#Isaias forecast track edging closer to Florida

As noted in the previous posts today, the forecast track for Isaias is balanced between the high pressure to the east, and an approaching trough/boundary to the west. As the afternoon progressed, in the great scheme of things that balance didn’t shift much – but it’s enough to cause angst and a threat to the Florida coast. With the new 5pm Official NHC Forecast (Key Messages regarding Hurricane Isaias (en Español: Mensajes Claves)), the current forecast is for Isaias to make a sort of landfall on the Treasure Coast of Florida near West Palm before scooting up the coast offshore from the GA/SC coast then North Carolina, brushing DelMarVa, NJ, NY/Long Island, and Cape Code before smacking Nova Scotia as a strong tropical storm. Here’s the obligatory map of doom (click for more detailed terror):

Despite the track changes, NHC also decreased the forecast intensity a bit. So the net effect isn’t actually that different from the 11am forecast in practical terms. This is a minor change, not a freak out ZOMG the forecast is blown! The Bahamas should still prepare for for Category Two conditions, the Florida coast south of Cape Canaveral for a Category one hurricane. the First Coast and Georgia/SC Low Country coast for tropical storm winds and an extra foot or two above normal high tides. The storm may slow down in the turn just offshore from Saint Augustine/Daytona, which will be unpleasant. You should also pay attention to your local emergency managers; most of the time they are advising you to do the safest thing. COVID19 is making Emergency Managers do what they should have been doing all along, be more selective and careful in their evacuation orders. The main thing to keep in mind is to shelter from wind (with an important caveat), evacuate from water. The caveat is the strength of your shelter; mobile homes can be deadly in even minimal hurricane force winds and the weaker tornado-like effects they spawn (much less actual tornadoes). If you are in a place with a flood risk from storm surge, move to higher ground. If in a mobile home or structure you have doubts about, then get out. But otherwise, with COVID, evacuating for convenience so you can avoid a few days of being without power isn’t an option.

I’ll again remind folks in the US not to overlook The Bahamas – unfortunately they are already experiencing the storm and are still recovering from Dorian. Please keep them in your thoughts and be prepared to help out in the aftermath. And don’t forget our fellow citizens in Puerto Rico – power is out over large areas of the Island, and there is considerable flood damage.

As the storm turns … a closer look at #isaias tracking

NHC hasn’t changed their forecast much from the 5am to 11am advisories, although there are now hurricane watches for the Florida coast coming with a slight track shift to the west (closer to the coast). You can see the latest key messages here: Key Messages regarding Hurricane Isaias (en Español: Mensajes Claves), and the forecast discussion at this link. In short, they are still trying to split the difference between conflicting model guidance. If this seems like a bad re-run from several storms from the last few years, you’re right. Let’s take a closer look at why.

Here is the current (11am Friday 31 July) satellite view. Infrared (cloud temperatures) are on the left, and visual (what you would see with your eyes from space) on the right. Notice the dark red and black in the IR image – those are very cold cloud tops, indicating intense thunderstorms. This is a sign of consolidation and strengthening. With care you can make out another feature that is responsible for the storm starting to turn to the NW and, it is thought, curve to the N then Northeast parallel to the US coast. If you look above Isaias, you can see a band of clouds stretching across the image from upper right to mid left. This is a frontal boundary, and represents the left hand edge of the Bermuda High. Using the GFS model, let’s look at the situation Sunday Afternoon at 2pm:

This graphic shows the winds driving the surface winds (850mb pressure level, or 5000 ft) as the colored image while the green arrows show the main steering winds at 18,000 ft (500mb). Notice the 500mb winds are pushing the storm to the Northeast and across Cape Hatteras. So in simple terms, the track is a balance between the storm’s natural motion to the NorthWest (towards the coast), and the “wind pressure” pushing it to the NorthEast and away from the US Mainland. This “push” will also cause shear, which will inhibit the storm getting much stronger than maybe a minimal category two – and may even knock it back to tropical storm strength by then.

