September 07, 2017

American Versus European Models and Irma's Big Turn

Hurricane Irma, which passed north of Puerto Rico last night, is a remarkable storm, with estimated sustained winds of 180 mph.  A category 5 hurricane and one of the strongest on record.  The radar image last night from the NWS radar on Puerto Rico showed a double eye structure, which is not uncommon during eyewall replacement cycles.

But what is equally remarkable is the predicted track of the storm, in which most model forecasts suggest will include a sharp right turn as Irma reaches Florida.   Such a turn has many implications for forecast uncertainty and the impacts of this hurricane on the U.S. southeast.

Let's look at the forecast tracks of Irma, first using the vaunted European Center ensemble in which many forecasts are made, each slightly different in terms of initial state and model physics.   Virtually all forecasts direct Irma WNW initially before turning right near Florida.

  But the tracks tends to spread out in time, with some just offshore, some over Florida, and some even over the Gulf.  The impacts, both in terms of wind, storm surge and precipitation would be very different depending on exactly which track the storm actually follows.

The U.S. GFS model, with less members in its ensemble and coarser resolution, is generally similar, but with more members offshore.

The general agreement between the U.S. and European models is considerable, and other national modeling systems are doing similar things.  So I think we have great confidence in the turn.

But why the sudden turn?

Because of the passage of an upper-level trough to the north, with Irma starting to feel the steering effects of the westerly flow of the midlatitudes.  The forecast 500 hPa chart (around 18,000 ft) with heights and winds illustrate the predicted environment as the hurricane approaches Florida.

The trouble with such tropical-midlatitude interactions as hurricanes move north is that such situations can produce a lot of uncertainty, something suggested by the ensembles.   It is sort of like jumping on to a rapidly moving merry-go-round.  You know generally where you are going once you get on, but you are not sure which horse you will end up on.  A very challenging forecast.   But with a storm this large and powerful, anywhere in southern Florida will have serious impacts.

Last night's high resolution European Model run was scary, with the storm making landfall near Miami and very strong onshore flow to its north, which would produce a large storm surge, with water being pushed up on the coast north of the low center (see below).  Storm surge prediction is something the National Weather Service is lagging behind in and no such guidance is available yet for Irma, based on the ensembles shown above.  When Irma is closer to Florida, surge forecasts will become available.

Accumulated precipitation from this European Center run?

5-15 inches.  Heavy but not like Harvey.  Why?  Because the storm is not predicted to stall.


  1. Cliff, huge amounts of precipitation headed for British Columbia this weekend. Can you please comment on the source?

  2. I was born in faulkton SD, the country town had a merry go round open to the public, they called it the carnival city, ;).
    Except I was raised in a hutterite colony ten miles from town, ma and pa never let me ride as that was too "worldy" and unbecoming to this clan, at 16 I had enough of the cult and left for good. Now any merry go round picture sets me off. :))

  3. Speaking of hype, meteorologists generally measure the strength of hurrucanes by their central pressure. Irma's is 927 millibars, which doesn't even put it within striking distance of the top 10.

  4. Place slipper - Low pressure won't kill you. Destructive force will. As an example avalanche's are rated by destructive potential, not mass density.

    That is a good proper risk rating, as we can already see in itscwake of destruction

  5. As usual Placeholder your statement is based on wrong information. Irma had a pressure of 914 at midnight and before weakening slightly to 175 mph winds this morning it had sustained 185 winds for 37 hours, which is the longest stretch of any storm since we have been able to reliably measure wind speed with satellites. The next longest is typhoon Haiyan at 24 hours. By many measures this is one of the strongest hurricanes we have seen, especially in the Atlantic.

  6. craiger77, satellite measurement of a hurricane's eyewall winds and central pressure are very much in the beta testing mode. Satellites do three things well:

    1. They give us a picture of the storm at a distance so that we can see largescale organization of the cyclone.
    2. They tell us the temperature of the cloud tops which strongly correlates with altitude, and therefore the intensity of the thunderstorms.
    3. They give us a direction and speed of travel.

