Written by Lav Khot, Josh Ward, Craig Oswald, and Sanjita Bhavirisetty, Washington State University – AgWeatherNet, March 9, 2026
This article introduces the updated Frost Forecast web portal that will delivery daily Spring frost forecasts in 2026 season and explains how growers can access Mesonet tower–specific inversion trends from the past 12 hours, along with forecasts for the next 36 hours, to support more effective frost‑mitigation decisions.
Frost Forecast Program
AgWeatherNet (AWN) began the Frost Forecast Program (formally known as the Fruit Frost Service) during the Spring of 2021, after taking the service over from Clearwest Agricultural Forecast Service. AWN Meteorologists shadowed Clearwest Meteorologists during the spring of 2020 to learn forecasting techniques applicable to accurately forecast overnight low temperatures in the sensitive fruit growing regions of Eastern Washington. Clearwest, consisting of retired National Weather Service (NWS) Meteorologists who had provided the service while under the NWS’s tenure, continued to provide the service beginning in 1996, after the NWS ended the service during its modernization in 1995. Prior to Clearwest, the service was initially started by the U.S. Weather Bureau in 1918 ( renamed as NWS in 1970). This service was started in 1918 due to demands from southern California citrus and vegetable growers who were being hurt by damaging frost events and needing predictions. The Weather Bureau expanded this service to Washington State in 1922 with the establishment of the Yakima frost district, followed by the Wenatchee frost district in 1923. A more detailed history of the fruit frost service can be found here (link opens in new window).
The current AWN Frost Forecast Program, supported financially in part by Washington State Tree Fruit Association, is committed to delivering frost-risk assessments to growers throughout the Central Basin and the Yakima, Wenatchee, and Okanogan valleys. This enables well-informed decision making regarding the implementation of active frost mitigation methods (e.g., wind machines, heaters, under- and over-tree sprinklers).
How It is Done
The daily frost-risk assessments are determined by taking into account the current bud stages of several tree fruit varieties, in addition to the synoptic, mesoscale, and microscale weather patterns across Washington state. AWN Meteorologists analyze Numerical Weather Prediction (NWP) model output, Automated Surface Observing System (ASOS) data, AWN Mesonet (60 tower) station inversions and 24-h forecasts (currently operational on 48 of those 60 towers), and AWN maintained frost stations data to accurately forecast the overnight low temperature across the fruit crop production areas mentioned above. This is done by continuing to utilize the ‘Key Station Forecast Concept’ as established by the Weather Bureau at the inception of the fruit frost service. The concept behind key station forecasts was developed from the knowledge that often a large difference in minimum temperature occurs from place to place on many nights, and under different weather regimes, even across a fairly small area. Temperature sensing stations have been set up at various cold and warm areas, and forecasts are being made for these sites. A grower can then determine which of these sites most closely matches their particular orchard location, then ‘key off’ the forecasts for a particular station to help determine the likely minimum for their orchard.
How to access the Daily Frost Forecast
On the AWN website https://weather.wsu.edu, registered users/growers can access the daily spring frost forecast (in English or Spanish) by navigating to the “Weather” tab and clicking on the “Spring Frost Forecast” button on the AWN Forecast Discussion card (Figure 1 right). Make sure to select your region using the drop down on the right side of the forecast page (Figure 2).
What is New
The AWN Frost Forecast page now includes a new summary table that concisely provides the latest data gathered by airports across the Central Basin and Yakima, Wenatchee, and Okanogan valleys. The new table also includes Growing Degree Days in addition to Blossom Degree Days, both of which are calculated using the latest airport data. The webpage used to report frost has now been relocated to the Frost Forecast page underneath the Forecast Discussion section. AWN has also added the option to report bud stages separate from the frost report page. Bud stage reports are highly encouraged by AWN Meteorologists as they allow them to know at which temperatures bud damage will begin to occur, and how severe it may be based on the low temperature forecast. Bud stages are typically highly variable across the different tree fruit growing regions, thus making grower submitted reports necessary for the most accurate forecasts possible. Changes to key stations this year are the addition of Omak Airport as a key station site in the Okanogan District. In the Wenatchee District, forecasts will no longer be produced for the Wenatchee.E site. In its place, Wenatchee-Pangborn Airport will be added as a key station site. Considering that Wenatchee.E is effectively colocated with the airport, no significant difference is expected in temperature observations between these locations. In addition to Frost Forecasts during the Spring season, AWN is considering providing this service to growers during the Fall season as well. Please fill out this form to help AWN gauge grower interest in this service.
