17 May 2020

Arthur forms off Florida coast and pushes the season into action

For the sixth consecutive year, a named storm has formed prior to the official start of hurricane season on June 1. The topic of my previous post on Tuesday has been remarkably well-forecast by models and did indeed organize into a tropical storm north of the northern Bahamas on Saturday (see Odds increasing of a pre-season storm... AGAIN).

It was first upgraded to Tropical Depression 1 on Saturday afternoon, then to Tropical Storm Arthur on Saturday night.  Maximum sustained winds are at 40 mph and it is not expected to strengthen much more, but will, as mentioned on Tuesday, bring higher surf to the southeast US coast... and a shot at tropical storm force winds along the North Carolina coast as it heads out to sea.  As such, tropical storm warnings are in effect for the portion of the coast highlighted in blue below. Impacts should be fairly limited to extreme eastern North Carolina beginning Sunday afternoon and extending through Monday morning. The latest forecast can always be found at https://www.nhc.noaa.gov/

As mentioned at the beginning, there has been at least one pre-season named storm every year since 2015, which is bringing the trend over the past 50 years earlier and earlier. While the date of first hurricane formation is still easily contained by the artificial bounds of hurricane season, tropical and subtropical storms are forming earlier.
The date of first named storm formation over the past fifty years (1971-2020) shown by the blue line and dots. June 1 is marked by the aqua line, the median date is marked by the pink line, and the linear trend through the data is shown by the dashed gray line. 
The next names on this year's list are Bertha, Cristobal, and Dolly. Perhaps they will wait until after June 1st!

12 May 2020

Odds increasing of a pre-season storm... AGAIN

A tropical disturbance is expected to form near Cuba or the southern Bahamas in the coming days... then drift across the northern Bahamas and potentially develop into a tropical or subtropical cyclone this weekend. If it earns a name, it would be Arthur, and 2020 would become the 6th consecutive year with a pre-season named storm.
As you can see in the chart above, the date of the first named storm in the Atlantic has decidedly been trending earlier over the past fifty years. This increases the odds of having pre-season (before June 1) activity. Since 1965, the bounds of the official Atlantic season are June 1 through November 30, but they were nudged around prior to that. There has been renewed debate regarding another nudge to the dates -- that's a topic for another day. Hurricane season was never intended to contain ALL activity, just the large majority.

Now on to the feature of interest, which doesn't exist yet. There has been agreement among global models of some sort of subtropical or tropical cyclone development over the Bahamas, but environmental conditions are marginal and the window of opportunity is brief. These forecasts from the most recent cycles of three global models are valid on Saturday evening, and are remarkably consistent with a weak but closed low pressure system over/near the northern Bahamas. Regardless of if it forms, it will track toward the northeast, away from land.

Forecasts of the low-level wind and surface pressure from the European (top), US (center), and Canadian (bottom) global models, all valid on Saturday evening. (tropicaltidbits.com)

The primary impact from this in the U.S. will be elevated chances of heavy rain in the southern Florida peninsula on Thursday-Friday. There could also be higher surf all along the southeast US coast through Monday.

Rainfall accumulation forecast through Sunday morning. (NOAA Weather Prediction Center)

So, would it be subtropical or tropical if it forms?  The difference is technical and purely academic, but since they are terms the public hears when storms are named and classified, what can it hurt to put this out there? The difference between the two types of cyclones is their structure: horizontal and vertical as well as thermodynamic and dynamic. Is the cyclone vertically aligned, are the strongest winds at the surface, are the strongest winds near the center, how deep is the cyclone, are there any frontal boundaries, etc, etc?  So this next figure is technical, but the summary (from the U.S.'s global model) is that it is expected to be marginally tropical in nature, but hovering right around no man's land. Cyclone classification definitely has grey zones, since all of the defining parameters are on a continuous spectrum. 

Cyclone phase space diagrams for the developing low pressure from the GFS model. Together, the two panels describe the key horizontal and vertical structure, both dynamically and thermodynamically. (Florida State University)

08 May 2020

"Cone of Uncertainty" Update and Refresher

Anyone who lives on a hurricane-prone coast or even watches television is familiar with the infamous "cone of uncertainty" produced by the National Hurricane Center.  It begins as a point at the current position of a tropical cyclone and expands to show the potential position in five days. It is called the "cone of uncertainty" because the further out in time you go, the more uncertain the forecast becomes... and it tends to look like a cone!