Something to keep in mind is how much forecasts have improved over the last 20 years. In the past a storm like this would have already triggered evacuations in across Florida and in to Georgia. Today we have a much better (if not anywhere near perfect!) ability to narrow down the options.

So the advice from 5am remains: Florida south of the Cape should prepare for strong tropical storm force winds and squally wind on the coast with tides running a couple feet above normal. The First Coast (Daytona/St. Augustine/Jacksonville) and Georgia/SC Lowcountry would be wise to prepare for the same, but more than likely it won’t be that bad. The upper SC and North Carolina coasts should prepare for a category one hurricane, although again it’s likely the worst will be offshore. After that, the storm may brush Cape Cod and Nova Scotia, but that’s still a few days out.

Not to overlook The Bahamas in all this – unfortunately they are already experiencing the storm and are being hit no matter what. Please keep them in your thoughts and be prepared to help out in the aftermath. And don’t forget our fellow citizens in Puerto Rico – power is out over large areas of the Island, and there is considerable flood damage.

Hurricane Isaias (AL092020) Fri 31 July Update

The US National Hurricane Center (NHC) has again somewhat changed the forecast overnight, especially intensity, and has upgraded Isaias to a hurricane. TLDR is things will be worse in the Bahamas, but for the Southeast US (GA/FL/SC) things aren’t that different from yesterday’s estimate; for North Carolina and the Northeast, there is a bit more risk, and Nova Scotia is still in the forecast. Here’s the details …

The latest forecast takes Isaias through the Bahamas peaking at category 2, then skims the US Southeast before side swiping Cape Hatteras and Cape Cod. Here are the estimated impacts in plain English using my TAOS/TC model, based on the official NHC forecast, click to embiggen:

Isaias is now approaching the Turks and Caicos and southern Bahamas as a minimal hurricane. Structurally, it continues to slowly consolidate, and starting to develop features of a more traditional tropical cyclone like a central cluster of convection and banding features. The official forecast is a bit of a compromise between conflicting model signals. These are in part due to the evolving structure, and different position estimates. Here is a map zoomed in on the position fixes over the last day, along with the track. AIRC are airplane fixes, the rest are satellite based fixes …

The way the forecast process works is that every six hours, the storm position and characteristics are assessed. That is then put into three broad classes of computer models, global, hurricane-specific dynamic, and hurricane-specific statistical/dynamic. The model tracks available after an hour or so (which includes models from the previous cycle) are all plotted and assessed, and the forecaster “cooks the spagetti” to create the forecast. Here is what the tracks for Isaias look like right now:

When looking at these models, several things pop out: with models from the previous cycles, we see how well they have performed. For example, right now the average 48 hour error for AL09 this morning for the Canadian Meteorological Center model is 126.6 nautical miles, the ECMF 89.2 nmi, the US GFS model nearly 200 miles, and so forth. The forecaster also looks at how well each model is representing the structure of the storm – that is a clue as to how well it might do in the future. All of that information is then blended into the official forecast. The present NHC forecast is balancing two scenarios depending on the strength of the Bermuda High – a big mound of air sitting over Bermuda around which the hurricane is being steered. Even small changes can shift the track 100 miles left or right – meaning closer to the US coast, or safely away.

As use can start to see, that takes a lot of experience and judgement. Each one of those lines tells a piece of the story – but just plucking one or two lines out and creating a narrative with it is irresponsible. This is why for the official word please take a look at the NHC’s great “Key Messages” products: Key Messages regarding Hurricane Isaias (en Español: Mensajes Claves). You should also pay attention to your local emergency managers; most of the time they are advising you to do the safest thing. That said, as I have expressed before, in the past many EMA’s have leaned towards over-evacuation, especially for weaker storms. COVID19 is making Emergency Managers do what they should have been doing all along, be more selective and careful in their evacuation orders. The main thing to keep in mind is to shelter from wind (with an important caveat), evacuate from water. The caveat is the strength of your shelter; mobile homes can be deadly in even minmal hurricane force winds and the weaker tornado-like effects they spawn (much less actual tornadoes). With COVID, evacuating for convenience (so you can avoid a few days of being without power) isn’t an option.

