A Stormy Start To The Holiday Weekend

Apr 03, 2023

by Frank Strait

May 26, 2023

12:00 AM

There are some changes to the forecast. The latest computer model guidance shows a slower progression of the developing coastal storm, which will cause rain and wind to linger along our coast throughout most of the day Saturday. The models have also shifted the storm's track slightly to the east, pushing more of the coastal flooding risk (which was limited, anyway) into North Carolina, reducing the potential for heavy rainfall across the Savannah River Basin.

A zoomed-in satellite loop shows the cold front's leading edge moving through North Carolina, and it will continue to push through the Palmetto State this evening.

A loop of true color satellite imagery from midday showing clouds associated with a cold front advancing southward through eastern North Carolina. Source: University of Wisconsin RealEarth

Behind the front, strong high pressure is building into the Great Lakes region, and the steep pressure gradient (a large change in pressure over a distance) between the high and developing storm will cause winds to increase across the state and coastal waters.

Our weather map will look something like this tomorrow morning:

A forecast weather map for Friday morning from the Weather Prediction Center (WPC) shows strong high pressure over the Great Lakes, a storm off the Southeast Coast, and tightly packed isobars between the two indicates the steep pressure gradient.

The worst winds come tomorrow and last through Saturday morning, with the peak Friday night. The latest computer model guidance shows less of a wind threat north of the Fall Line, thanks to the eastward shift in the low's predicted track. However, winds will still be strong along the coast from the Charleston area northward, roughly northeastward from the I-26 corridor, toward the Grand Strand and interior Pee Dee. Winds will peak between 45-50 mph if the computer guidance is good. Elsewhere, gusts will peak in the 35-40 mph range.

It still looks like most of the state could get drenched, but the eastward shift affects the rainfall forecast, too. Areas near the Savannah River will see less rain, likely 1-2 inches, while the Pee Dee should get the most, likely 2-4 inches, and locally higher amounts. However, recent weeks have brought above-normal rain to the western part of the state, while it's been dry over most of the east. As for timing, the rain becomes heavy along our coast later Friday and spreads inland overnight, reaching the Upstate Saturday.

WPC's Excessive Rainfall Outlook for Saturday shows most of the state covered by a slight risk (level 1 of 4) for flash flooding.

The coastal flooding risk is lower than we usually see with these events. As mentioned yesterday, winds will be mostly offshore, and we're in a period of low astronomical tides. However, minor coastal flooding is in the forecast along our coast during the overnight high tides tonight and Friday night, with Friday's expected to be the worse of the two.

The wild card remains whether the coastal storm gains enough tropical characteristics to become worthy of a name. The chances of this becoming a subtropical storm look lower today than yesterday, but it isn't zero. Should it become a subtropical storm and follow the more eastward track, the potential for stronger onshore winds and a small storm surge would be mainly in North Carolina but may affect some or all of the Grand Strand.

WPC's forecast weather map for Sunday morning shows the storm ashore over the Pee Dee region.

I want to emphasize that Saturday's weather looks quite ugly across the state. Between the wind, the rain, and the unseasonably chilly air flowing into the state, it's unsuitable for outdoor activities; it's uglier than Mr. Happy Face (the reigning World's Ugliest Dog). The current forecast has highs for Saturday ranging from only the high 50s over parts of the Upstate to around 70°F along the Grand Strand. It's way cooler than we expect in South Carolina in late May, and we might beat the pants off some record-low-maximum temperatures for the date.

Forecast high temperatures for Saturday from the National Weather Service. Source: WeatherBELL

For example, the lowest high temperature on record for Saturday, May 27, for Columbia Metro Airport (CAE), is 68°F, and for GSP Airport is 65°F. Current forecasts have a high of 63°F for CAE and 59°F for GSP.

Thankfully, conditions on Sunday and Monday look better. Neither day will be entirely rain-free, though. Spotty showers will linger on Sunday as the coastal low moves away, and Monday will feature mainly afternoon showers and perhaps a thunderstorm as an upper-level disturbance moves through. Sunday's highs range from the middle 60s in the Upstate to the mid-70s along the Coastal Plain, and Monday looks even warmer (though still below average); highs will range from the lower 70s in the Upstate to around 80 in the Lowcountry.

by Frank Strait

May 26, 2023

12:00 AM