Virginia and the northeastern states as well. Looking at Figure 2, the cold front moved through the southeast region from 0Z March 31st to 06Z April 1st and eventually led to the colder temperatures observed at 12Z April 1st. Figure 2 (top left) and figure 2 (top right) show the cooler drier air behind the cold front in Alabama that moves over the warmer moister air that is ahead of the cold front. There is also a change in the wind direction and speed between in front of the cold front and behind it.
Figure 2: Surface analysis map showing the different temperatures, dew points and wind speed across the United States. The different fronts can also be seen, specifically the cold front moving across the southeastern U.S. from 18Z Wed Mar 31 (top left) to 12Z Thur Apr 1 (bottom right).
Moving into the upper levels, the 250 mb map (figure 3) shows the jet streak present across the United States with strong wind speeds due to the temperature gradient with the cold front. The jet streak implies there is strong vertical wind shear. Looking at figure 3, the subtropical jet streak in the Texas gulf area, connects into the jet streak throughout the Midwest area. This helped guide the squall line indirectly east. Also, looking at the right jet entrance region, due to the squall line being present in that area, it was impacted by synoptic scale lifting. Lastly, looking between the large ridge over the western U.S. and the deep trough over the eastern U.S., there is a high-pressure system observed over Nebraska which shows a textbook subsidence region from upper-level dynamics and this is what is impacted the southeast region on April 1st and into the coming hours. The 500 mb vorticity map (figure 4), showing the rotation of flow in the atmosphere, shows the trough feature moving east of Atlanta taking the vorticity maximum along with it. This is what is brining these clear skies since there will be no precipitation due to this as Atlanta sits at the base of the trough currently and eventually will be east of it bringing negative vorticity advection over Atlanta and in turn downward vertical motion being experienced.
Figure 3: 250 mb map from 03Z April 1st, showing the jet streak throughout the entire United States, specifically noting on the subtropical jet over Texas and the gulf and the stronger winds coming off of the trough into the northeast region.
Figure 4: 500 mb relative vorticity map from 12Z April 1st showing the trough present and the vorticity max moving east of the trough and Atlanta
Focusing lastly on the downward vertical motion that will exist over Atlanta as stated previously, the 850 mb temperature advection map (figure 5, left) shows the cold air passing through and the downward vertical motion or subsidence that will be present. In figure 5, in the blue fill patterns on the backside of the cold front due to the strong temperature gradient experience downward vertical motion. This can be confirmed by looking at figure 5 (right), the 850 mb vertical velocity map where the gray areas show downward vertical motion which maximizes cold air advection. There is a line from Louisiana through Mississippi through Alabama, where there is negative values of vertical velocity shown by the gray areas. This lines up with cold air advection being maximized. Downward vertical motion is also exhibited in association with the cold frontal passage. In turn, this explains the lack of clouds in the sky as cold air advection was still being experienced throughout the entire day.
Figure 5: (Left) 850 mb temperature advection map showing the cold air passing through and the downward vertical motion or subsidence that will be present. (Right) 850 mb vertical velocity map showing in gray downward vertical motion which maximizes cold air advection or upward vertical motion in red