The Central Bahamas should be preparing for a Category 2 hurricane Saturday – you don’t have long, so get wrapped up now! For GA/SC (except perhaps north of Myrtle Beach), impacts are likely to be minimal. In fact, for the Georgia coast, on the current track we may not even get any rain if we are in the subsidence zone at the fringe of the storm! My only concern is for some shallow coastal flooding in the usual places if there are sustained onshore winds in the event the storm moves slower than the official forecast (which the ECMF model is predicting). For the coast of Florida around West Palm by Sunday, and for Myrtle Beach to the Virginia Border late Monday into Tuesday, strong winds and rain bands are possibly added to that hazard. The beaches will be hazardous for the next week with rip currents so keep that in mind …

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Isaias evening update (Thu 30 July)

It seems that the center of Isaias is reforming just off the north shore of the Dominican Republic. What that means in short is that it seems to have survived more intact than had it gone a bit further west, and the likely track has shifted a bit. NHC has also changed their forecast philosophy a little (Key Messages regarding Tropical Storm Isaias (en Español: Mensajes Claves)) and now forecast the storm to become a minimal hurricane – but not much more than that since conditions aren’t terribly favorable. So what does this mean? Not all that much. It makes the scenario where the storm come up mid-Florida much less likely, so the worst of the storm will stay offshore from them the Bahamas will take the brunt, but Isaias is fast moving and this is no Dorian. Nevertheless, The Bahamas should prepare for hurricane conditions for late tomorrow through Sunday. The coast of Florida is now under a tropical storm watch, and the Fort Lauderdale to Cape Canaveral area may see some stormy weather if the storm slows and turns in 48-72 hours just offshore as forecast. For GA/SC, maybe breezy, some gusty rain right on the coast, depending on the timing of the onshore winds and tides low lying areas that get water with above normal tides might see a little flooding, but this track is far enough offshore to not be much risk – IF the storm goes as planned. Consider this an opportunity to think through your hurricane plans for a real storm threat. Hatteras is more likely to get side-swiped, but they are used to this sort of thing. The Canadian Maritines might get a strong blow in aboot six days.

A brief COVID19 update

While we are watching AL09/Isaias and the invest area behind it AL93 (not likely to bother anybody any time soon) so as to be getting our freak-out on for hurricane season, there is also a lot of COVID news and data, although most of it can be summed up as nothing much has changed, wear your mask in public and exercise good hand hygiene. The science has moved forward in several areas; there is a potentially important new paper analyzing what happened on the Diamond Princess cruse ship entitled Mechanistic Transmission Modeling of COVID-19 on the Diamond Princess Cruise Ship Demonstrates the Importance of Aerosol Transmission. Although it is still in peer review, it’s an interesting bit of work. The bottom line is it emphasizes the importance of droplets and aerosols in transmission (don’t be confused, aerosols are just fine droplets, which cloth/procedure masks will stop; we’re still not talking about actual airborne transmission which would take N95 or better). Although the uncertainty bands are really wide and make me a bit queasy, the conclusion using their median values that 80% of transmission is via the direct respiratory route, and only 20% via fomites (surfaces) meshes with other research.

The data remains fuzzy, but some interesting stuff is settling out. In Georgia, the positive test ratio seems to be leveling out at around 12%. Of those testing positive, about 10% end up in the hospital, and about 2% end up deceased. If we revisit the forecast made at the end of May, it shows that as the infections have likely moved into the younger populations the fatality rate has indeed dropped – but not nearly as much as the optimists wanted to believe. Another factor is that the long-term complication rate for younger people who catch this thing seems high – there aren’t statistics yet, but things like lung scarring, cardio problems, and even neurological issues seem to be cropping up. So catching COVID19 may be far from fatal for a younger healthy person, but the long term consequences may be significantly more than the “just the flu” crowd is thinking. And that’s the biggest problem with this virus: it’s not obviously bad enough to cause everybody to get serious about it, but it is bad enough to cause serious consequences if we don’t …

Opinion: I just don’t get the opposition to masks other than a stubborn “I’m not going to if those guys like it”. If you really want things to get back to normal, masks and some common sense will clearly cause the transmission rates to plummet within 3-4 weeks. There are very few economic activities that won’t benefit from that, and ones like restaurants/bars can adapt better.

