A quite complex upper level pattern exits with 2 progressive northern stream troughs, one over the High Plains & another coming into the Pacific Northwest, a 3rd slower moving one over the southern Rockies and a 4th in the central Gulf of Mexico. This ridge will quickly erode as the southern Rocky Mountain trough tracks into the southern plains tonight. A few showers will develop in the coastal waters as moisture from the Gulf trough is drawn into the southern plains trough. I have pretty much kept the previous forecast for Wednesday with no rain in coastal Mississippi and 30-40% rain chance to the west.
The Pacific Northwest trough will be reaching the Great Lakes by midday Thursday. A frontal boundary will accompany this feature and push into the forecast area later in the day on Thursday. It will likely provide enough lift to produce rain coverage approaching 50% in north and eastern zones. There`s still a little uncertainty how far into the Gulf the boundary will go before stalling. A reinforcing cold front will swing through Saturday evening as a weak trough racing across the country merges with another diving south out of Canada across the southeastern US. This boundary will flush out any residual coastal moisture but provide little cooling as ridge quickly builds in from the west.