A dome of surface high pressure along the Eastern U.S. seaboard and a building middle and upper tropospheric ridge in the Southern U.S. will maintain fair and dry weather today and tonight. We will continue to see mid and high-level clouds stream across the region well out ahead of the storm system we're watching in the Southwestern U.S. These clouds are not expected to produce any rain in our area right away. As the high pressure slides off the Atlantic coast, the winds will turn to the southeast, bringing us above-average temperatures. Highs should reach the mid 70s away from the coast this afternoon. The southeasterly winds and clouds will slowly increase late tonight, so temperatures will bottom out around 50 degrees at about midnight, then slowly rise thereafter.
The elements for Sunday's storm are in place as the models have indicated for several days now. A powerful upper-level low is spinning over West Texas and Southern New Mexico. It is already producing a large area of rain and storms over much of the Lonestar and Sooner states and reaching as far east as the lower Mississippi River Valley. Meanwhile, another potent disturbance is zipping southeastward over the Northern Rockies. These two systems will merge (more technically referred to as "phasing" in meteorology) on Sunday. An area of low pressure is expected to gradually organize in Eastern Texas later this afternoon, then intensify more rapidly as it translates north-northeastward into Central Missouri on Sunday morning. The increasing pressure gradient between the low to our west and the high to our east will force the winds to pick up out of the south over our region. Consequently, dewpoints will be on the rise and destablization will commence; however, most of the models hold the stormy weather off until after dusk. We could see a couple of insignificant brief showers (especially in our western viewing area) during the afternoon before the heavier weather arrives.
Sunday night will be the main event. The center of low pressure will strengthen even more as it slides across lower Michigan. The trailing cold front will reach as far south as the Central Gulf coast. Broad cyclonic flow through a deep layer of the atmosphere will promote large-scale ascent in an environment that will be weakly unstable, but strongly sheared. The model forecast profiles are showing the low-level winds increasing to between 50 and 55 knots (about 3,000-5,000 feet above ground level). It will not take much to transfer those winds down to the surface in a convective downdraft with that kind of environmental flow. As a result, damaging straight-line winds appear to be the biggest threat with these storms, especially if they form into a squall line as indicated by some of the early high-resolution model runs. The biggest question remains whether discrete storm cells can develop ahead of the line. If they can, tornadoes will become a greater likelihood since the boundary layer flow will remain somewhat backed ahead of the cold front. The magnitude of the shear suggests that if tornadoes do form, they could be strong.
The highest probabilities of severe weather are expected to be along and west of U.S. Highway 319, meaning the best threat will be in the central and western Florida Big Bend and Southwest Georgia. The short range numerical model guidance forecasts the instability to be the strongest in these areas Sunday evening. The substantial loss of daytime heating will cause the storms to slowly diminish as they move into a less unstable environment in the eastern Florida Big Bend and South-Central Georgia after midnight. In these areas, only isolated damaging winds and a brief/weak tornado are possible in the strongest remaining thunderstorm cells.
The rain will be ending from west to east early Monday morning. The operational and experimental NAM models have been consistently a little slower in moving the rain out of our eastern viewing area. Even so, partial clearing will take hold Monday afternoon along with a brisk west-northwesterly wind. The winds should ease enough for a light to moderate freeze on Tuesday and Wednesday mornings, before the airmass moderates again by Wednesday afternoon.
The next in what seems like a never-ending series of Pacific systems will be moving into the Southern Plains or Lower Mississippi Valley on Thursday. The GFS is slower than the ECMWF with this next shortwave. This time, the GFS has considerable support from its ensemble members and also the Canadian global and UKMET models. Since the ECMWF is so reliable, the Storm Prediction Center has already placed a large portion of our viewing area in a risk for severe storms late Thursday or Thursday night. However, I'm going to adopt a "wait and see" attitude and accept the slower models at this time.