04 August 2020

12-day tour ends for Isaías as it heads into Canada

As forecast, Isaías did briefly intensify to a Category 1 hurricane before making landfall near Myrtle Beach SC on Monday night. During the day on Tuesday, it accelerated northward through NC, VA, MD, PA, NY and will cross into Canada tonight as it transitions to an extratropical cyclone. It left the African coast on July 23rd -- twelve days ago -- but has spent only half that time as "Isaías".

Isaías is now the fifth named storm to make landfall on the continental US this season already... there have only been nine named storms in total, and it's only August 4th!  This radar loop from Wilmington NC shows the evolution of the storm as it made landfall... this and many other radar loops of the storm are available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/ -- including a 3-day loop than spans its trek from the Bahamas to Virginia.

As usual, rainfall is the greatest hazard posed by this landfalling tropical cyclone.  Over the past three days, I have measured nearly 5 inches of rain at my house near Miami, but higher totals have been measured in the mid-Atlantic and northeast US states... preliminary estimates are in the 8-10 inch range in parts of MD and PA. Additionally, between Monday and Tuesday, there have been 27 tornado reports from North Carolina up into New Jersey.

Wilmington NC, which was hit by the eastern eyewall of the hurricane as it made landfall, experienced its largest storm surge in recorded history, beating the previous record set just two years ago by Hurricane Florence.

As of Tuesday afternoon (5pm local), Isaias is still a tropical storm with 65 mph peak sustained winds; it's centered near Albany NY and is racing toward the north-northeast at 40 mph. The list of tropical storms that have passed within 150 miles of where Isaias is on Tuesday afternoon is slim, but certainly includes ones you've heard of. The six most recent are Irene (2011), Gloria (1985), Frederic (1979), David (1979), Belle (1976), and Agnes (1972).

Invest 94L, which I've been mentioning for several days now, is located just south of Bermuda but is looking less and less likely to develop. Elsewhere, the Atlantic basin is quiet for the near future.

03 August 2020

Isaías taking aim at mid-Atlantic and northeast US states

As of Monday morning, Isaías has not regained hurricane intensity, but is really close.  The very warm Gulf Stream water is giving it lots of fuel, but strong wind shear is constantly trying to tear it apart -- a battle of conflicting environmental influences. Hurricane warnings are in effect for northern South Carolina and southern North Carolina... tropical storm warnings extend up into Massachusetts, and storm surge watches/warnings span the coastlines of SC and NC.  As I mentioned in Saturday's post, this storm will be impactful along the entire US east coast.

Even if Isaias does briefly strengthen to a hurricane again before landfall, the greatest hazard by far will be rain, followed by storm surge, followed by wind. The following map shows the flash flood risk associated with Isaias' trek along the coast over the next couple of days.

And the map of peak storm surge values again confirms how widespread the effects of Isaias will be.

Landfall is forecast to be near Myrtle Beach SC on Monday evening, and Isaias could be a minimal hurricane by then. There are long radar loops available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Elsewhere, there is still that tropical wave north of the Lesser Antilles (Invest 94L) with a decent chance of development according to NHC.  The support for its development is dwindling in the models though. Should it form and get named, the next name is Josephine. It will remain out over the open ocean, perhaps bring some weather to Bermuda, but even that is a pretty low chance.

02 August 2020

Isaías weakens along Florida coast, hazards continue up US east coast

During the day on Saturday, Isaías got completely overwhelmed by a combination of dry air and wind shear... suffocation and decapitation. While that doesn't immediately "kill" a tropical cyclone, it weakens it in a hurry. It was downgraded to a tropical storm on Saturday evening, and remains a tropical storm with 65 mph peak sustained winds on Sunday morning, though those winds are well offshore. It is not expected to regain hurricane intensity (but briefly could because it's so close to the 74 mph cutoff between a tropical storm and a hurricane).

Since passing over the warm Gulf Stream ocean current, vigorous thunderstorms have re-erupted and persisted over the center, breathing new life into the struggling storm. In the enhanced satellite image at the beginning of the post (from Sunday morning), the mid-level dry air is shown by the oranges to the west of the storm, and the cold cloud tops associated with the strong thunderstorms near the center are the bright blues/greens/reds.

