WeatherTiger's Tropical Storm Ian Forecast Analysis for September 24th

Ian gets named; breaking down four scenarios for possible impacts on Florida next week.

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THIS POST IS OUTDATED. Please click below to read our newest (9/26) Hurricane Ian discussion:

WeatherTiger's Hurricane Ian Forecast Analysis for September 26th (PM)

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Here are some facts about Tropical Storm Ian we did not know 24 hours ago.

First, we know it is named Ian. Small steps.

Second, we know that Ian is a major hurricane threat to Florida. And finally, a portrait is beginning to emerge of both the storm’s internal structure and the steering winds that will set its path across the eastern Gulf in the upcoming five days.

What we still don’t know is what region of Florida will see the worst impacts from Ian, and when those impacts will take place. Yet, some general scenarios are taking shape as possible outcomes. Today, I’ll be sketching out what those major scenarios are, and laying some baseline odds for how likely each of them are moving forward.

11am NHC track/cone. Click here for latest map.

As of the NHC 11 a.m. Saturday advisory, the broad center of Ian is located about 300 miles south of Jamaica, with maximum sustained winds of around 45 mph. Ian has nudged a bit further south than expected in the last day, and some further jumps in the location of the surface circulation are possible into Sunday as the storm’s structure continues to slowly improve. This is normal for a developing storm.

Wind shear has decreased over Ian in the last 24 hours, with deep convection now much closer to the developing circulation center instead of displaced well to the west. As Ian moves generally west at 10-15 mph though Sunday afternoon, winds will slowly increase as the storm’s lower and mid-level circulations start to line up with one another.

The low-level and mid-level centers are not stacked now (left), but likely will be by early Monday, as shown in the 06z HAFS-S model.

By late Sunday or early Monday, those circulations are likely to be stacked on top of one another as Ian starts to curl northwestward across the western Caribbean in response to a dip in the jet stream over the eastern United States.

That is when the fireworks may start. A structurally sound Ian moving into a favorable environment for strengthening is expected to result in a period of rapid intensification that may last into Tuesday. The NHC forecast calls for peak Category 3 intensity, but stronger outcomes are certainly on the table if Ian pulls together this weekend.

Over the last two days, the GEFS ensembles have trended slower to move troughing out of the eastern U.S.

There are two primary questions that will determine who gets the worst of Ian. The first is how far west Ian gets as it curves across Western Cuba or the Yucatan Channel before turning north on Tuesday. The second is how strong the eastern U.S. trough will be mid-week, and how quickly it departs the East Coast. As of Saturday afternoon, there are hints that Ian will make it a bit further west, as well as a model trend towards longer-lasting eastern troughing, though more shifts remain possible.

Let’s see how those possible variations in short-term track and long-term steering add up, four quadrant threat matrix-style:

NHC forecast changes since yesterday, with today’s position (X) jumping south of previous forecasts.

Scenario 1: Keys and South Florida (5% chance)

This scenario requires both a stronger trough and a more northward initial track, in which Ian is crossing west-central Cuba on Monday and then races northeast across the Keys and South Florida on Tuesday.  The trend is away from this possibility, but it continues to bear mention, as if it occurred Ian would be bringing impacts to urban centers while likely still at elevated intensity. We’ll see if there is any hint of a swing back in this direction, but for now, the threat is diminishing in Southeast Florida.

06z Euro Ensemble member positions on Wednesday morning.

Scenario 2: Southwest Florida (30% chance)

This case, in which landfall takes place between Naples and the northern Tampa Bay metro area, is possible either if Ian adjusts further east but the troughing is a little weaker, or if Ian glides towards the western tip of Cuba but the steering troughing trends stronger. The most probable timing of a potential landfall in this region would be Wednesday, with support from the 06z Euro Ensembles

Friday’s NHC forecast tracks had a most probable strike in this region; while there have been adjustments north and west since, the uncertainty in Ian’s initial position and the evolution of the steering pattern mean that Southwest Florida is not out of the woods. This is particularly true as while Ian’s intensity may crest on Tuesday, major hurricane strength is still favored to persist into Wednesday. Central Florida and the Space Coast, this general scenario includes a Charley-type track, so watch for trends stronger with the troughing as a sign of danger.

All ensembles average track from the 00z model cycle, nudging west since yesterday and pointing towards a region north and west of Tampa.

Scenario 3: North of Tampa and faster (35% chance)

What if Ian continues sliding further south and west through Monday, but the steering trough remains strong or trends stronger? In that situation, Ian will have more time over the eastern Gulf of Mexico on Tuesday and Wednesday, but still accelerate north and northeast towards a possible landfall on Thursday. The general target area in such a situation would begin north of the Tampa Bay area and extend into the Big Bend or eastern Panhandle, depending on the vagaries of trough evolution and how wide a swing Ian actually makes into the Gulf.

By late Wednesday, the hurricane’s formerly cushy environment may start to degrade, with increasing wind shear out of the southwest pushing drier air into the storm’s core. This would likely induce a weakening trend on Thursday, though a faster track into North Florida or west-central Florida might limit how much weakening occurs prior to landfall. A quicker landfall driven by a stronger trough would likely deal a punishing blow to the area in wind and surge. The Saturday afternoon NHC track forecast has this general scenario as the favored outcome, and for now, it’s also where a plurality of model guidance is coming in.

12z GEFS members have shifted somewhat west. Positions shown on Thursday afternoon.

Scenario 4: North Florida and slower (30% chance)

This outcome is the murkiest of all. If Ian passes southwest of Cuba late Monday and a weaker trough pulls away from the East Coast more quickly, the storm’s forward motion might slow down midweek as it turns north over the eastern Gulf. This would potentially postpone landfall until Friday (or beyond), as shown on the 12z GFS and some of the 12z GFS Ensemble members, and most likely target somewhere in the Panhandle and east-central Gulf Coast. This category also includes the outside possibility of extended meandering in the central Gulf.

Shear increases dramatically near Ian by Wednesday.

.If there’s any good news to be gleaned this outcome, it’s that dry air and shear should be majorly impinging on an Ian still over the Gulf on early Friday. While this unfavorable environment would probably erode storm structure significantly, impacts, especially in terms of surge and wave action, could still be severe in North Florida with a delayed landfall. And even lower-end winds can have significant impacts in the Panhandle, particularly inland.

12z GFS shows lots of dry air wrapping into Ian’s core by late Thursday.

Overall, the shift south on Saturday morning tilts the odds more towards a westward track, but we are still in the timeframe in which more forecast surprises could happen. While forecasters know more than we did 24 hours ago, we don’t know enough yet to confidently project who will get the worst of Ian and how bad it will be. In the meantime, keep preparing, Florida, and keep watching the skies.

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