It finally looks like we'll be getting the much needed rainfall this week as a early season tropical feature takes shape in the Gulf of Mexico.
The models are showing a surface low forming in the east Gulf off the west coast of Florida today. Satellite imagery show a large area of disturbed weather over the state of Florida, as well as offshore to the west of the state this morning. This area is getting stirred up from an upper level low that is currently spinning over the Gulf and this is slowly transferring to the surface. Depending on the model, this low will move up the west coast, take a turn west, circle back out into the Gulf and then back up across Georgia. There is some consistency in the models for this development, both from the numerical models as well as the ensembles, so it is looking like the drought conditions for Florida and parts of Georgia will be wiped clear.
The Weather Prediction Center's Day 2 Outlook talks about the potential of 3"+ of rainfall over areas of the southeast.
As the closed mid level low crosses the Gulf Coast and reaches the Southeast states during Day 2, a low level southwest flow continues to transports 1.75/2.00 inch precipitable water air (which is between two and three standard deviations above the mean) across much of FL, as well as GA/SC into NC. Model soundings showed moderate instability over these areas, and the moisture and instability is expected to feed mainly diurnal convection. Slow cells motions associated with the mid level system could result in local 3.00+ inch rainfall amounts over GA/SC/NC, and despite the fairly high three hour flash flood guidance (see image below), may pose a flash flood threat during Day 2. Thus, a large Marginal Risk was placed here. Further south over the FL Panhandle and portions of southern AL, a surface system associated with the mid level low meanders just offshore. There is some spread on how fast the system moves northward, but there is some signal that the low level southeast flow ahead of it could begin to transports higher moisture to the Gulf Coast. While the highest QPF totals were kept offshore during Day 2, there could be enough convection across the Gulf Coast for localized flash flooding issues, so a Marginal Risk was placed here for Day 2. (WPC Quantitative Precipitation Forecast Discussion)
The surge of moisture will slowly begin through the day today and will take precipitable water values over the 1.5" range for the southeast for the rest of the week and through the weekend. Toward the end of the extended period, the GFS is showing the potential for another tropical type system, so it is something that may need to be watched as time goes on. According to Joe D'Aleo at Weatherbell, the 500mb day 8 and day 11 analogs from CPC had years with 8 tropical events, 1 tropical storm and 2 hurricanes.
There are areas across the southeast that are in drought conditions but most of the area has actually had normal to above normal rainfall over the last 30 days.
Here are the slides from the latest severe weather briefing. First, the SPC discussion
Strong tornadoes, very large hail, and damaging winds are expected across parts of the Tennessee Valley and Southeast during the late afternoon and evening. ...TN Valley and Southeast...
Have upgraded to Moderate risk for the increasing likelihood of several tornadic supercells between 21-03Z, centered on northern AL and portions of adjacent states. Expansions to Enhanced and Slight risks have also occurred across parts of GA and SC for this evening and tonight. Lowest confidence remains on northern extent of the strong tornado risk where the degree of surface-based destabilization is quite uncertain.
An elevated storm cluster is ongoing across far northern MS to the Mid-MS Valley, still removed from appreciable surface-based instability that is present near/south of the warm front in north-central MS/AL. See MCD 144 for additional short-term information. Guidance is highly varied in the degree of warm front movement today, which may be in part explained by differences in the handling of the ongoing convection. The NAM robustly destabilizes into middle TN but has a distinct absence of convective precip where convection is ongoing. While the RAP has substantially less buoyancy in TN with SBCAPE >500 J/kg remaining largely south of the state border. While low/deep-layer shear and hodograph size will become increasingly favorable for rotating storms, confidence in the degree of northward destabilization in the late afternoon across TN is low.
However, farther south in parts of MS/AL/GA, confidence is greater in moderately large buoyancy developing with MLCAPE of 1500-2500 J/kg becoming common south of the warm front. The northern extent of this instability plume with enlarged low-level hodographs should render the potential of several tornadic supercells during the late afternoon and early evening, a couple of which may be long track. Morning CAMs suggest the southern extent of probable supercell development will be across north-central AL, with convection becoming increasingly sparse to the south/southwest.
During the evening, convection should become increasingly clustered/semi-discrete, but a risk for a few strong tornadoes should continue across parts of northern/central GA near the warm front. Damaging wind/tornado risk should persist but become increasingly localized across parts of SC tonight.
Here's a great resource for those that are more meteorologically inclined. These two images are from the Louisville NWS forecast office, and they show what parameters are needed to support various forms of severe weather. To me, this is awesome information for a weather novice as well as a seasoned meteorologist.
In order to get severe weather, there are several ingredients that need to come together, a warm moist atmosphere and a method to generate lift. If the winds turn with height, you get helicity or rotation.
The latest NAM 3km high resolution model is really bumping up the severe potential on Monday, but not for everyone. The NAM has been the stronger of the models to this point.
The images below show a weak wedge pushing into the east and north metro area Monday evening, keeping temperatures and dew points down. Also notice that the composite severe parameters keep the worst of the severe chances outside the wedge.
There is still plenty of time for change, but in the scenario the NAM is showing, the north and northeast areas of north Georgia would not be in the worst location for severe. However, for those areas outside the wedge, the severe parameters are much more disturbing. The Supercell Composite alone is near the top of the scale while the Significant Tornado parameter runs 4-6. This is just one model and one run, and as you know, things can and will change, so a few more model runs will be needed before specifics will be known.
Everyone needs to have a plan for Monday evening.