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.