Atlanta will become victim to a cold air damming event during the weekend of April 8th, 2023, where precipitation, high winds, and cold temperatures will plague our region. In this case, I will detail the synoptic environment regarding jet streak dynamics, trough ridge dynamics and QG-theory. In Fig. 1, an initialized GFS-model run on April 6th, 2023, at 1800 UTC shows a strong upper-level jet streak positioned over the northeastern states with winds reaching up to 90 knots on April 8th, 2023, at 12 UTC. To the northwest of the jet streak, a surface anticyclone (blue H) is positioned just north of the Great Lakes with a pressure of ~ 1036 mb. Through time, the surface anticyclone will head to a region of maximum ageostrophic convergence by following 1000-500 hPa thickness contours with lower values to the left. A four-quadrant figure is provided to show the different quadrants of the jet streak where the left jet-entrance region is the top-left quadrant where ageostrophic convergence will provide continual support to maintain the strength of the surface anticyclone through subsidence (downward vertical motion). The surface anticyclone will propagate towards this region. Regarding trough ridge dynamics, a weak upper level is positioned over central CONUS. To the right of the base of the trough, ageostrophic divergence will promote upward vertical motion due to mass continuity where there is surface convergence between the base of a trough and the crest of a ridge. This will in turn support lifting for possible storms in our area as the jet propagates eastward.
Fig 2. Shows 500 hPa ascent (blue) and cyclonic relative vorticity in 10-5 s-1 where blue ascent depicting upward vertical motions on is seen over Alabama due to ageostrophic divergence associated with the trough, ridge dynamics discussed in the previous paragraph. A longwave ridge is also shown barreling up from the upper Midwest into northern Canada. At the crest of this ridge, this is an area of relative vorticity minima due to super-geostrophic winds in relation to trough, ridge dynamics. By following the 900-500 hPa thickness contours on Fig. 3, as thermal wind flows parallel to thickness contours with lower values to the left, the direction of thermal wind will be northwesterly and westerly towards our region of interest. By the Trenberth form of the QG-omega equation known as the “absolute geostrophic vorticity by the thermal wind.” calculated by the equation: σ(Δ2+f0/σ ∂2/(∂p2) = 2f0 [-VT∙∇(ζg+f), areas of positive omega (upward vertical motion) and negative omega (downward vertical motion) can be plotted. When the right hand of the equation is < 0, there will be negative absolute geostrophic advection by the thermal wind due to the thermal wind advecting more values of negative vorticity from the vorticity minima over northern Canada into northeast CONUS and subsequently maximize omega creating downward vertical motion. This can be seen in Fig. 3 where there are blue colors in the vicinity of the surface anticyclone denoting downward vertical motion (positive omega). The Trenberth form of the QG-omega equation will, in addition to the left jet entrance region discussed above, will aid in the maintenance of the surface anticyclone strength of ~ 1036 mb.
As the surface anticyclone moves towards New York State, Fig. 4 shows a forecast run for 1800 UTC 8 April 2023. The anticyclone has now progressed towards a region favorable for continual maintenance discussed in Fig. 1 with the left-jet entrance region and Fig. 3’s negative absolute geostrophic advection by the thermal wind supporting downward vertical motion. The now 1032 forecast mb surface anticyclone’s flow (clockwise) will provide northeasterly and easterly winds from the Atlantic Ocean that will begin to be blocked by the Appalachian Mountains and deflected parallel to the Appalachian Mountains. The air approaching the Appalachian Mountains from the Atlantic Ocean is less dense and warmer than the surrounding air, so it will begin funneling denser, cooler air downstream (southwest) where Atlanta and surrounding areas will experience rainfall amounts ranging between an inch or two, gusty winds reaching up to 30 mph, and what could very well be the last “cold-air outbreak” of this Spring with maximum and minimum 24-hour temperatures in the low 40s for April 8th, 2023! This is known as cold air damming where the “C” shaped MSLP pressure contours in the southeast is a clear signature of this event.
Figure 4: A MLSP GFS-model map initialized at 1800 UTC 6 April 2023 where MSLP is plotted in the solid black contours every 4 hPa, precipitation shaded in the fill pattern along the x-axis in mm/6h, 850 hPa temperature in the dashed contours with at or below freezing values in blue and above freezing values in red every 5℃, and 10-m winds in the black barbs in knots at 1800 UTC 8 April 2023. The blue ‘H’ represents the surface anticyclone positioned over New York, moderate precipitation plagues the southeast, and the ‘C’ shape of the MLSP contours depicts a cold air damming event. (Alicia Bentley)