| Vertically stacked low pressure persists in the mid Mississippi River Valley this morning. A mid/upper level shortwave impulse is skirting around the southeastern flank of the parent low and is tapping into upper jet energy primarily in two locations. The first is in KY / TN on the right horseshoe of partially phased jet energy surrounding the upper low, while the second was in association with left exit region of a strong jet streak along the gulf coast. As the jet streak translates northeast, this places the metro in a somewhat more favorable position for divergence aloft thereby increasing ascent in the low and mid levels. Surface instability remains limited, as weak in situ cold-air damming and reduced insolation have left cool and stable conditions in the surface to 850 mb layer. As a result, the primary risk is for elevated convection producing hail. For Saturday, upper low finally begins moving, as it rotates southeastward through the base of an upper trough axis keeping cyclonic flow in place for the metro. Despite cold frontal passage Friday night, the upper low will keep mean troposphereric moisture content relatively high, resulting in widespread cloud cover and scattered showers. Precipitation is most likely across the northern metro and mountains where orographic lift will enhance rain chances. For air quality, mixing depth on Saturday expands markedly in association with increasing winds aloft and frontal passage. With progressively cleaner boundary layer expect PM 2.5 concentrations to easily remain in the green. SPM
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