How to Read the Pollen Forecast


Step 1: Follow the hyperlinks to get to your FORECAST of interest.

Step 2: Note the DATE and LOCATION at the top of each forecast.

Step 3: Page down on the large horizontal (xy) map showing the pollen cloud pathway/trajectory. The SOURCE end of the trajectory is indicated by a green dot. Time/position nodes show six-hour segments along the pathway (up to 48+ hours). Latitude and longitude coordinates of the source are given on the left margin of the map. The small graphic below the xy map shows the vertical movement of the spore cloud center.

Step 4: Review the REGIONAL WEATHER and TRAJECTORY WEATHER information.

Step 5: Study the OUTLOOK carefully. The likelihood of pollen exposure along the forecast pathway is based on several things: weather favorable for pollen release at the source, lack of rainfall along the pathway, favorable weather for deposition. You should ALWAYS take into consideration your local weather and conditions.

Step 6: Click on the highlighted labels to find additional source forecasts for that date and other dates.



Each Mountain Cedar Forecast includes descriptions of factors pertinent to pollen transport, a general outlook assessing the risk of exposure, and a map showing the flow away from known populations of mountain cedar with a single, centrally-located trajectory.

The maps show the motion of a floating particle that is released into the atmosphere on the date and time given in the second line of the map headings. The larger upper map shows the horizontal motion; the small rectangular lower map, the vertical motion. If you imagine the particle to be at the center of a pollen cloud, then the forecast trajectory indicates the future pathway of the center of that pollen cloud. The release time is in Universal Time on a 24-hour clock, and will be set to correspond to about 11a.m. local time. A green dot denotes the starting point for the trajectory (exact latitude and longitude are written along the left-hand side of the map). After the trajectory starts, there are time/position markers along the pathway at 00Z, 06Z, 12Z, and 18Z. A '+' gives the position, and the asterisk and the hash marks in the lower map correspond to the starting point and time/position stamps in the top map. In the headings, remember that the SECOND line has the start date and time for the forecast trajectory. The third line has some information about the weather data used as input to the trajectory model, and it can be ignored for most purposes.

The Regional Weather section gives a broad view of the weather conditions existing immediately prior to and during the forecast period. Notes on jet stream winds, approaching fronts, temperatures, etc., will be found here. The Trajectory Weather section focuses on the conditions near the forecast track of the spore cloud center. This information is specific to each particular trajectory.

The Outlook portion of the Forecast combines all the biological and meteorological elements into an evaluation of the risk of pollen exposure associated with that source or group of sources. There is a short overall statement usually separated with several asterisks, followed by comments concerning pollen release at the source, possibility of future deposition, and opportunity for exposure.

THINGS TO REMEMBER: The forecast trajectories and the resulting Outlooks can vary from reality in a number of ways. They will be most useful if you keep in mind the following:

1. Pollen release from the male cones typically occurs in late morning. The trajectory starts at 11 a.m. which should coincide with the timing of pollen release. However, pollen released at other times of the day may follow other tracks, especially if the weather situation is changing rapidly. In addition, we have frequently recorded nighttime pollen peaks in the mountain cedar area. This may occur when the pollen, released earlier in the day, settles back down to earth.

 2. The pathway you see on the map is the anticipated path for the pollen cloud CENTER. The pollen cloud will actually spread away from the center as it travels. As a result, areas on either side of the trajectory pathway may also be in the exposure zone.

 3. There is a limit to the detail and accuracy of the weather forecasts. If you are in a potentially high risk area, be sure to pay close attention to your local conditions! Check the current weather using the weather links on the forecasting homepage.

 4. Finally, these forecasts can be looked at from a different perspective. They can't tell you exactly where the pollen will go, what the exact weather will be, or if you'll definitely be exposed to pollen in your area. They can, however, do a good job of telling you where the pollen WON'T go, what the weather WON'T do, and when you likely WON'T have to worry about exposure. This information should be helpful in making decisions about allergy control measures.


This page has been modified from How to Read the NCSU Blue Mold Forecasts

 To learn more about trajectories, go the NOAA Air Resources Laboratory