Isaias 11am Thu Update

Not a lot of changes … here’s the 11am satellite view, Infrared on the left, visual on the right:

The nearby land/mountains of Hispaniola (DR/Haiti) and Puerto Rico are interfering with the storm, and will for the next day as it moves north. Two key questions are how much that will disrupt the storm, and where it will reform after it passes. That will control the future path and how close it gets to the US and Canadian Maritime Provinces, and how bad, especially in the Bahamas. THis should happen overnight, so by in the morning (Friday) we should have a better picture. Again keep in mind that the forecasts keep the storm below hurricane force (and the models like HWRF that do spin it up a bit more don’t turn it into a major hurricane), so the guidance from this morning still holds: weaker structures, right on the coast in low lying areas are the things to worry about. Everybody else should be ok aside from power outages and debris …

Here’s the morning update for more info.

Isaias (AL092020) Morning Update (30 July 2020)

Overnight some important developments with Potential TC 9: for starters, NHC has decided it now meets the criteria as a tropical storm (warm core, closed circulation, winds over 35 knots) so the system is now Tropical Storm Isaias. Now that the system is consolidating, the models can finally get a better grip on the track, and it is more likely to swing up the US East Coast. As a reminder, your best summary of official information are the “Key Messages” products from NHC. Here’s the links: Key Messages regarding Tropical Storm Isaias (en Español: Mensajes Claves). The biggest immediate threats are to Hispaniola/Puerto Rico in the form of heavy rains. For the US East coast, it’s a bit more complicated, but not freak-out worthy. Here’s the details …

As I write this just before 6am the sun isn’t up enough for a good visual band image, so here is a 3.9um Infrared image, with surface pressures in millibars from the GFS model in green. Remember all of the images here can be clicked to see at full resolution …

Compared to yesterday Isaias is much more consolidated, and the models are able to produce a more reliable track. The “steering currents” – the winds from the surface up through 20,000 feet or so – are pretty strong, so the main track models (in color in this map) are fairly consistent. Don’t be distracted by the cloud of faint gray lines, those are ensemble members (I’ll do a post explaining that in more detail on Patreon). While it is possible Isaias will brush the US coast, the more likely scenario is for it to stay just offshore and brush by Cape Hatteras. The reason is clear from the above image: that big high pressure system (marked by the H1022 above the storm) is providing the main steering. Winds around a high circulate clockwise in the Northern Hemisphere, so the storm is being force around the left side of the high. Here’s the track models:

Impacts: Puerto Rico is experiencing heavy rain and wind gusts up to 50mph. Economic impacts there are likely to approach $100 Million or more, worse because of the still ongoing recovery from Maria and the recent spate of earthquakes. As for the US East Coast, here are the forecast impacts:

Notice that the worst of the storm stays offshore, and that while it gets close to hurricane status, Isaias isn’t forecast to become a hurricane. On this track and forecast, Isaias could cause between one and two billion dollars in impacts to the US, likely at the lower end of that. I expect damage totals to be under-reported because insurance will likely cover very little of the impacts. On the “plus” side, economic activities like tourism and service industries are already depressed, so impacts there are far smaller than a weekend storm would normally cause, but in a way that’s not a good thing because those struggling to stay open will be hurt.

