The process of forecasting temperature begins with a detailed assessment of the current synoptic-scale weather pattern. Pay close attention to both the vertical and horizontal temperature distribution across the fire weather district. Determine current temperatures for both the fire weather zone and the key station by analyzing surface charts, coded surface observations, raob soundings, GOES-9 soundings, current RAWS observations, and any other sources of data available at the forecast office. Study the maximum and minimum temperatures over the past 24 hours and make note of changes or trends. When analyzing RAWS observations, pay close attention to differences in temperature due to changes in elevation, aspect, exposure to wind, nearby bodies of water, etc. Make note of marine air......Also note changes in the 1000-500 mb and 1000-850 mb thickness over the past 24 hours. A local study used by Olympia Fire Weather shows a very high correlation between 1000-850 thickness, cloud cover and/or weather, and 2:00 p.m. temperatures for the day. Precipitation, the amount or areal extent of cloud cover, and boundary layer and surface winds can modify surface temperatures. Note the amount of cloud cover is critical when forecasting surface temperatures. The amount of solar radiation reaching the earth’s surface - or terrestrial, longwave radiation escaping the earth’s surface - is highly correlated with cloud cover. Broken, opaque cloud cover can reduce temperatures by as much as 5 degrees and overcast opaque cloud cover can reduce temperatures by as much as 15 to 20 degrees. At night, cloud cover results in higher temperature than would be expected under clear sky conditions.

STEP #2 - FORECASTING SURFACE TEMPERATURES

CHANGES IN 1000-500 MB AND 1000-850 MB THICKNESS

Thickness between constant pressure levels is a measure of the average temperature within the layer. Higher thickness values correspond to warmer temperatures and visa versa. During the summer fire season, forecast 24 hour change in the 1000-500 mb thickness is a good indicator of surface temperature change. Generally speaking, every 10 meters of change in the 1000-500 mb thickness corresponds to a 1 degree change in the surface temperature when skies are clear. Because the thickness and areal extent of marine air in western Washington is a critical factor in determining daily temperatures and relative humidities, the Olympia Fire Weather office uses 1000-850 mb thickness as a tool in forecasting temperatures for the individual fire weather zones. A PC application program used by the Olympia Fire Weather office uses historical fire weather data to predict 2:00 zone averages of temperature, humidity, and wind speed, using the predicted 1000-850 mb thickness and time of year as input supplied by the user. The program searches the database for matches and then computes zone averages of 2:00 p.m. temperatures, relative humidity, and wind speeds, stratified by sky condition and weather.

850 MB TEMPERATURE AND 24 HOUR CHANGE

Look at 850mb gridded data and forecast soundings to determine forecast changes in the 850 mb temperature. Temperature changes at this level in the atmosphere are good indicators of surface temperature changes in western Washington, since most forested areas are below 7,000 feet. Changes of plus or minus 5 degrees centigrade can also be a good indicator of air mass changes and potential changes in the surface temperature.

MODEL STATISTICS AND GRIDDED DATA OUTPUT

Look at MOS (FWCSEA, FWCBLI, FWCSMP), FOUS (NCMFRHT72 and NMCFRH72), TRAJECTORY FORECAST (NMCFTJ50), and the FREEZING LEVEL (NMCFOH43) forecast guidance available for the area. MOS guidance gives temperature forecasts at 3 hour increments out to 48 hours. However, note that most MOS guidance is for low elevation stations and must be adjusted for elevation, slope, and aspect. Note any changes or trends in the forecast temperatures, wind, sky cover, and precipitation.

In addition to the above guidance, NWSFO Boise has also developed RAWS MOS guidance based on hourly weather observations and NGM gridded model data. MOS equations for various forecast parameters were developed using 5 years of June through September hourly weather observations from the RAWS fire weather station located at Finney Creek in the North Cascades. This new product will provide 3-hourly forecasts of temperature and maximum and minimum temperature through 48 hours for Finney Creek.

STEP #3 - FINNEY CREEK CLIMATOLOGY

Charts and graphs have also been developed by WSO Olympia and NWSFO Boise which show the climatology of Finney Creek. View the station catalog for Finney Creek (available on CD-ROM) which shows 5-day running means of 2 p.m. temperature and maximum and minimum temperature. Frequency histograms of maximum and minimum temperatures for Finney Creek are also available on your local Fire Weather home page.

STEP #4 - 2:00 PM FIRE WEATHER OBSERVATIONS

Study today’s 2:00 PM fire weather observations available in AFOS file NMCFWOOLM. Look at the observed temperatures in zone 658 and surrounding zones, along with the reported maximum and minimum values during the past 24 hours. Identify trends which can help you with the narrative forecast for tonight and tomorrow. Be sure to notice differences in temperature due to station elevation, location, cloud cover, etc. Remember...ridgetop and mid-slope sites will often have higher temperatures at night than valley locations, especially under clear to partly cloudy skies.

STEP #5 - ADJUST TEMPERATURE FORECAST FOR PRECIPITATION

Adjust your temperature forecast trends lower if you expect to forecast precipitation in tomorrow’s forecast. MOS will typically over forecast temperature when precipitation is expected at the time of the observation, especially in mountainous locations.

STEP #6 - ADJUST TEMPERATURE FORECAST FOR SURFACE WINDS AND AIR MASS STABILITY

Unstable air and wind promote mixing of the air mass. Temperature changes in the air mass aloft can better mix to the surface when the air mass is unstable. In mountainous terrain, changes in the thickness or temperatures aloft may have little effect on surface temperature, especially in late summer and early fall. Strong winds also promote better mixing and usually (except for foehn winds) have a cooling effect on surface temperatures. Light winds limit mixing, resulting in warmer surface temperatures. Changes in the wind direction can have a dramatic effect on surface temperatures. For example, offshore flow as opposed to onshore flow can change temperature by as much as 30 degrees in zone 658, especially at lower elevation sites exposed to the intrusion of marine air. Closely monitor RAWS observations for zone 658 to get a feel for how changes in wind speed and wind direction can effect relative humidities under certain weather patterns.