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How to Read the Forecasts

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Forecasts are issued three times per week. Monday and Wednesday forecasts will cover transport events for the next two days. Those on Friday's will cover the next three days.

Main Forecast Page

The Current Forecast can be viewed by clicking on the link in the left-hand side menu.

Clicking on a link to a particular forecast day will lead to the appropriate Forecast Summary page for that day's forecast. If needed, Epidemic Updates or Forecast Notes will be highlighted near the top of the page. The Forecast Summary for the stated day / date follows. This consists of a weather description for the next several days and the Risk Prediction (Outlook) for the next two (or three) day's transport events. The Risk Prediction / Outlook on the Forecast Summary page represents the risk to cucurbits from ALL of the known sources and is the most important part of the forecasts.

Below the General Weather and the combined Risk Predictions is a list of the known sources. A source in this list may be a single diseased field or plant bed; it may represent several sources in a single county; or, it may represent a number of sources in a number of different counties. The latter grouping is usually sources that are geographically clustered together, though the clustering may be quite loose at times. In general, the source groupings are made so that they are as natural as possible and promote the greatest understanding of the forecasts.

Each source / source region in the list is a link. Following the link will provide details about the transport events from that particular source / source region.

Detailed Source Pages

The links for the individual sources / source regions will contain specifics for the source or sources in question. Note the Date Issued, the Disease Locations (counties/states), and the Trajectory Start(s). Directly under this information is a Trajectory Map. This map shows the path that airborne spores will take when released from the source (or sources) on the indicated date and time. (see example).

A Trajectory Map has several parts. The larger upper map shows the horizontal motion; typically, the small rectangular lower map shows the vertical motion. If one imagines the release point to be the center of a spore cloud, then the forecast trajectory indicates the future pathway of the center of that spore cloud. Along the bottom of the map there is some information about the atmospheric transport simulation.

The release time is in Universal Time on a 24-hour clock, and will be set to correspond to about 10am or 11am local time. In the headings, the SECOND line has the start date and time for the forecast trajectory.

A black star denotes the starting point for the trajectory. After the trajectory starts, there are time/position markers along the forecast pathway. The larger symbols are the 00Z (00 UTC) markers, which corresponds to 7pm EST or 8pm EDT. The smaller symbols are in 6-hr intervals. A time scale is shown below the lower map.

Alternate Displays

Presentations on the larger horizontal map vary depending on the status of the epidemic and the forecast situation. Other trajectory maps include:

Multiple sources on one map: These appear during heavy epidemics. All the trajectories are for the same day and color (see example). 

Trajectories from one source at multiple starting heights: Trajectory height can be assigned. Generally, trajectories start at 200m above ground. However, multiple starting heights of the trajectories may be run to aid analysis and evaluation of the transport events. Sometimes it is helpful to post these in the forecasts; for example, when there is significant horizontal spread of the airborne spores. A maximum of three heights is allowed; The red trajectory is the lowest starting height, the next-higher is blue, and the highest is green (see example)

Fully Detailed Forecasts

At various points during the disease season, the individual forecast pages will feature expansive details about the transport events from that source / source region. There are sections on the Regional Weather, the Trajectory Weather, Trajectory Confidence, and the risk of epidemic spread (Outlook) for that source. These are described below. (see example).

The Regional Weather section gives a broad view of the weather conditions existing immediately prior to and during the forecast period. Notes on general weather patterns, main weather features, temperatures, etc., can be found here.

The Trajectory Weather section focuses on the meteorological conditions near the forecast track of the spore cloud center. This information is specific to each trajectory.

Trajectory Confidence is based upon the forecaster’s assessment of atmospheric complexity pertaining to that case, his/her forecasting experience, and model comparison / evaluation. It describes the general quality of the forecast pathway simulation, using the ratings of low, medium, and high. With continuing improvements to the HY-SPLIT model, most ratings will be High. Sometimes there are combination ratings, such as, “High first 12 hours, then low”, but there is never an increase from the initial rating. However, even the Low and Medium rated trajectories may be helpful.

The Outlook portion of the forecast combines all the aerobiological elements into an evaluation of the risk of disease development associated with that source or group of sources. The Threat is given first. It is a measure of source strength, describing the source's potential contribution to the spread of the epidemic. The given threat rating is color coded corresponding to the likelihood of infection. Red represents the greatest threat, followed by blue and then green. Risk Predictions and the factors pertinent to potential disease development due to that source follow the Threat. These factors include sporulation at the source, survivability of the airborne spores, possibility of future deposition, opportunity for infection, and other information that enhance the understanding of the forecast. Local weather and conditions should always be taken under consideration.

For more information visit our Interpreting Threat and Risks page.


The forecast trajectories and the resulting Outlooks are most useful if you keep in mind the following:

  1. Sporulation occurs at night and spores are released anytime from 8 AM through 1 PM. The trajectory start at 10 AM is just before maximum spore release. However, spores released at other times of the day may follow other tracks, especially if the weather situation is changing rapidly.

  2. The pathway you see on the map is the anticipated path for the spore cloud CENTER. The spore cloud will actually spread away from the center, similar to a smoke cloud. The areas on either side of the trajectory pathway will also be vulnerable.

  3. There is a limit to the detail and accuracy of the weather forecasts. If you are in a potentially higher risk area, be sure to pay close attention to your local conditions!

  4. Finally, these forecasts may be viewed from a different perspective. They can't tell you exactly where the spores will go, what the exact weather will be, or if you'll definitely receive viable spores deposited on your fields. They can, however, do a very good job of telling you where the spores WON'T go, what the weather WON'T be, and when you likely WON'T have to worry about inoculum arriving in your fields from known sources. This information should be helpful in making decisions about control strategies using protectant fungicides.

If you have any questions, please contact the Forecast Center.