There have been minimal impacts on the east coast of Florida because the thunderstorm activity and stronger winds have been confined to very close to the center or east of the center.  However, the northern Bahamas have received very heavy rain and fairly significant storm surge. A 1-day radar loop from Miami shows the (lack of) structure as it made its closest approach before heading north.  This and other long radar loops can be found at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Tropical Storm Isaías will continue to bring periods of rain and minor coastal flooding to the eastern Florida coast from West Palm Beach northward.  A 2-4-foot storm surge is possible up to Cape Fear, NC as it tracks along the coast over the next couple days. The greatest hazard will be heavy rain and flash flooding in the Carolinas, the mid-Atlantic states, and then the northeast U.S. through Wednesday.

The map below shows the full path that Isaías took, beginning when it left the African coast back on July 23rd. It battled dry air the entire time, but was able to briefly reach Category 1 hurricane status immediately after leaving Hispaniola.

Tropical Depression 10, which was out near Cabo Verde, has already dissipated and never became a tropical storm. The tropical wave near the northern Lesser Antilles, identified as Invest 94L, still has a good chance of forming this week, but model guidance is in agreement on it turning north into the open Atlantic.  The next name on this list is Josephine, and the record earliest formation of the "J" storm is August 22nd.

01 August 2020

Hurricane Isaías approaching Florida, Tropical Depression 10 forms

Hurricane Isaías is still tracking over the Bahamas, and has encountered that dry air and wind shear that I mentioned in yesterday's post. The combination of those two things has unquestionably overwhelmed the boost given by very warm ocean water.  In this long radar loop below, we can see a lack of symmetry, and an erosion of the western and southern portions of the hurricane's eyewall and rainbands as it approaches Andros Island. In short, it's having serious problems, which is great news for Florida.
There are long, updating radar loops available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

As of Saturday morning, Isaías is a Category 1 hurricane with 80 mph peak sustained winds. It is forecast to weaken some more as it heads north, but will still be impactful along the entire US east coast over the next 4-5 days, particularly in the storm surge and coastal flooding department. Hurricane warnings are in effect for the northern Bahamas and much of the eastern Florida peninsula. The center of the storm will track extremely close to the Florida coast, perhaps making landfall, or just missing, very similar to Matthew in 2016. And like Matthew, it seems likely to make landfall in South or North Carolina as a strong tropical storm or Category 1 hurricane.

Zooming in to just the next three days and the tropical storm wind speed probabilities and their arrival times, we see the highest risk along the east coast of the central Florida peninsula, but 30%+ all along the southeast coast.

Places near the center could see 2-4 inches of rain, and 2-4 feet of storm surge along that same part of the east-central peninsula. Fortunately, there is not any reasonable model guidance to suggest that the Carolinas will be dealing with a strong hurricane when it arrives on Monday.  It's also moving fast enough that widespread flooding is not a big concern.

In the far eastern Atlantic, near Cabo Verde, Tropical Depression 10 formed on Friday evening.  It is not expected to last very long, and may never reach tropical storm status. Also, a strong tropical wave centered about 600 miles east of the Leeward Islands is likely to develop this week. As of now, model guidance is in good agreement on it tracking toward the northwest and turning northward before reaching the U.S. 
Either one of these two could be the recipient of the next name on the list, Josephine. The record earliest date for the "J" storm to form is August 22 (Jose in 2005), so there is no doubt at all that 2020 will crush that record too.

31 July 2020

Hurricane Isaías poised to strengthen over the Bahamas

Late Thursday, after the center had spent a few hours over the ocean north of Haiti, Isaías was upgraded to a hurricane, the second of the season.  It is expected to strengthen some more over the warm water around the Bahamas. The numerous small flat islands that comprise the Bahamas do not offer resistance to a tropical cyclone like Puerto Rico and Hispaniola do. Hurricane warnings are in effect for the Bahamas, and a tropical watch for the southeast Florida peninsula. We can expect the watches to crawl northward along the US east coast as the day goes on, and parts of southeast Florida could see their watches get upgraded to warnings.

Confidence is growing that the center of Hurricane Isaías will remain just off the east coast of Florida, but be close enough to bring wind, rain, and storm surge, particularly in the central and northern portions (where a landfall is not out of the question). There is also increasing confidence from consecutive model cycles that South Carolina or North Carolina could be looking at a hurricane landfall on Monday. Very few deterministic model runs or ensemble members keep the center of the hurricane completely offshore now. The following map shows the probability of locations experiencing tropical storm force winds within the next five days as well as the most likely arrival times of those winds.

