textproduct: Columbia

WHAT HAS CHANGED

Aviation discussion updated for 12Z TAFs.

KEY MESSAGES

- 1. Periods of moderate to heavy rain expected Wednesday night into Thursday with severe weather possible.

- 2. Rain chances pick back up late week and into the weekend.

DISCUSSION

Key Message 1: Periods of moderate to heavy rain expected Wednesday night into Thursday with severe weather possible.

Overview: A cold front will work across the Southeast Wednesday through Thursday coinciding with strong upper support and strong moisture advection. Periods of heavy rain are possible with a flash flooding threat limited to mainly urban and flood prone areas. There is a marginal threat of severe weather as two main rounds of convection are expected to move through the forecast area. Thunderstorms ahead of the front should push across the FA on Wednesday night/Thursday morning potentially followed by a second round of convection later in the day as the front approaches. Fairly breezy winds will also be possible through the day Thursday where a Lake Wind Advisory may be needed.

Rainfall: As ongoing convection moves across the Southeast on Wednesday, strong moisture advection and shortwave energy should sustain thunderstorms through the night as the line moves into the forecast area. The 35-45kt low level jet that develops overnight will promote heavy rainfall and potentially training cells. The line of storms will propagate south and east through the night so the overall motion of the system should prevent widespread flash flooding, however urban and other flood prone areas will still be at risk. Mean QPF amounts from the NBM range from 0.5 to 1 inch with highest amounts to the west, possibly due to CAMs trying to propagate the deepest convection into southern GA overnight. We expect locally higher amounts with some PMM values in the REFS around 1.5 inches in the CSRA.

Severe Threat: Multiple rounds of thunderstorms are expected, first Wednesday night/Thursday morning with another round possible Thursday afternoon. The overall severe threat is somewhat muted by the nocturnal timing of the first round and the uncertainty in instability re-developing ahead of the second round. Poor lapse rates overnight should hinder mixing stronger winds to the surface, making flooding the greater threat overnight. The deeper moisture will shift east during the day with westerly flow allowing more discrete convection just ahead of the front. Drier air aloft and slightly steeper lapse rates could allow for a downburst or hail threat but instability is expected to be weak with mean REFS sbCAPE values less than 500 J/kg. The severe threat in the afternoon will be largely dependent on the first round of convection and lingering cloud cover and rainfall through the morning.

Key Message 2: Rain chances pick back up late week and into the weekend

While much of the forecast in the extended remains on track the biggest change is the reduction of PoPs for Friday as model consensus narrows to a drier solution. A progressive pattern for remainder of the forecast period appears likely. The most active period looks to be this weekend into early next week with a deep upper trough working across the eastern US supporting a high chance of PoPs from Saturday through Monday.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

VFR conditions expected to continue Wednesday, cold front bringing rain and possible restrictions tomorrow morning.

Southerly winds shifting southwesterly this morning will increase as the inversion breaks with gusts around 20 knots expected. Moisture will continue to increase with a line of showers and potentially thunderstorms approaching the terminals associated with a front. While isolated showers and storms are possible this evening, the most likely timing of widespread rain and associated restrictions is tonight into Thursday morning. While the timing of the line does not favor widespread thunderstorms, there remains low chances for thunder to impact the terminals. Confidence remains too low to include thunder for now but moderate to heavy rain will likely lead to at least temporary visibility restrictions.

EXTENDED AVIATION OUTLOOK...System brings a chance for restrictions once again with showers and thunderstorms starting Thursday morning. Additional rain chances this weekend.

CAE WATCHES/WARNINGS/ADVISORIES

SC...None. GA...None.