Bottom line for Georgia, Florida, and South Carolina, the key take-aways are that while it may get windy, and some rain bands on the coast, those should be no worse than sustained summer thunderstorm type conditions (IOW, a day-long thunderstorm, or hot and sticky nor’easter). A random tornado can’t be ruled out, but again that’s not terribly different in risk from a springtime convective system. If the storm wobbles closer to shore, it will be weaker (due to land interactions). If it is farther offshore, the effects on the left hand (weak) side of the shore rapidly diminish. If you live right on the coast, in areas that flood with unusually high tides, you should probably take precautions. As usual, following the recommendations of your local emergency managers is probably the best bet. Given the COVID-19 situation (especially in Florida), evacuations should be very limited: only those in immediate risk of flooding, and unsecured mobile homes.

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Latest on AL92 (Potential TC Nine)

Potential TC 9 continues to continue. It is dumping a lot of rain and causing flash floods across parts of the Caribbean (including Puerto Rico) with some gusty winds. But it’s still technically not a tropical storm. The forecast situation is still murky. If it stays a broad system, it will likely just continue on across Hispaniola (Haiti/Dominican Republic) and eastern Cuba, at which point the models are fairly consistent taking it up across Florida. This is the scenario NHC likes … ironically if it gets more organized, then the high mountains of Hispaniola will tear up the circulation more than if it stays as is – so if it gets stronger in the next 12 hours, it may actually be weaker over Florida (especially inland). For folks in Georgia, this is almost certainly going to be a wind/rain event, perhaps some limited coastal flooding in the usual spots, unless something really weird happens. Don’t worry about the cone of shame.

As usual click to embiggen. This may look bad, but notice the impacts are mostly minor – some power out, trees down, branches breaking. Messy and somewhat expensive (since more than likely almost none of it would be covered by insurance) but not dangerous.

It is likely that overnight the system will become organized and tropical enough for NHC to finally declare it Tropical Storm Isais. But don’t be surprised if you wake up in the morning and it’s still PTC9 …

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PTC#9 Mid-day “nothing has changed” report

Not too much changed with PTC#9 at the 11am advisory … will do a full update on the blog later today. Here’s the latest TAFB analysis … two things to note if you embiggen is the sort of oblong shape to the system, and the big high pressure system to the north.  That high pressure ridge (think of the “H” as the top of a hill) is causing the storm to move around the left hand side – that is what is guiding the storm right now.  So at the moment, the thinking is still that the system will stay weak, and should track south of Puerto Rico (but that may not help much if the winds and rain cross the Island).  After that, hard to say at this point … stay tuned!

 

AL092020 – Still “Potential” (Wed 29 July AM)

“Rules! We’ve got some rules around here”  And so it is with what is and isn’t a Tropical Cyclone (the technical name for depressions, tropical storms, and hurricanes).  The system NHC is tracking as AL09, “Potential Tropical Cyclone Nine,” doesn’t meet the formal criteria to be a tropical storm, even though it is producing tropical storm force winds in places.  As of this morning though there are thunderstorms (convection) flaring up near the center, indicating better organization.  It will likely be declared Tropical Storm Isaias later today.  Here is what it looks like at sunrise … left is infrared, right is visual (still a bit dark).  Click to embiggen:

So what does that mean in practical terms?  It means the wind field is broader than a typical tropical storm – but not as strong in the core.  It is slower to organize even with somewhat favorable conditions. It also makes forecasting much harder because we don’t have a good center fix and motion.  Depending on where the center finally coalesces,  On the current track across the Greater Antillies (Puerto Rico/Hispaniola/Cuba), the high mountains should inhibit much strengthening.  The official forecast keeps AL09 as at best a middling tropical storm.  But that can still cause a lot of misery, especially in Puerto Rico, as they have are still struggling to recover from Maria or the recent earthquakes.  Beyond that, it is likely the storm will curve north into Florida.  But it wouldn’t surprise me if it doesn’t survive the trip over Hispaniola and Cuba intact, or enters the Gulf of Mexico and strengthens.  Or something else (yeah, that’s a lot of help).  That’s how it goes with disorganized storms …

On this track it would cause upwards of $100 Million in damage across the Caribbean, and maybe $700 million in impacts in Florida.  A big question is if it triggers evacuations and shelters – that has major implications for the COVID19 pandemic we will look at later today …

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