Temperature Inversion trends and 36-h Forecasts
Temperature Inversion
Basics on temperature inversion, and AWN Mesonet tower station specific inversion strength forecasting (at 1-h interval for next 36-h) efforts are detailed in Amogi et at. (2025). In nutshell, inversion strength refers to the temperature difference between a warmer air layer above (30 ft) and the cooler air at, or just above the canopy height (5 ft). A larger temperature difference indicates a stronger inversion. Stronger inversions offer greater potential for warming by mixing the air. Wind machines are typically most effective when inversion strength is at least 2oF, as they typically raise the surface air temperatures by about 50% of the inversion strength.
Using conditions illustrated in Table 1, AWN has rolled out tower station specific advisory to help growers in deciding when to or not to mix the air using wind machines. More details on wind machine operational guidance are in Amogi et al. (2024).
Table 1. Conditions to determine air mixing advisory within orchards.
| Scenario | Tair @ 5 ft. | Tair @ 30 ft. | Advisory |
|---|---|---|---|
| Inversion 1 | ≤ 32 °F | > 32 °F | Mixing could help |
| Inversion 2 | ≤ 32 °F | ≤ 32 °F | Mixing could make it worse |
| No inversion | Tair @ 5 ft. > Tair @ 30 ft. | – | Mixing not advised |
Accessing Inversion Trends and Forecasts
Currently, the inversion forecasts for next 36-hours, at 1-h interval, are operational for 48 matured (i.e., with historical data for above 1 year) tower stations within AWN network. These forecasts are available from the Weather dashboard (figure 3; accessed by opening a station view and clicking the Inversion info icon in the left “Weather panel”). The chart (figure 4) displays the last 12 hours of observations and the next 36 hours of forecast, and it uses shaded bands to indicate “Mixing advised” (green) when conditions suggest mixing could help, and “Mixing not advised” (red) when mixing is unlikely to improve conditions.
The same inversion plot is also available on all crop specific cold hardiness model pages and can be accessed by clicking on the “View Inversion” button on the bottom right of the cold hardiness plot.
Contact
For feedback on these tools, and any new feature you would like to see, do reach out to us at weather@wsu.edu
Lav Khot
Director, AgWeatherNet
lav.khot@wsu.edu
509-786-9302
Josh Ward
Field Meteorologist and Forecaster, AgWeatherNet
joshua.ward1@wsu.edu
509-786-9288
Craig Oswald
Meteorologist and Forecaster, AgWeatherNet
craig.oswald@wsu.edu
Sanjita Bhavirisetty
Applications System Analyst / Developer
r.bhavirisetty@wsu.edu
509-786-9360
Funding and acknowledgements
AgWeatherNet’s frost‑forecasting program is supported in part by the Washington State Tree Fruit Association. Several of the tools and products it used in the process have been developed through projects funded by NSF/USDA NIFA CyberPhysical Systems (Award No. 2021‑67021‑34336), the Washington State Department of Agriculture Specialty Crop Block Grant, and USDA NIFA 0745. We acknowledge AWN Field team for deploying the tower stations and efforts of Dr. Basavaraj Amogi, Dr. Matt Cann, and Jaitun Patel to operationalize the weather forecasts.
Additional information
- Amogi, B., G.-A. Hoheisel, and L. Khot. 2025. A New Tool to Estimate Inversion Strengths. Available at: https://treefruit.wsu.edu/article/a-new-tool-to-estimate-inversion-strengths/
- Amogi, B., and L. Khot. 2024. Temperature Inversion Forecasts Available for AgWeatherNet Mesonet Tower Stations. Available at: https://treefruit.wsu.edu/article/temperature-inversion-forecasts-available-for-agweathernet-mesonet-tower-stations/
- Oswald, C., J. Holcomb, Bud Graves, and M. Cann. 2022. WA State Fruit Frost Forecast Service History. Available at: https://weather.wsu.edu/api/v1/resources/frost-history
