Rainy Weekend and Start to the Week – Saturday 10:15 a.m.
September 29, 2012 Leave a comment
An upper level disturbance over Texas is the partial remnant of a pacific tropical system. This feature slowly moves toward Central Alabama through Tuesday. The synoptic setup and evolution looks pretty good amongst the models before departures show up by Wednesday, mainly in the timing of the system finally lifting out of the area. This is somewhat odd due to the fact that the kicker northern stream system is handled fairly well as another cold front enters the picture Friday with cooler conditions as a trough develops over the heart of the US.
Model consensus and run to run performance has not been great over the past several days, but some trends are starting to appear. To some extent, the NAM/CMC are still trending slower and more south with the surface low while the GFS/ECMWF/UKMET have a more northern solution, which is much closer to the suite ensemble solutions.
Models have trended a bit stronger with the upper system approaching the past run or two. The surface low will allow a tropical airmass to overspread Central Alabama with precipitable water values in excess of 2 inches. It appears there will be sufficient frontogenetic forcing for the possibility of some locally heavy rain. Local streams are running near normal for most places except southeast and east, where drought conditions persist. The heaviest rainfall will depend on the track of the low, upward motion/forcing, where a deformation zone sets up, so the exact timing and placement still remains a challenge. Rain amounts may reach 1 to 3 inches with locally higher amounts, with most falling Sunday into Monday.
The severe weather threat is less defined but certainly not a zero chance. As the surface low approaches, the wind fields at all levels increase quite dramatically, at times exhibiting nice right turning hodographs. Therefore, bulk shear/relative humidity values would indicate the potential of some organized activity and rotation as a large warm sector develops. But there are a few factors that may inhibit this stronger activity from developing at all. The lapse rates throughout are rather poor. CAPE values are also rather low, even zero on some soundings as the atmosphere is completely saturated most of the event. Clouds and rain will decrease surface heating and with highs only forecast around 80 at best. The severe threat will be highly dependent on mesoscale effects/surface low track. So there is a marginal threat of tornadoes and damaging wind gusts, but at this time, localized flooding appears to be the higher threat.