Prior to reaching the network of U.S. radars, there will be radar coverage of it from Cuba and the Bahamas: http://bmcnoldy.rsmas.miami.edu/tropics/radar/

As of Friday morning, the official forecast shows Isaías reaching Category 2 intensity over the central Bahamas, then starting to weaken. There's actually not even much model support for it reaching the Category 2 mark. While the ocean temperatures are plenty warm to sustain a very strong hurricane, increasing vertical wind shear should limit how much intensification will take place, and there is a band of dry air ahead of it which could start to work its way into the circulation.

In terms of Accumulated Cyclone Energy (ACE), the 2020 season is at about 238% of average for this date, using the past fifty years as the baseline for the average.  The last season to be higher than 2020 by now was 2008 and then 2005 before that.  ACE is a commonly-used metric for tropical cyclone activity, and is calculated by adding up the squares of the intensities of every storm, every six hours -- it's a crude way of measuring the combination of intensity and duration, and is independent of the actual number of named storms.

30 July 2020

Isaías finally forms near Puerto Rico

A week after departing the African coast, the tropical wave we've been patiently watching was finally upgraded to Tropical Storm Isaías on Wednesday night.  This is the season's ninth named storm and the earliest ninth named storm on record -- the old record was set on August 7th, 2005 by Irene. So not only did it break the record by over a week, the *average* date of the ninth named storm formation (over the past fifty years) is September 26th, nearly two months from now! So, 2020 has now bumped 2005 off the record list for the earliest E, F, G, H, and I storms.

As of Thursday morning, Isaías is nearly on the shores of Hispaniola and has 60 mph peak sustained winds. Some strong rainbands have been impacting Puerto Rico resulting in flash flooding and power outages. The wind field is very large for a new tropical storm, but also very lopsided. It will undoubtedly weaken during the day as the center passes over the large mountainous island. Then it is expected to emerge over the southern Bahamas on Friday morning and have the opportunity to start re-organizing.  

Tropical storm warnings cover the Virgin Islands, Puerto Rico, Dominican Republic, Haiti, and the southern 2/3 of the Bahamas... the northern 1/3 of the Bahamas is under a tropical storm watch. A chunk of southeast Florida will likely be added to that watch list shortly. Long radar loops from Puerto Rico and the Bahamas are available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Models are gradually coming together in showing a track that recurves toward the north over the Bahamas and scrapes the east coast of the Florida peninsula. Like Matthew four years ago, just a little wiggle makes a difference in what is experienced on land. But unlike Matthew, this isn't a Category 4 hurricane! Based on the current NHC intensity and track forecast, which are admittedly highly uncertain because of the upcoming interaction with Hispaniola, here are the most likely arrival times and probabilities of tropical storm force winds along the track.

The intensity guidance from the dynamical models has been fairly consistent with showing this becoming a strong tropical storm or Category 1 hurricane, but it's a tough call not knowing what condition it will be in by Friday morning.

The ocean temperature along the forecast track is very warm (>29°C, 0.5-1.5°C above average), including the energy-rich Gulf Stream that follows the shape of the southeast US coastline. However, a limiting factor to keep Isaías in check is vertical wind shear which is expected to ramp up beginning this weekend as a trough approaches from the west.

Since the forecast is more uncertain than usual, we need to pay extra close attention to this storm for any changes and to be prepared if it comes your way.  You can always find the latest at https://www.nhc.noaa.gov/graphics_at4.shtml?start#contents

Turning our eyes eastward, there's a weak tropical wave near 40°W that has a slight chance of developing in the coming days, but should also remain over the ocean... it's something to be aware of anyway. The next name on this year's list is Josephine.

29 July 2020

Tropical disturbance crosses into Caribbean

Potential Tropical Cyclone Nine (formerly Invest 92L) has still not developed into a tropical storm, but crossed over the Leeward Islands on Wednesday morning.  Aircraft and radar data confirmed the lack of a closed low-level circulation -- a key requirement for any tropical cyclone. It's quite close to being classified as a tropical storm though, and that could easily happen later today. If named, it would be Isaías.

