Whither Alberto?
The (sub)tropical system will have indirect impacts on our weather for the rest of the long weekend
Updated on May 27, 2018 at 9am with additional forecast details.
Subtropical Storm Alberto marks a rudely early start to the 2018 tropical cyclone season. Its primary impacts of rain and surge will be felt along the Florida, AL, and MS Gulf Coasts over the next few days. Current thinking is that it will fully transition to a tropical system (right now it is still being somewhat driven by upper-air influences that prevent it from taking on fully tropical characteristics) and strengthen some as it heads north.
As of 8am on May 27, it is doing just that: Winds are up to 45mph and it is moving northward at 15 MPH. By the current NHC forecast, this will put Alberto onshore in the Florida panhandle sometime Monday afternoon. Alberto is continuing to get a little more organized, and may complete a fully tropical transition just before coming ashore. Thankfully, the faster forward speed means there is less time for Alberto to organize and strengthen over warmer-than-normal Gulf waters.
If you have travel plans to the Gulf of Mexico over the next few days, watch Alberto closely and pay attention to local media and authorities.
Impacts on the Lowcountry
Alberto’s effects will be felt well away from the center. Working in conjunction with a subtropical ridge of high pressure to the east, it will funnel a deep plume of tropical moisture northward into the area starting Sunday. We’ve been in quite a moist tropical airmass for the last week and a half as it stands. Alberto’s arrival along the Gulf coast will only supercharge this airmass.
The loop above shows precipitable water, which is essentially the measure of how much water that can be wrung out of a column of air. Precipitable water values will surge over 2" starting early Sunday, which is extremely soggy for this time of year (but normal for a tropical system). This will set the stage for periods of heavy rainfall and possibly some localized flooding.
Rainfall: Timing and impacts
Sunday (updated 5/27 9am): Showers and thunderstorms are expected to develop Sunday morning along the leading edge of the push of higher moisture content and push northward with time. We’ll start the day with scattered showers and maybe a few rumbles of thunder. As moisture continues to push northward across the area, we will see an uptick in shower and thunderstorm activity, with the best chance of heavy rain coming later Sunday into early Monday morning. My advice is to have outdoorsy stuff starting to wrap up between 2–4PM. I’m still giving a fairly vague range of times because Alberto’s forward speed has been increasing this morning, and this may move timings up a bit. Overall, though, there is good agreement in the guidance suite that rain will begin to get more widespread by 4PM.
Monday (updated 9am 5/27): We will catch a little break early Monday morning in the wake of the first round of showers and thunderstorms. Thereafter, expect off and on rainfall with some locally heavy downpours at times. It will not be an all day rain, but if you venture outdoors, be just as prepared to head indoors. I can’t rule out some thunder, either, especially if breaks in the cloud canopy develop.
Tides & flooding threat
One thing we will need to watch carefully for Sunday night/early Monday morning is the potential for localized flooding with the heavy rain band. The current thinking is that there is a distinct possibility heavy rain could coincide with the 7:40 PM high tide. In addition, onshore winds from the south and southeast will be favorable for pushing that tide a little higher (though it is worth noting that storm surge is not expected to be a concern here). High tide combined with heavy rain always is a concern for downtown Charleston, and tomorrow night will be no exception, particularly with tide levels that could approach 6.9' or higher. (“King tide” flooding generally starts around 7' mean lower low water.)
While it’s not yet clear that flooding will happen, be prepared for the possibility by doing a few simple things, especially if you live in peninsular Charleston:
- Take some time to clear storm drains around your home or business. Clear storm drains give the roads a fighting chance to stay above water especially in the hardest downpours.
- If you live in a known flood-prone area, be thinking about where you might move your car if flooding poses a more clear threat.
- Tell your neighbors about the threat of flooding so that they can take the same steps. Help them to prepare if they need it.
- Keep an ear to trusted weather sources (including yours truly, if you so desire) for weather updates and alerts from the National Weather Service.
Winds
As of 9am 5/27: Winds will not be a major factor with Alberto, but given its eastward trend, we will be susceptible to some breezy conditions especially at the coast, where winds could gust 30–40 MPH at times. Widespread issues due to wind are not anticipated, but with wet soils due to heavy rain, a fallen tree or two cannot be totally ruled out.
Tornadoes
As of 9am 5/27: With Alberto’s center trending a little further east and the resulting favorable juxtaposition of wind shear and instability, there is potential for a tornado or two on Monday. Tornadoes with tropical systems are often brief and relatively weak, but they can still cause damage and need to be taken seriously. It will be a good idea to make sure you can receive weather alerts on Monday, especially if you will be trying to fit some outdoor activities in.
Bottom Line
- Best rain chances come roughly late Sunday afternoon through Monday morning. It won’t rain all day on either day, and there may very well be periods where you can get outside a little (just be ready to get back indoors!)
- Rain will be heavy at times, with upwards of 3–4" possible in some spots within a very deeply tropical airmass.
- We will need to watch the 7:40 PM high tide on Sunday very closely for the potential for flooding along the coast as well as in downtown Charleston.
- Winds could turn breezy at and near the coast, but aren’t expected to play a major role (though downed trees due to wet soils will be possible).
- There is a small risk for a tornado on Monday, but widespread severe weather is not expected.
I’ll have quick updates as needed throughout the day Sunday and Monday on Twitter.
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