Issued: 8pm on Sunday, September 13th 2020
Technical Forecast Discussion
Short Term (Sunday 9-13-2020 through Tuesday 9-15-2020)
Waves of weak PVA will keep relatively lower heights over our forecast area, as a ridge is built off to the west due to strong NVA over the next 24-48 hours. A strong cold core high driven by negative shear VORT aloft, along with overall net convergence downstream of the ridge to our west will begin to make its way into the area over the next day or two. This will assist in SFC CAA behind the cold front that passed through this morning, as well as promote large scale clearing. The primary source of CAA will be meridional flow out of the north. Monday will be a beautiful day to be out and about for this time of year–highs will be in the middle and upper 70s, with mainly clear skies. This fall-like pattern looks to remain fairly intact through the end of this forecast period. A few clouds may be stirred up due to a weak VORT axis advecting itself downward into our region on Tuesday, but with a lack of WAA aloft, any type of negative omega generated will likely not be enough to produce any precipitation. Highs on Tuesday may be a few degrees warmer due to flow shifting to the SE, but nothing summer-like (like last week) looks to be possible. SE’ly flow will be driven by cyclogenesis due to the right entrance of a newly-formed jet streak. This will form due to height gradients becoming tighter as the polar and subtropical branches of the jet merge as a result of the ridge upstream from our area. This cyclogenesis will bring about frontogenesis of both a warm and cold front. As the warm front will be off to our north, slightly warmer sfc air will be advected in Tuesday afternoon and evening. Skies should remain mainly clear to partly cloudy.
Long Term (Wednesday 9-16-2020 through Saturday 9-19-2020)
What was a fairly zonal pattern will once again become active and meridional. A strong VORT max, generated by both shear and curvature effects will begin to be advected downward. This digging will be caused by positive differential thickness advection in the mid and upper levels. The base of this feature should be dug down into the state by late Thursday night into early Friday morning. Meanwhile, at the surface on Wednesday, WAA should briefly return temperatures to the 80s. This will be caused also by NVA extending the influence of a ridge slightly up into the southern part of the state. This ridging will be short-lived however, as a VORT max resulting from the remnants of Hurricane Sally propagates north and eastward. This feature should reach the Tennessee valley Thursday night. Associated positive differential vorticity advection and aloft WAA may stir up precipitation chances Thursday, both convective and stratiform. However, these chances will be relatively low due to the weak magnitude of the WAA. Meanwhile off to the north and west, A digging of a trough due to PVA and positive differential thickness advection will begin to occur. The axis of this positively tilted trough should reach the state by Friday evening. This will greatly cool the forecast area down, with likely highs in the 60s for Saturday. At the SFC, a cold core high associated with the left entrance of a jet streak will provide mainly clear skies for the area going into the early weekend.