The forecast beyond Thursday is extremely fuzzy with this, because 1) models don't generally handle disorganized tropical disturbances too well and 2) it will encounter Hispaniola on Thursday. This large mountainous island could be the end of something that barely got started (remember Erika in 2015?). Looking at the latest batch of ensemble tracks from the global model ensembles, we see none that reach hurricane intensity south of northern Florida. Members that stay further south and cross over more land stay very weak, while members that turn north are able to intensify more. For the south Florida concern: very few end up near south Florida, and among the ones that do, they are weak. If it were to reach south Florida though, it would be Saturday, or as late as Sunday morning.

Moving on to the suite of deterministic model runs, more of these pass south of the Bahamas and eventually travel roughly along the Florida peninsula. But, although not shown here, these are all tropical depressions or tropical storms the entire time.

And to summarize everything, I'll wrap up with the official forecast from NHC. They have the unpleasant task of making a 5-day deterministic forecast for all the world to see. The tropical storm warnings (blue) and watches (yellow) are pretty straightforward, but the track and intensity forecast from Thursday onward is in their words "more uncertain than usual". Keep in mind that the size of the cone of uncertainty is fixed for every forecast of every storm all season long, so it does not reflect actual current uncertainty. Similarly, the intensity markers (D, S, H, M for Depression, tropical Storm, Hurricane, Major hurricane) do not reflect any intensity uncertainty.

28 July 2020

Warnings issued ahead of tropical disturbance

The tropical wave I first mentioned last Thursday has still not developed into a tropical cyclone, but it's getting close. Now centered about 550 miles (1 day) east of the Leeward Islands, this disturbance is forecast to become Tropical Storm Isaías this week. (that's pronounced ee-sah-EE-ahs)

"Potential Tropical Cyclone 9" (formerly "Invest 92L") is moving quickly toward the west; tropical storm warnings have been issued for the Leeward Islands, Puerto Rico, and Dominican Republic.  Note that a "potential tropical cyclone" is the same critter as an "invest"... both are disorganized systems that are not yet tropical depressions. However, the PTC label was developed in 2017 to facilitate the issuance of watches and warnings related to a system that had not yet formed (like this one). Prior to that invention, they had to wait until something formed to be able to warn on it.

Model guidance is pretty scattered with this, so the static-sized cone doesn't adequately capture the present level of track uncertainty, and the "S" doesn't adequately capture the intensity uncertainty. The positions and intensities on Friday, Saturday, and Sunday are big question marks, so don't take them literally. A reconnaissance flight into the area is planned for Tuesday afternoon, and that will reveal some details on the structure and the data that are collected will be assimilated into the evening's suite of model runs.

One big thing working against PTC9 is a huge mass of dry air... a Saharan Air Layer surge.  It has been battling against that since it left the African coast, and it doesn't seem like it will escape it any time soon. This map shows the center of the developing storm with a red I (Invest), a background satellite image, and then the yellow-orange-red shading indicates increasing amounts of dry, dusty air.  Forecast models keep that dry air really close for the next several days.

Furthermore, if it takes a track close to or over the big islands like Puerto Rico and/or Hispaniola, that will also disrupt intensification.  If it tracks a bit north of those islands, it could get stronger. That leads to the track scenarios. The more southern tracks that pass over Puerto Rico, Hispaniola, and even Cuba show a storm that remains very weak, or even dissipates.  A nudge northward includes some land interaction, then over the Bahamas. And further north misses most land interaction but still passes over the Bahamas.

Then there's a lot of spread regarding a track that continues into Florida, one that recurves slightly up into the southeast US, or one that recurves completely before reaching the U.S. Since we don't even have a tropical cyclone yet, it's too early to have confidence in forecasts that far out.  But for timing purposes, a south Florida encounter would be on Saturday-Sunday, while a Carolinas encounter would be Monday-Tuesday.

These track maps show output from the ECMWF and GFS global model ensembles; the tracks are colored by intensity, with Category 1 hurricanes starting at yellow. In addition to these, there are several deterministic runs that are used, but I haven't shown them here yet.

26 July 2020

As Hanna makes landfall, focus shifts to east Atlantic

Hanna made landfall on Saturday afternoon at about 5pm local time on Padre Island, Texas as an upper-end Category 1 hurricane.  It continued to intensify right up until landfall, reaching peak sustained winds of 90 mph. The Corpus Christi area was in the northern eyewall (with the onshore winds) and as a result experienced the worst of the storm surge, which was about 6 feet.

