Monthly Archives: July 2013

The bright side of the rain: Cooler temperatures

One nice side effect of this very active weather pattern so far this summer has been its moderating effects on temperatures: This year’s warmest day was June 13 with a high of 96 degrees. In 2012, we hit 98 degrees (highest temperature last year) four times: June 30, July 9, July 26, and July 27.

Remember the seemingly endless flood of alerts about heat advisories last year? This year, the National Weather Service in Charleston has only issued one heat advisory for the Charleston metro area; this heat advisory came on June 13 (coinciding with the warmest day of the year). Contrast that to last year, when in addition to four heat advisory days (June 29, July 2, July 26, and July 27) we also had two Excessive Heat Warning days (June 30 and July 1). As you may recall, 2012 was also the summer with 32 straight days of 90 degrees or better at the airport from June 28 to July 29. (See this Weather Underground chart for more information, and see the note below on what constitutes a Heat Advisory or an Excessive Heat Warning.)

More cooler than normal weather is expected to persist for at least the next week as an upper-level trough hangs tough in the eastern part of the country, bringing unseasonable cool to much of the Northeast and parts of the Midwest; this trough will help continue to route disturbances our way, stirring up showers and thunderstorms in the afternoons.

Note: The National Weather Service in Charleston, SC issues a heat advisory when heat indices are expected to top out between 105-114 degrees for two hours (this threshold is raised to 110 degrees after July 1). An Excessive Heat Warning is issued when it is likely that heat indices will reach or exceed 115 degrees. (Source: NWS CHS Public Information Statement)

Another day, another flood

Earlier this afternoon, heavy rainfall parked itself over downtown Charleston, and you probably know by now what happened next…

The College of Charleston observatory recorded 2.08″ of rain in a little over an hour this afternoon; radar estimates neared 3″ just north of there.

Here’s a list of flooding reports relayed by the National Weather Service.

Unfortunately, we’ll need to keep a close eye on how things evolve tomorrow as the requisite moisture will once again be in place for potentially heavy rains and flash flooding in parts of the Lowcountry. Rain chances will then steadily decrease as the week goes on.

Clouds holding temperature and rain in check thus far

Satellite, radar, and temperatures at 11am 7/21/2013.

Satellite, radar, and temperatures at 11am 7/21/2013.

Plenty of cloud cover has pinned our temperatures down some into the low 80s; this is helping to keep instability in check and rain from developing. Clouds are a little thinner near the coast, though, and this is allowing temperatures to head up a few more degrees. Radar is starting to show some activity trying to kick off there. Storm motions are quick this morning but high-resolution models suggest they will slow down a bit by this afternoon; still, expect storms to move around 10-15 MPH.

Locally heavy rainfall will be possible with any storm that can get going, so we’ll have to watch carefully as we are still contending with rather soaked soils.

Perigean spring tide starts tonight

We’ll need to keep an eye to the evening high tides for the first part of the week, starting tonight, for the potential for minor coastal flooding around the time of high tide. Today’s winds are offshore, which should help keep tides below the 7′ threshold for shallow coastal flooding. If things change, the NWS will issue a Coastal Flood Advisory. High tide tonight is at 7:45.

Regardless of tidal anomalies, we’ll definitely want to keep a close eye on any heavy rain that may approach the area at times of high tide as it won’t take too much to cause problems.

Off and on rain for the weekend

Yes, unfortunately, more rain is in the cards for the weekend. Best rain chances are set up for Saturday where we’ll see some upper disturbances swinging through generating showers and thunderstorms. Locally heavy rain will certainly be possible with even a shot at some minor flooding given the potential for very slow storm motions. Saturday won’t necessarily be a total rainout — rain will be off and on — but you’ll certainly want to make sure your inclement weather plans for tomorrow’s outdoor activities are firmed up. Rain will suppress temperatures a few degrees below normal, too.

Rain chances are slightly lower for Sunday, but locally heavy rainfall and perhaps a couple stronger storms are in the cards.

Rainfall surplus update

After a trace of rain at the airport today, we stand at 39.77″ for the year. That’s 13.77″ above normal for this point in the year. Given rainfall forecasts for the weekend, it’s likely we’ll breach 40″ of rain this weekend. For reference, we see 51.03″ in a typical year (normals derived from 1981-2010). Last year, we only got 43.97″ of rain. (Source: Annual Climate Report, NWS Charleston, SC.)

Off and on rain through the morning, improvement this afternoon

Off and on rain continues for much of us this morning. Some showers will have heavy downpours and gusty winds. Overall, it will be a bit breezy today. Of note: The National Weather Service canceled the Flash Flood Watch for the Charleston area this morning, so there’s much less concern over flooding rains for the area at this point.

