Changes in Uncertainty for Hurricane Florence: Are We Communicating this Well Enough?

Hurricane Florence has been a study in contrasts.   For the last several days its track has been one of the most predictable on record.  Most major models have been spot on, the various ensembles have been tightly clustered, and the uncertainty in the forecast tracks is low.

Then as the storm approaches the Carolina coast everything changes:  the predictability of the storm is greatly reduced with all kinds of possible forecasts, including stagnation along the coast.

Why did this happen and how can my profession communicate such changes in predictability better?

To see the issue, here is an ensemble forecast from the European Center system showing the 51 forecasts started at 5 AM PDT on Monday.  Remember, that in an ensemble system a forecast model is run many times varying the initial conditions and model physics--showing the range of potential events.  This figure shows the probabilities of the storm being at any location, based on the ensembles.  Offshore there was little uncertainty in track but the the tracks really diverge near the coast.

A similar situation is true for the NWS global model ensemble (GEFS)

In contrast, the tradition way of showing uncertainty by the National Hurricane Center shows a progressive increase of uncertainty (see below).  This is because they simply use historical errors over time based on many forecasts.  The current figure communicates a loss of skill over time regarding the track forecasts, but there is a huge loss of information found in the ensembles...and such uncertainty information is very valuable.  Ensemble-based diagrams are far better.

Why were the forecast tracks reliable and tightly clustered out over the Atlantic, but all over the place near land?   It has to do with large-scale (or synoptic) steering flows.   To first order, you can consider hurricanes as huge tops that are pushed around by the large scale flow.  During the past few days, Florence has been steered westward by high pressure to the north of the storm, as illustrated by the sea level pressure map for Monday at 5 PM PDT.  The "L" due east of southern Florida is Florence.

The upper level (500 hPa) map for  11 AM on Monday shows high pressure north of the storm that was helping steer Florence to the northeast.

Similarly for 11 AM PDT on Tuesday.  A high pressure area to the north was effectively steering the hurricane towards the coast.

 But the situation on Thursday at 11 AM PDT is  very different.  A large ridge of high pressure is over the eastern US, without much flow over the SE U.S.  And weak high pressure surrounds the storm--thus there is little steering flow to push the storm in any direction. 

The dangers of a stagnating storm are substantial, particularly the potential for heavy precipitation, with some models going for 20-30 inches in some locations. 

Our models are now capable of providing useful information of how hurricane track uncertainty will change in time...the challenge is to find ways of better communicating the information.








from Cliff Mass Weather and Climate Blog https://ift.tt/2OerrE9