A "cone of uncertainty" for Hurricane Irma (left) and Hurricane Harvey (right). Both cones are from 2017 and are therefore identical to each other in their construction. 
The size of the cone is fixed for every storm during the entire hurricane season, but the size slowly evolves from year to year. If the storm is moving quickly, the cone will appear more elongated and if the storm is moving slowly, the cone will appear more compact... but it's the exact same cone.  The examples shown above are from Irma (left) and Harvey (right); both storms were in 2017, so both cones are identical in their construction.

The cone is updated each year prior to the start of hurricane season, and it almost always shrinks each year too.  Hurricane track forecasts are gradually improving, meaning that in general, there is less uncertainty where a storm will track now than there was a decade ago.  In fact, a two-day forecast now is as accurate as a one-day forecast was a decade ago, and a five-day forecast now is more accurate than a three-day forecast was two decades ago!  The map below shows a sample satellite image with the new 2020 cone overlaid on the 2015, 2010, and 2005 cones for comparison.  Improvements are getting increasingly challenging to achieve because there can never be a perfect forecast of a chaotic system like the atmosphere.

So just how is the size updated each year?  The National Hurricane Center uses its own track forecast errors over the previous five years to calculate a circle at each "lead time" (1 day, 2 days, ... 5 days).  The size of that circle is designed to enclose the position of the storm's center with 2/3 probability, meaning that there's historically a 1/3 chance the storm will track outside the circle at that time.  Lines connecting the various circles complete the shape of the cone. [Note that the 2020 cone size is thus determined from all track errors during the 2015-2019 seasons.]

Since the cone is so widely used yet sometimes misunderstood, here are some key refreshers:
  • The cone does not tell you anything about where impacts will be experienced.  Even for a perfect down-the-middle track forecast, impacts such as strong wind, heavy rain, storm surge, and tornadoes will extend beyond the cone.
  • The cone does not tell you anything about the size of the storm.  Regardless of how strong they are, hurricanes come in a wide range of sizes.  Recently, NHC has added the observed size of the wind field to their cone graphics to help illustrate this (see the Irma and Harvey examples above... the orange and red shading indicates the extent of tropical storm and hurricane force winds at the time the forecast was issued).
  • The cone does not tell you anything about the actual uncertainty associated with the forecast. Since the size of the cone is fixed, it cannot become more narrow or broad to accommodate a more or less predictable environment.
  • Nothing magically happens at the edge of the cone. If a hurricane is approaching and you are scrutinizing each new forecast to see if you are inside the cone or not, you are missing the point of it.  It is arbitrarily chosen to be the 67% historical probability threshold... a 75% probability cone would be larger, and a 50% probability cone would be smaller.
  • If you use the cone graphics from NHC, there is some information about intensity provided. At each forecast point, there is a letter written inside the black dot corresponding to a general intensity range: D (tropical depression), S (tropical storm), H (hurricane), and M (major hurricane (Category 3+)).  But keep in mind that there is uncertainty associated with the intensity forecasts too!
To think about a cone of uncertainty for intensity, consider this: averaged over the past five years (2015-2019), the mean error in a 1-day forecast is +/- 9 mph, the error in a 3-day forecast is +/- 15 mph, and the error in a 5-day forecast is +/- 20 mph.  But there is also a wide range of values that go into those averages, meaning that there is a small probability of a very large error and a small probability of near-zero error.

Other important terms:

Storm Surge Watch: the possibility of life-threatening inundation from rising water moving inland from the shoreline generally within 48 hours.

Storm Surge Warning: the danger of life-threatening inundation from rising water moving inland from the shoreline generally within 36 hours.

Hurricane Watch: sustained winds of 74 mph (64 knots or 119 km/hr) or higher are possible. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.

Hurricane Warning: sustained winds of 74 mph (64 knots or 119 km/hr) or higher are expected. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.

Tropical Storm Watch: sustained winds of 39-73 mph (34-63 knots or 63-118 km/hr) are possible within the specified area within 48 hours.

Tropical Storm Warning: sustained winds of 39-73 mph (34-63 knots or 63-118 km/hr) are expected somewhere within the specified area within 36 hours.

Strong winds and thunderstorms arrive well before the center of the storm (sometimes a couple days), so when the time comes, be sure to plan and finalize your preparations prior to the expected arrival of tropical storm force winds, not the expected arrival of the center.

And if you missed it earlier, I have some general hurricane information and preparedness tips at http://bmcnoldy.blogspot.com/2019/04/2019-hurricane-season-intro-local.html