The process of forecasting surface wind direction and wind speed begins with a detailed assessment of the current synoptic- scale weather pattern. Pay close attention to both the vertical and horizontal wind distribution across the fire weather district. Determine the current wind speed and direction for the fire weather zone and the key station by analyzing surface charts, coded surface observations, raob soundings, GOES-9 winds, profiler data, WSR-88D VAD wind profiles, pireps, current RAWS observations, and any other sources of data available at the forecast office. Study surface pressure gradients across the area and note any changes or trends in the gradients over the past 24 hours. When analyzing RAWS observations, pay close attention to differences in wind speed and direction due to changes in elevation (ridgetop vs valley), aspect (north, south, east, and west), the surrounding terrain and steepness of the terrain, etc. Note: knowing the topography at the fire weather station can oftentimes help explain any inconsistencies in the observations. Also note the boundary layer winds across the area and determine whether or not significant changes have occurred in the past 24 hours.

STEP #2 - FORECASTING SURFACE WIND SPEED AND DIRECTION

BOUNDARY LAYER WINDS AND 24 HOUR CHANGE

Boundary layer wind charts are an excellent tool in forecasting surface winds. Charts through 48 hours are available on AFOS, PCGRIDDS, and the HP workstations. Use the Meso-ETA or the MM5 models for boundary layer wind forecast since both models have much better terrain resolution across western Washington than do the early ETA or the old NGM. Tracking 24 hour changes in the boundary layer winds can provide a good first guess for your forecast.

PRESSURE GRADIENTS AND 850 MB WINDS

Pressure gradients usually correlate quite well with reported surface winds at fire weather stations located in the major, low-elevation valleys of the North Cascades...especially when the orientation of the valley is perpendicular to the direction of the pressure gradient. Complex terrain and steep topography can shelter many locations from strong winds even when pressure gradients across the area are quite strong.

Strong winds at 850 mb can lower to the surface through the downward transfer of momentum caused by daytime surface heating. Monitor observed and forecast winds at this level paying close attention for low level jetstreams. Forecast changes in wind speed and direction at 850 mb should be used in formulating forecasts of surface winds for the narrative fire weather forecast and the point forecast for Finney Creek. Winds at this level can also provide a good first guess ofthe ridgetop winds, i.e., 3,000 to 7,000 ft winds.

MODEL STATISTICS AND GRIDDED DATA OUTPUT

MOS surface wind forecasts (i.e., FWCSEA, FWCBLI, FWCSMP, etc.) can provide a good first guess for surface wind forecast. However, keep in mind that most MOS guidance is available for only the lower elevation of your area. FOUS products (NCMFRHT72 and NMCFRH72) provide useful forecasts of boundary layer winds trough 48 hours. Use forecast guidance on upper level winds (NMCFD1FA6, NMCFD2FA6, NMCFD3FA6) as a first guess for ridgetop winds.

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 wind direction and wind speed through 48 hours. Note...RAWS observations of wind speed and direction are 10 minute averages, therefore RAWS MOS wind forecasts are 10 minute averages of of wind speed and direction.

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. wind speeds. Frequency histograms of wind speed and direction at 09Z and 21Z 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 winds in zone 658 noting any differences in the winds due to topography, slope, aspect and elevation. Ridgetop and mid-slope winds will often have stronger winds than valley locations. Identify trends which can help with the narrative fire weather forecast.

STEP #5 - ADJUST WINDS FOR TOPOGRAPHY (ASPECT, SLOPE, ELEVATION AND SHAPE)

Adjust your wind forecast to take into account how topography changes both wind speed and direction. For example, south facing slopes typically report higher wind speeds since they receive more solar insolation. Wind speeds on slopes also increase as the steepness of the slope increases.Higher elevations and ridgetops normally experience strong winds because friction becomes less of a factor. The shape of the landscape can act to channel winds in directions completely different than the free air winds. Knowing the topography surrounding the fire weather stations in your district can help explain inconsistencies in the observations. Also remember the typical slope and valley wind regime when formulating your forecast. Winds are usually upslope/upvalley during the day and downslope/downvalley at night.

STEP #6 - ADJUST WINDS FOR SKY CONDITIONS

The amount of cloud cover can alter surface winds by diminishing the amount of solar insolation. Upslope, upvalley and downslope, downvalley winds are usually not as strong when skies are cloudy. However, lessening the effects of thermally induced winds call allow a greater contribution of the larger scale, pressure gradient winds. Under clear to mostly clear skies at night though, winds due to large scale pressure gradients rarely mix to the surface due to the formation of nighttime surface inversions in the valleys.

STEP #7 - ADJUST FOR AIR MASS STABILITY

Unstable air allow more of the winds aloft to mix down to the surface. This can result in gusty, erratic surface winds and at times strong winds when there are strong winds aloft at ridgetop level. On the other hand, stable air usually favors lighter, more steady surface winds.