Over the past two days (Friday morning through Sunday morning), preliminary rainfall estimates show a peak of about a foot near Brownsville, with scattered rain gauge data showing 8-10 inches, and a larger swath of at least 3 inches.

These storms that rapidly intensify as they're about to make landfall are a challenge, to say the least. This chart shows the 6-hourly observed intensity in the thick black line (note that these are not necessarily the intensities at the advisories), then each forecast made by the National Hurricane Center is shown in the colored lines. I added the time of landfall as a reference point.  This storm far exceeded model guidance and human forecasts.  Fortunately, it didn't have more time to work with or it certainly would have gotten even stronger.

Hanna is now a weakening tropical storm over northeast Mexico, and continues to dump rain but will lose tropical cyclone characteristics by Monday. There are a couple long radar loops covering Hanna's landfall at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Now on to the tropical wave I've been mentioning for a few days... it left the African coast on Friday and continues to show signs of gradual organization.  NHC is giving it a 60% probability of becoming a tropical cyclone within the next two days, and a 90% probability within the next five days. Models support this high likelihood.  It looks ragged on satellite images so far on Sunday, but regardless, it's on track to reach the Lesser Antilles on Wednesday-Thursday, likely as a tropical storm.

Water temperatures are very warm ahead of it and across the deep tropics, quite a bit warmer than average for this time of year (2-3°F). We think of ocean temperatures warmer than about 26°C as sufficient, 28°C as plenty warm, and 30°C+ as rocket fuel. 


Long-range models are trending toward keeping Invest 92L (likely future Isaias) toward the north end of the Lesser Antilles, then Puerto Rico and Hispaniola, as opposed to remaining in the deep tropical Caribbean. This latest example from the 50-member European global model ensemble illustrates the general behavior: members that remain far south tend to dissipate or remain very weak in the Caribbean, but roughly 1/3 of members pass north of Hispaniola and are able to become stronger. Among those, some turn northward more quickly and other more slowly. The "L" symbol positions on this map are valid on August 3rd. As with any long-range model product, never take it literally. The details are irrelevant, but the patterns and trends are useful.


25 July 2020

Hanna becomes first hurricane of the season as Gonzalo falls apart

As alluded to in yesterday's post, Hanna did indeed reach hurricane intensity, and with just hours remaining until landfall, it's still strengthening. As of the 15 UTC advisory, peak sustained winds are up to 80 mph.  It is moving toward the west at 7 mph, which brings it to landfall in the mid-afternoon timeframe. I have a variety of long radar loops available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Storm surge could reach 3-5 feet north of the center (where the wind is onshore) and parts of south Texas could see rainfall totals up to 18 inches.

Over the past fifty years, the median date of the first hurricane is August 6th, so Hanna is about two weeks ahead of par (the average date is August 15th).  There's virtually no trend over this period in this date, and in the fifty years from 1971-2020, no hurricane has formed prior to the start of hurricane season (I omit Alex in January 2016 because meteorologically, it was a post-season storm of the 2015 season).

It's quite rare to get hurricane landfalls in this stretch of the Texas coast between Brownsville and Corpus Christi... the last three were in 2008 (Cat2 Dolly), 1999 (Cat3 Bret), and 1980 (Cat3 Allen).  Now we can add Hanna (Cat1 in 2020) to the list. Harvey made landfall as a Category 4 hurricane just north of Corpus Christi in 2017.

Further east, and on the other end of the spectrum, Tropical Storm Gonzalo is disintegrating as it reaches the Windward Islands. It is still officially clinging to tropical storm status, but it is hard to spot if you didn't know where to look!  It is forecast to degenerate into an open wave later this weekend off the coast of Venezuela.

Finally, Invest 92L remains a feature of interest in the far eastern Atlantic. It will be slow to get going, but there is solid agreement among models that it will develop and track through the deep tropics. It will reach the Lesser Antilles on Wednesday, and then in the 7-10-day range, the track spread naturally increases dramatically, so it's certainly something to keep an eye on. If named, it would be Isaías (ees-ah-EE-ahs).  This name was introduced in the 2014 season (Ike was retired in 2008), but never used because that season ended with Hanna.