    The reason that NOAA operates the not-inexpensive Hurricane Hunter fleet is to establish central pressure and maximum winds.

  7. Yes, it strengthened to 914mb, which puts it at the tail end of the top 10. And speaking of the top 10, there aren't any easily comparable measurements much before WW2. No need to worry, though: The global warming cult will invent numbers when they think they need to.

  8. On a lighter note:


    Curious about your thoughts on the last two paragraphs in this piece. It presents a rationale for the climate links being discussed in relation to current events.

  10. On another note, there's precious drops of aqua coming out of the sky in Bellingham. The shrubs are thirsty, The ground is parched
    Let us rejoice for the fall rains have begun

  11. Don, a better link to that cartoon is:
    This allows you to mouseover the cartoon and get the additional punch line.

  12. what do you think about these unusually strong solar flares happening now?

  13. Don, you interrupted PS. He was about to tell us his memories pre WW2.

  14. Meanwhile here in the Seattle area, fantastic weather! Clean air, cloudy, a little mist, 70F. Perfect!

  15. Remember when the Worldwide Church of Global Warming regularly issued pious warnings about the difference bewteen climate and weather?

    Not anymore. Now, every storm is proof of your religion.

    Folks, you had no principles or integrity then, and you don't now. Facts didn't matter then, and these days you just make up the numbers. And you actually wonder why you are greeted with overwhelming disbelief outside of the fever swamps of Seattle?

  16. What is abundantly clear is how important modeling is. The downside (of not having spent $$ on the latest computers and modeling ... and... of course the jury is still out... but it seems the European model seems closer to the expected path) is if Irma continues to move further West, the NEXT time we have a similar hurricane, less will obey the mandatory evacuation orders, especially if the gas shortage causes deaths on the roads.
    Aslo, to Cliff's repeated admonitions to those who link climate change to Irma ( as the mayor of Miami is doing now), we ( the "scientifically" minded) need to do a better job informing about climate change... what is and what is not.

    1. Exactly. Those that link every storm to climate change are misinformed and only hurt the climate change advocates credibility. Many times its political with left leaning politicans talking about sonething they know nothing about often with the sensationalist media giving them a platform in the form of front page news. The scientific community also has to do a better job on distinguishing which catostrophic storm/ climate events are caused by normal natural variability and non human based climate change and which ones are specifically caused by anthropogenic climate change. We all know the Earths climate always changes and often to extremes from the tropical planet of the Paleocene-Eocene Thermal Maximum to the many ice ages that almost covered the entire planet with ice.The answer that I think is still rather unclear is that what type of climate and weather events are specifically a result of anthropogenic climate change. You cannot just say that every strong hurricane or changing climate event (for example the blob) is the fault of anthropogenic climate change without proper scientific research into that specific event.

  17. No need to worry. I hear the deer are bunching up

  18. Perhaps the hurricane models would be better if so much money had not been put into the so-called Climate Change models which have a record of awful predictions. The Climate Change models put in bad and inconsistent physics and then expect to be taken seriously. As an example, NASA on the same web page says that the atmosphere absorbs 29% of the thermal longwave infrared radiation emitted by the surface of the Earth in one diagram and 90% is absorbed according to another diagram on the same page! What settled science that is. One diagram is plausibly based on science and actual measurements holding an energy supply rate of 58% of the energy rate supplied by the sun, while the other says the atmosphere holds energy at 158% of the energy supply rate from the sun. Consistency is not for a faith-based viewpoint on the physics of the atmosphere and the Earth's climate as held by the advocates of catastrophic man-made global warming.

  19. No need to worry. I hear the deer are bunching up

    More reliable than your cult, whose predictions have failed to come true. But Seattle "progressives," having abandoned traditional religion, bitterly cling to their new one and demand that everyone else pay tribute.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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