Frequently Asked Questions:

What does the diagram show?

If you are viewing the Ensemble Control or Mean IVT AR Landfall or Inland AR tool, you are looking at the magnitude of integrated vapor transport (IVT) at different latitudes along the U.S. West Coast from “today” through the next 7 days at 3-hour increments. The landfall locations are the black dots in the right-most panel. The IVT magnitude is shaded on a scale from <200 kg/m/s to >1000 kg/m/s. The right map-panel graphically depicts the total 7-day time-integrated IVT for that location in millions of kg/m.

If you are viewing either of the Probabilistic AR Landfall Tools, these contain the probability of having “atmospheric river” conditions, (i.e., high water vapor content) at different latitudes along the U.S. West Coast from “today” through the next 7 or 16 days at 3-hour increments. Evidence suggest that IVT magnitudes >250 kg/m/s are typically consistent with “AR conditions”, wheras IVT magnitudes >500 kg/m/s are consistent with strong ARs. The values of 150 kg/m/s should be used for inland locations. The landfall locations are the black dots in the right-most panel. The probability is shaded on a scale from 0% (blue) to 100% (purple).

The diagram also graphically depicts the number of hours a location along the coast may expect to see AR conditions at different fraction thresholds: Gray bars for number of hours with a probability >50%, black bars for number of hours >75%, and red bards for number of hours >99%. The higher this number, the longer AR conditions are likely and the more precipitation may be expected!

Why is this diagram useful?

These diagrams display the magnitude and probability of AR conditions at the coast, and at inland locations, and thereby provide a representation of whether or not high-impact precipitation events may be likely. As displayed, the diagram is also capable of providing information on the timing, landfall, intensity, coastal propagation, inland penetration, and uncertainty of AR conditions.

How do you read the diagram?

The main diagram (left display) is read from *right* to *left* at a given latitude in order to indicate how many days from today AR conditions are likely at the coast. By plotting the display from *right* to *left*, the display shows you “what is in the pipeline” as storm systems generally move from west to east.

What is meant by AR conditions?

AR conditions are defined as vertically integrated water vapor flux values generally exceeding 250 kilograms per meter per second. This threshold is generally associated with a landfalling atmospheric river and is capable of producing precipitation along the coast and in interior locations.

How is the probability determined?

The probability is derived from the National Center for Environmental Prediction (NCEP) Global Forecast System (GFS) Ensemble Prediction System. The NCEP-GFS Ensemble contains 20 perturbed models and one control model. These 21 models all predict variables that can be used to compute the vertically integrated water vapor flux. The number of models that predict “AR conditions” at the grid point along the coast divided by the total number of models (21) is one model’s depiction of the probability of having AR conditions at the coast. Data beyond day-7 are interpolated every 3 hours from six-hour data.

What did the display look like on *some date in the past*?

The diagram is archived with each 6-hour run of the NCEP GFS Ensemble. For past diagrams, please contact the diagram developer, Jason Cordeira, at j underscore cordeira at plymouth dot edu.

Still have more questions?

Please feel free to contact the diagram developer, Jason Cordeira, at j underscore cordeira at plymouth dot edu.