Relative Humidity Forecasts

Step #1 - INITIAL ANALYSIS

The process of forecasting relative humidity begins with a detailed assessment of the current synoptic- scale weather pattern; pay close attention to both the vertical and horizontal extent of moisture in the fire weather district. Determine current dew points values for the fire weather zone and for the key station by analyzing

surface charts, coded surface observations, raob soundings, GOES-9 soundings, RAWS observations, and any other sources of data. Look a the upstream air mass to see if relative humidities or dew points are significantly different from the current conditions in your area. Identify 24-hour changes in dew points and relative humidities and note the maximum and minimum RH for the past 24 hours.

Step #2 - FORECASTING RELATIVE HUMIDITY

Boundary layer relative humidity charts are an excellent tool in forecasting surface relative humidity. Tracking 12 and 24 hours changes in the boundary layer relative humidity can provide a good first guess of the surface relative humidity. The chart valid for 12z tomorrow can provide useful guidance on forecasting the actual relative humidity, as well as the humidity recovery, at ridgetop or high-elevation fire weather stations.

Look at 850mb gridded data (i.e., dew points, relative humidity, dew point depression, mixing ratios, etc.), along with forecast soundings to determine forecast changes in the vertical profile of moisture.

Look at MOS (FWCSEA, FWCBLI, FWCSMP), FOUS (NCMFRHT72 and NMCFRH72), TRAJECTORY FORECAST (NMCFTJ50), and the FREEZING LEVEL (NMCFOH43) forecast guidance available for the area. On the MOS forecasts, relative humidities must be calculated from the forecast temperature and dew point. Use the prevailing dew point with the forecasted maximum and minimum temperatures to calculate a maximum and minimum relative humidity for the next day. Note any changes or trends in the temperatures, dew points, or relative humidities.

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 relative humidity, along with forecasts of maximum and minimum relative humidity through 48 hours.

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. relative humidity and maximum and minimum relative humidity for Finney Creek. Frequency histograms of various weather parameters 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 relative humidities and dew points 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 relative humidity due to station elevation or location. Remember...ridgetop and mid-slope sites will often have lower relative humidities at night than valley locations, especially under clear to partly cloudy skies.

Step #5 - ADJUST RH FORECAST FOR PRECIPITATION

Adjust your relative humidity trends higher if you expect to forecast precipitation in tomorrow’s forecast. MOS typically under forecasts relative humidity during periods of precipitation, especially in mountainous locations.

Step #6 - ADJUST RH FORECAST FOR SURFACE WINDS AND MIXING

Wind effects mixing of the air mass. Strong winds promote better mixing and can bring either drier air or more moist air drier air down to the surface. You should consider both wind speed and the moisture content of the free air when making forecasts of surface relative humidity. Light winds retard mixing, resulting in less contribution of free air relative humidity to the surface relative humidity. Wind direction can also have a profound effect on surface relative humidity. For example, the difference between onshore flow and offshore flow can drastically change relative humidities over a short period of time by 50% or more. Watch for presence of our easterly, foehn-type winds across the Cascades. Closely monitor RAWS observations for zone 658 to get a feel for how changes in wind speed and wind direction can affect relative humidities under certain weather patterns.

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	Please send comments or questions about this fire weather training module to John Werth, NWS Seattle, WA. email: john.werth@noaa.gov phone: (206) 526-6095 ext 251