This morning: Radar, satellite, and model trends suggest heaviest rains will increasingly be found west of I-26 as the deepest moisture moves west; this includes Folly Beach, James Island, Johns Island, West Ashley, Ravenel, Hollywood, Kiawah, Seabrook, Wadmalaw, and pretty much all of Dorchester County including Summerville, Ridgeville, and St. George. As high pressure builds in, the threat for the heaviest rains will move out of this area as well. All in all, most of us should salvage a pretty decent Sunday afternoon and evening before rain chances head back up along the coast for a spell tonight.

Highs will be tough to pin down with off and on rain but it wouldn’t be out of the question to see upper 80s if the sun peeks out enough. Expect breezy east winds today as high pressure muscles its way in from the northeast.

Charleston Rainbows

Storms moving onshore this evening helped set up a rare rainbow before the rain hit around 7:50 PM. Here’s a few pictures tweeted to @chswx as well as others I found and quickly authenticated.

Flash Flood Watch extended through Sunday morning

The NWS is keeping the Flash Flood Watch around a little bit longer…with the watch now going until Sunday morning as the threat for additional heavy rain continues overnight.

This afternoon, showers and thunderstorms continue to develop over the Tri-County area with heavy rain at times. Moncks Corner and surrounding areas have already received 2″ of rain this afternoon per radar estimates.

Sunday will mark the beginning of improvement as the upper system that has fed our on-and-off lousy weather moves back westward and high pressure builds back in. By early next week, we will be back in a typical July pattern again with highs in the low 90s and scattered afternoon thunderstorms.

Flash Flood Watch extended through Saturday evening

Earlier this afternoon, the National Weather Service extended the Flash Flood Watch through 6PM Saturday. Plenty of deep moisture will continue to be available, allowing for very efficient rainfall production which may cause localized flash flooding. Earlier this afternoon, there were some brief road closures in downtown Charleston thanks to a couple inches of rainfall about two to three hours after high tide. It’s not out of the question to have a repeat possibly as early as overnight tonight.

Keep an ear out to the forecast and to your reliable sources of weather info in case Flash Flood Warnings are issued for the area.

One positive from the rain is that it’s not been sweltering hot at all — in fact, today’s high of 81 was a new record low maximum temperature for the date.

Flash Flood Watch until 6PM for the Charleston metro area

Radar rainfall estimates through 5:30 Friday morning.

Radar rainfall estimates through 5:30 Friday morning.

A Flash Flood Watch is up for the Charleston metro area through 6 PM as the stage has once again been set for the potential for very heavy rainfall in many locations. Allendale County took a very hard hit last night with the radar estimating upwards of 9.5″ of rain in the far northern end of the county just since late last night. Flash Flood Warnings in effect earlier for northern Dorchester County have expired.

The potential is there today from anywhere for 1-3″ of rain with local amounts exceeding 4″. At a minimum, make sure you have a solid set of indoor plans today and go easy on the roads when it’s raining.

Residents of low-lying coastal areas should watch time of high tide (11:21 PM) very carefully as any heavy rain setting up near the coast along that time will most certainly cause some flooding problems.

Stay close to reliable weather sources today in case Flash Flood Warnings are issued. Most recent iPhones and Android phones will be able to relay Flash Flood Warnings through the government’s WEA alerts (think of it as a mobile Emergency Alert System). I’ll carry Flash Flood Warning updates on Twitter automatically and on this blog as time permits.

Chantal's circulation dissipates, but wet weather remains in the forecast

At 5PM, the National Hurricane Center discontinued advisories on Tropical Storm Chantal as Hurricane Hunter aircraft were unable to find a closed circulation as it raced toward Hispaniola. Regeneration appears highly unlikely as the storm is forecast to continue to encounter a hostile, highly sheared atmosphere. (Then again, never say never with these…the NHC is going to keep watching it, and I will too.)

Weather Prediction Center's precipitation forecast from Wednesday 8PM to Monday 8PM.

Weather Prediction Center‘s precipitation forecast from Wednesday 8PM to Monday 8PM.

With that in mind, the stage is still being set for another round of very heavy (and possibly flooding) rain for the weekend. The Weather Prediction Center is thinking we could see another 3″ of rain through Monday evening. We remain almost 15″ above average for rainfall at this point in the year with 38.68″ of rain to date; if this forecast verifies, we’ll breach 40″ of rain by July 15. (According to the annual climate report, in an average year, we see 51.03″ of rain.)

At a minimum, be thinking about weekend plans that can most easily be accomplished indoors; downtown residents, especially those in low-lying areas, will want to be thinking about some higher locations to park cars as that threat is once again going to be with us through the weekend. Don’t be surprised to see a Flash Flood Watch issued at some point. Keep in mind that several rivers around the area, including the Edisto River at Givhans Ferry and the Santee River at Jamestown, are in flood stage (with the Edisto in moderate flood). Expect additional rainfall to exacerbate the flooding situation on those rivers.

NWS Charleston’s current forecast thinking is that we will return to typical July weather in the first part of next week, with highs in the 90s and showers and thunderstorms in the afternoon. Stay tuned for updates…