Tropical Storm Irene, with 60 MPH winds, is getting its act together over the Greater Antilles right now as it moves towards the west-northwest, threatening the US Virgin Islands, Puerto Rico, and eventually the island of Hispaniola (housing the nations of the Dominican Republic and Haiti). Hurricane warnings are in effect for the aforementioned islands as the National Hurricane Center expects Irene to become a hurricane as early as tomorrow morning.
Anyone from the northern Gulf Coast all the way up through Atlantic Canada should keep an eye on this system. Jump the fold for a few of the possible scenarios.
Tropical Storm Irene at 7:45PM EDT
(millwx beat me to posting about this by 6 minutes. Go check out his analysis as well -- it's really good.)
Right now, there are really two things that will drive the intensity and direction of the storm: land interaction and how soon a high pressure system over the Atlantic curves Irene into the United States. When it comes to the forecast models, they have just as many differing opinions as there are here on DailyKos. Every one has a different bias and a different way of seeing the world. Only rely on official National Hurricane Center forecasts to decide what you should do to protect yourself and your family. Don't go into panic mode because one of 20 models shows a category 5 hurricane slamming you six days down the road.
Here is the official NHC forecast. Click to enlarge.
The forecast points in the map above are the NHC's best guess as to where the center of Irene will be at the forecast intervals (12, 24, 48, 72, 96, and 120 hours). The cone around the forecast track denotes the uncertainty surrounding the long range forecast, which in this case, the NHC says is fairly high:
THERE IS A FAIR AMOUNT OF SPREAD IN THE TRACK GUIDANCE MODELS...ESPECIALLY AT DAYS 4 TO 5. FOR EXAMPLE...THE GFDL MODEL TAKES IRENE TO THE WEST OF THE FLORIDA PENINSULA WHILE THE HWRF MODEL TAKES IT THROUGH THE NORTHWESTERN BAHAMAS. INTERESTS IN FLORIDA ARE ADVISED NOT TO FOCUS ON THE EXACT FORECAST TRACK BECAUSE OF THE INHERENT UNCERTAINTIES IN LONGER-RANGE FORECASTS.
The reason for the uncertainty in the long-range forecast right now is because of a high pressure system over the western Atlantic, which is steering Irene along towards the west-northwest right now:
Click to enlarge
The NHC expect that several shortwave troughs will chip away at the ridge over the southeastern United States, and create an opening for Irene to start moving northwest-to-northward to impact the southeastern United States. The question isn't if it will happen, but how soon will the ridge erode to allow Irene to come north?
There are three scenarios. The first is the most likely, where the ridge slowly erodes and allows Irene to hit Florida. The second is also very possible, where the ridge erodes quickly and allows Irene to stay over water longer and hit Georgia or South Carolina (this would be bad). The third and least likely (again let me emphasize LEAST likely) scenario is that the ridge holds strong and allows Irene to bypass the bulk of Florida and head into the Gulf of Mexico to post a threat to Louisiana, Alabama or the FL Panhandle.
Scenario 1 (Most Likely): Irene hits southern Florida and tracks up the length of the Peninsula.
A good chunk of the computer models, as well as the official National Hurricane Center forecast, expect that the ridge will break in time for Irene to make a turn and impact heavily populated southeastern Florida, traverse the length of the state and move into the southeastern United States. As of right now, this is the most likely scenario, but it's not written in stone. Lots can happen between now and late this week to cause this forecast to bust.
This scenario is best illustrated by the above forecast from the NHC, as well as by this "spaghetti model" from Florida State University (the NHC forecast is the black line). The spaghetti model takes all model runs on the hurricane and puts them on the same map. The result looks like a bunch of spaghetti (hence the nickname) and gives an idea as to whether or not the models are predicting the storm in sync, or if there's a difference in their forecasts. In this case, there's quite a bit of difference.
Model image from FSU. Click image to enlarge.
.Scenario 2: Ridge breaks more quickly and Irene stays just offshore from Florida, and impacts Georgia or South Carolina.
This is the scenario we don't want. To put it simply...conditions are favorable for Irene to strengthen. It's expected to have low shear, good upper-level outflow (acts as an exhaust...gets rid of air to let in more warm moist air to strengthen), and warm waters to feed it. The longer it stays over warmer waters (like those off the FL/SE coast), the more of a chance it has to strengthen. More of the models have been trending towards this possibility, so we'll have to see what the NHC says at 5PM EDT when the next advisory comes out. Several of the reliable models have this happening, so it might be more likely than any of us would like to think.
A few (read: 2) of the models show Irene going west of Florida and posing a threat to the northern Gulf Coast, probably from Mobile east to the bend between the Peninsula and Panhandle of Florida. Since only one or two models shows this right now, it's probably not that big of a threat, but it's still not completely out of the question so it's worth mentioning.
Irene's intensity at US Landfall depends almost wholly on two factors: How much will Hispaniola/Cuba rip it apart, and how long will it spend over water before impacting the US? Under Scenario 1, Irene will probably reach minimal hurricane strength (Cat 1) before landfalling on southern Florida. Under Scenario 2...I'd prefer not to think about that, but suffice it to say that it will be a formidable hurricane by landfall. Scenario 3, while unlikely, would pose a problematic hurricane to the Gulf Coast.
Here's the 8PM EDT NHC forecast regarding Irene's intensity. Keep in mind that this, just like the storm's track, can greatly vary as time progresses.
Click to enlarge
If you live anywhere on the East Coast, it's imperative that you keep an eye on Irene's movements. Be sure to follow Hurricane Kos here on DailyKos so you know exactly when any of our Hurricane Kos team members post.
Here are some links to help you stay ahead of the storm.
National Hurricane Center
National Weather Service
Storm Prediction Center
College of DuPage GOES Satellites
Wunderground Tropical Site
Florida State University Tropical Forecast Models
DailyKos Tropical Weather Link Library
5:31 PM PT: I've updated the satellite image, track map and intensity forecast to reflect the 8PM EDT update from the NHC. Irene now has 60 MPH winds.