I hate to be the bearer of bad news, but we'll have to deal with some ugly weather this weekend. While the entire state will feel the effects of a storm system starting Thursday, the primary area of concern will be along our Coastal Plain.
To explain where all this is coming from, let's begin by looking at visible satellite imagery.
True color satellite imagery from this morning centered on South Carolina. Source: University of Wisconsin RealEarth
The first player in our late week and weekend drama is a cold front now crossing the Great Lakes, which you can see as a band of clouds crossing that area. This feature will move through South Carolina tomorrow and become stationary over our coastal waters. Two other players are disturbances over Mississippi and Alabama and another over eastern Texas. The two will join forces and spawn a storm that will form near Florida late tomorrow, then move toward us by Saturday morning. One more weather feature to mention is an intense high pressure area that will build into the Great Lakes region by Friday morning.
So, our weather map will look like this Thursday evening:
Forecast Weather Map valid at 0000 UTC Friday, 8 p.m. Thursday EDT, from the Weather Prediction Center.
By this time, the front and developing storm near Florida will generate showers along our coast. The difference in pressure (pressure gradient) between the storm and the high-pressure area over the Great Lakes will cause strong winds Thursday night over our coast and coastal waters. The strong winds continue through Friday and probably into Saturday morning as the storm strengthens and moves closer. Early indications are that gusts will peak around 50 mph close to our coast and perhaps over much of our Coastal Plain. It will also be windy across the rest of the state, though not as extreme, with peak gusts of 35-40 mph.
The coastal waters will experience strong-gale-force winds from Thursday night to Saturday, possibly even storm-force. A small craft advisory and gale watch are already in effect for our coastal waters for the rest of this week and Saturday, and there is a gale warning for the offshore waters (beyond 20 nm from shore).
Rainfall totals statewide range from one to two inches from Thursday through Saturday, with isolated amounts higher along our coast; however, computer models differ on how much rain will fall at any particular point. The best chance for higher rain totals is in the Pee Dee region, where some models indicate up to six inches by Sunday morning.
Coastal flooding should not be a major concern thanks to the fact we're approaching a first quarter moon phase, when the astronomical tides will be near their lowest, and the winds will come mainly from the north-northeast, blowing offshore, pushing the water out to sea. So, while there is some concern for coastal flooding and beach erosion from this storm, the risk is not as high as it can be. However, minor coastal flooding is possible at high tide on Friday and Saturday.
Now, to address the elephant in the room. Sometimes storms like this gain tropical cyclone characteristics as they pass over the Gulf Stream. It's a concern with this one because the offshore waters are warmer than usual for May.
A plot of sea surface temperature anomaly from the UK Met Office's OSTIA system, which uses satellite data and coastal observations to determine sea surface temperatures for the world's oceans and other large bodies of water like the Great Lakes. Source: WeatherBELL
Waters beyond the Continental Shelf are running 1-3°F warmer than average, and there is a chance that this storm will become a subtropical (hybrid tropical and extratropical) storm as it approaches our coast Friday night into Saturday. There would be two consequences to this. First, it would enhance rainfall. Second, it would cause the winds to turn onshore to the right of where the center comes ashore, resulting in some areas seeing a second period of stronger winds. Those winds would produce a small storm surge of a foot or two.
There is a lot of uncertainty on this issue. For starters, the storm may not become subtropical. The National Hurricane Center says there is only a 10 percent chance for a named storm to form. Second, whether subtropical or not, the center may track into North Carolina instead; if so, the onshore wind and storm surge concern would be a problem for the land of vinegar-based sauce instead of us.
The 2 p.m. Wednesday Tropical Weather Outlook from the National Hurricane Center indicates a 10 percent chance for a named storm to form over our coastal waters.
As for Sunday and Memorial Day, we should see improvements. However, a lingering upper-level disturbance over the area will bring the potential for mainly afternoon showers and thunderstorms, like what we often see in midsummer. Temperatures, however, will not feel like the unofficial start of summer. Saturday looks unseasonably cool, especially in the Upstate, where it could stay in the 50s and low 60s all day (ouch, that's like February). Sunday and Monday look warmer, but temperatures will still be below average for late May, with highs in the 70s over most of the state; parts of the Lowcountry may reach 80°F.
Currently, the confidence level is medium to high on the rainfall and wind potential and low on the potential for subtropical storm effects. The picture should get clearer over the next day or two, so stand by for updates.
Visit www.hurricane.sc to find hurricane preparedness information from SCEMD so you're ready for this or any other storm that might affect us through the upcoming hurricane season.
