And so it begins. The National Hurricane Center has issued its first advisory on newly-minted Subtropical Storm Alberto in the western Caribbean. The storm could intensify into a strong tropical storm as it approaches the northern Gulf Coast early next week. Heavy rains, flooding, and rip currents are likely across the southeast through next week, an especially tricky situation given the influx of tourists for the holiday weekend.
The first advisory from the NHC on Alberto shows the storm making landfall somewhere between New Orleans and Mobile during the day on Monday. Everyone from central Louisiana to the Florida Peninsula is within the cone of uncertainty. The cone is the historical track error in the previous forecasts for a storm. The center of a storm historically stays within the cone 66% of the time. The wind, rain, and storm surge can extend hundreds of miles from the center of a storm, however, and that's why the entire southeast is in play for foul weather from this storm.
Subtropical?
Don't let the term "Subtropical Storm Alberto" trip you up. A subtropical storm is a cyclone that has both tropical and extratropical characteristics. Tropical cyclones are driven by thunderstorms around a low-pressure center at the surface and their wind field is usually compact. Subtropical storms derive some of their energy from upper-level winds and their surface wind fields are usually more spread out or detached from the center of circulation.
The difference between a tropical storm and subtropical storm is technical and has little practical use for people in the path of the storm. It's the NHC's job to adhere to the "well, technically" aspect of storms like this. Don't focus on the name. Alberto will likely transition to a fully-tropical system this weekend and it'll still have the same threats and hazards no matter what you call it.
Rain
Heavy rain is the greatest concern with this storm as Alberto taps into deep, tropical moisture flowing across the southeastern United States. The Weather Prediction Center's latest rainfall forecast (issued early Friday morning) shows a slug of 3-5" of rain covering most of the southeast.
Rain associated with Alberto will start this weekend in Florida and gradually work its way inland as the storm pushes inland early next week.
The heaviest rain will likely be focused on the eastern side of Alberto when it makes landfall. The storm will likely slow down when it approaches the coast, exacerbating the heavy rain in areas where rain bands start training (repeatedly moving over the same areas). Some spots caught under training bands could see rainfall totals approach double digits by the middle of next week.
I talked a bit yesterday about how the southeast has already been soaked by heavy rain over the past couple of weeks. Most of us have seen several inches of rain and some areas have seen more than half a foot of rain from recent thunderstorms. The ground is already saturated and it won't take much of a downpour from Alberto to cause flooding concerns in low-lying areas.
Wind
The threat for wind damage is a growing concern along the northern Gulf Coast where Alberto makes landfall. Alberto is expected to be a high-end tropical storm when it makes landfall on Monday with sustained winds somewhere around 65 MPH. Forecasters will fine-tune the storm's intensity as we get closer to landfall and models get a better handle on the storm.
It's important to point out that the NHC's forecast discussion with the first advisory calls this forecast "conservative" in its intensity as the storm approaches landfall on Monday. The storm will slow down as it enters an environment favorable for strengthening once it reaches the northern Gulf of Mexico on Monday. There's an outside chance that Alberto could push hurricane strength at landfall if current forecasts hold true.
The risk for power outages will grow as the storm grows stronger. That sounds like an unnecessarily obvious thing to say, but all the rain over the past few weeks has loosened the soil enough that it won't take much for trees and power lines to start falling down. Widespread power outages are possible. It's a good idea to make sure you're prepared with batteries, battery-operated flashlights, food that doesn't need to be cooked (ravioli and Pop Tarts are your BFFs in the dark), and bottled water. Even a little bit of cash wouldn't hurt if you're lucky enough to have it—your credit/debit card does no good when the power's out.
Storm Surge
A storm surge is possible at the coast when Alberto makes landfall. The threat for a deeper surge will grow as the storm slows down and grows stronger near landfall. The northern Gulf Coast is an especially sensitive spot for storm surge. New Orleans is firmly within the cone of uncertainty and it would only take a small nudge to the west in Alberto's track to make flooding in New Orleans a serious concern.The National Hurricane Center started issuing storm surge watches and warnings last year to make residents aware of the threat for coastal flooding independent of the storm's intensity. There aren't any watches or warnings in effect for the United States right now, but they'll almost certainly be issued this weekend as the storm draws closer.
Surf/Rip Currents
It's a holiday weekend and thousands of people are heading to the beach to mark the beginning of the summer season. There will be lots of people at the beaches even as the storm approaches, and many of them will still go in the water even though the risks of rough waves and rip currents will be well advertised.I'll crib what I wrote yesterday about the threat for rip currents and how to deal with them:
Rip currents will be a growing threat as the winds pick up ahead of Alberto. Rip currents are fast currents of water that pull water away from the beach. Rip currents occur when waves crash directly on the beach rather than at an angle—you can sometimes spot these currents by looking for calm spots mixed in with the waves.
Don't go in the water if you're at the coast for the holiday weekend and there are red flags flying at the beach. Rip currents don't suck you under like they show in the movies. People drown in rip currents because they can't swim, panic, or can't tread water anymore due to exhaustion or waves. If you're ever caught in a rip current, either tread water and calmly signal for help or swim parallel to the shore until the pull stops, and then swim back to the beach.
Tornadoes
Tornadoes are a common part of landfalling tropical cyclones. The greatest threat for tornadoes exists in the right-front quadrant of a landfalling storm—in Alberto's case, this will be the eastern side of the storm. Tornadoes in tropical cyclones happen quickly but they're dangerous because they can touch down without much warning at all.The Storm Prediction Center has issued a marginal risk for severe weather in Florida on Saturday as Alberto passes into the Gulf of Mexico and its rain bands start overspreading the peninsula. The tornado threat will continue early next week farther north when Alberto makes landfall.
Updates
The National Hurricane Center issues full forecasts every six hours (5 AM/PM and 11 AM/PM Eastern Time) and will issue intermediate updates every three hours (2 AM/PM and 8 AM/PM Eastern Time) as long as there are watches and warnings in effect.As always, I'll keep you updated with maps and analysis here on DAMWeather and on Twitter @wxdam. Thank you as always for your continued support.
[Maps: Dennis Mersereau | Satellite: NOAA | Wind Prob. Map: National Hurricane Center]
Please consider subscribing to my writing on Patreon. Reader-funded journalism is more important than ever and your support helps fund engaging, hype-free weather coverage.
0 comments: