1998/99 Mountain Cedar Pollen Forecasting Project

Preliminary Analysis

February 9, 1999


Dr. Estelle Levetin, The University of Tulsa

Dr. C. E. Main and T. Keever, North Carolina State University


The first year of the Mountain Cedar Pollen Forecasting Project has been completed. Our goal has been to provide pollen transport forecasts in order to alert patients sensitive to mountain cedar pollen when the pollen was moving into their area. Developed jointly by scientists at The University of Tulsa and North Carolina State University, the forecasts were based upon the presence of pollen at the source and meteorological conditions favorable for pollen release and transport. Meteorological information was obtained locally from the Oklahoma Mesonet and via the Internet from the National Weather Service and the National Centers for Environmental Prediction. Trajectories were provided by Air Resources Laboratory of the National Oceanic and Atmospheric Administration. Forecasts were produced daily during December 1998 and January 1999. We hope the forecasts of the airborne movement of this potent allergen were beneficial to allergy sufferers and concerned health care providers!

Pollen from mountain cedar or ashe juniper (Juniperus ashei) is one of the most potent allergens in North America. Populations of mountain cedar trees occur in the Arbuckle Mountains of south-central Oklahoma and in the Edwards Plateau region of central Texas. Plants produce abundant pollen that is easily dispersed by wind. Day-to-day variability of pollen release is governed by several meteorological factors, the most important of which are temperature, relative humidity, surface moisture, and wind speed. In addition to being very allergenic, mountain cedar pollen is also unique due to the time of pollen release. Pollination occurs during December and January, with the peak often occurring the last week of December. It is responsible for mid-winter hay fever problems in the south-central region of the United States.

Creation of the forecasts was a multi-step process. Each day, forecast air-parcel trajectories were generated from several representative sites where mountain cedars are known to occur. Austin, Junction, and San Angelo were chosen from the Edwards Plateau in Texas. Sulfur, Oklahoma was selected from the Arbuckle Mountains. Conditions for potential pollen release and transport along the forecast paths were evaluated using past, current, and forecast weather data gathered over the Internet. Also involved in the decision-making process were such factors as the amount of pollen available to be released, the size of the source region, the duration of favorable release conditions that day, and how closely the forecast trajectories approached densely-populated areas. These elements were then synthesized to produce a judgement of the overall threat for that day's situation, either Low, Moderate, or Serious.

During December two favorable periods for pollen release occurred. During the first period from December 13 to December 17, conditions were most favorable on the 17th. This episode also held the greatest possibility for long-range transport of pollen. Airborne pollen spread northeast from both sources, affecting large areas of western north Texas and Oklahoma. The second episode was December 27 through December 31, during which favorable weather was largely limited to the Edwards Plateau region with peak conditions on the 29th and 30th. Wind directions were variable due to the passing of dry fronts; little long-range pollen transport was likely. Airborne pollen influenced various areas of Texas from day to day. Release from the Arbuckle Mountains in Oklahoma was possible between the 27th and the 29th. However, with favorable release conditions confined to a few hours each afternoon and the pollen source area being small, this location represented a Low threat.

After the first several days of January, an unusually dry and mild regime existed over the south-central United States. In Texas, where pollen-release weather and transport dominated from January 5-27, the peak period of favorable weather occurred between the 10th and 21st. Conditions varied from one side of the Edwards Plateau to the other at other times. In almost all cases, the more favorable conditions occurred more frequently in the western sections of the Plateau due to higher humidities in the east.

In the Arbuckle Mountains source area, frequent pollen-release weather occurred during the same January 5th to 27th period, though not to the extent found in Texas. There were more intrusions of cold and/or wet weather. The longest favorable period occurred from January 11-21 with peak release conditions occurring from the 15th to the 20th. This coincides with the time of best release conditions for the Edwards Plateau. Both the Texas and Oklahoma source regions experienced unfavorable conditions for pollen release after January 27 that lasted through the end of the forecasting program for 1998-99.

There was a variety of potential transport pathways exhibited by the forecast trajectories from both the Edwards Plateau and the Arbuckle Mountains. Most transport was to the north and east at varying rates including some trajectories that traveled great distances. Some pollen was transported south but only a few trajectories had a predominantly western track. The forecast trajectories typically had a duration of 48 hours and many showed a change in direction due to passing fronts, sometimes more than once. Washout of airborne pollen occurred periodically. However, the extent to which this occurred is difficult to determine and may only be possible with a broader sampling network.

Another notable point was the occasional tendency for trajectories from the east and west portions of the Edwards Plateau to take different paths. In most instances the Austin source trajectory followed one track while the Junction and San Angelo trajectories behaved differently (but similar to each other). These cases usually resulted from a passing front or a developing low pressure area to the north. The Austin trajectories appeared to be more readily influenced by wind flows ahead of the fronts or developing systems. Trajectories from the other two locations tended to begin in the same direction but then failed to get caught up in those same flows. They usually would travel slower and/or shift direction.

In general, conditions for pollen release were more favorable in January than in December and occurred more frequently on the Edwards Plateau than in the Arbuckle Mountains. Moderate or Serious threats were present for the Edwards Plateau about 3/5 of the season, compared to about 2/5 of the season for the Arbuckles. Though there were periods of favorable weather in December, jet stream patterns were even more conducive to warm, dry weather in January. The Edwards Plateau region experienced favorable to very favorable weather for all except the first four and last four days of that month.

Favorable conditions were less frequent in the Arbuckle Mountains mostly because of the greater number of weather systems influencing that region. This meant more precipitation and/or incursions of cold air that often missed the Edwards Plateau. When unfavorable weather governed release in both regions, favorable conditions returned more quickly for the Edwards Plateau area but tended to linger in the Arbuckle Mountains.

The weather related to pollen release is still under study as pollen trap counts become available. There should be more expansions or additions to this summary as more post-season analysis takes place. We suggest you periodically return to check for updates! All pollen forecasts have been archived and the complete set of day-by-day maps, forecasts, and other information of interest can be reviewed via the Mountain Cedar Pollen Forecasting website: http://bio44.utulsa.edu/homepages/levetin/FORECAST.html.