DESCRIPTION
WINDIPs provide upper-air winds at "ridgetop" (either
700 or 850 mb) and 300mb heights - as forecast by the NWS RUC (Rapid
Update Cycle), ETA, NGM (Nested Grid Model), AVN (Aviation), and MRF
(Medium-Range Forecast) models - and a rudimentary prediction of
mountain wave occurrence based upon empirical criteria.
Forecasts for the ETA, NGM, and AVN models are given at 12 hr
intervals out to each model's forecast limit (48 hrs for the ETA and
NGM models and 72 hrs for the AVN model), allowing results from
different models to be compared and so providing an indication of
prediction reliability/uncertainty. The RUC model provides
forecasts at 3 hr intervals, allowing better timing of the jet
maximum. The MRF model provides 4 to 10 day forecasts, allowing
longer-term planning. Each model's first forecast line gives the
forecast period (in hrs) and date/time, with the corresponding 300mb
and "ridgetop" wind predictions on the following two lines. Each
wind forecast consist of three predictions: a wind speed (in kt), a
wind direction, and a windspeed component perpendicular to the assumed
ridge axis (in kt). An initial asterisk (*) or plus
(+) is also present if the predicted windspeeds meet the
empirical wave criteria. An asterisk (*) indicates that the
winds are roughly perpendicular to main ridgeline, whereas a plus
(+) is used when the perpendicular criteria are not met and
the wave is thus more likely to be spotty and occur in unusual
locations, e.g. downwind of individual peaks or minor ridgelines
having an orientation differing from the main ridgeline. An
initial alert line summarizes the times and models for which wave is
predicted within the next 72 hrs
Please note that the primary purpose of this program is
to provide advance warning of wave events rather than to
predict wave on the actual day that it occurs (it was originally
developed to help me decide when it was worth making a day-long drive
to Minden to fly wave). And it provides no information on the
lowest or highest altitudes at which wave will occur.
Wind Prediction Format
Line1: WINDIP header line giving station identifier and date
Line2: Alert summary line listing models forecasting wave for each forecast period
Line3: Header line giving ridgeline axis direction and script start-end date:time
Line4: RUC model forecast times ala: ff=mm/dd:hhZ
where ff=forecast_period(hr)
mm=month dd=day hh=hourUCT [ff=00 for
initial analysis]
Line5: RUC 300mb forecast winds ala: ww dd uu
where ww=wind_speed(kts) dd=wind_direction(true)
uu=ridge_wind_component(kts,perpendicular_to_ridgeline_axis)
Line6: RUC "ridgetop" forecast winds ala: ww dd uu
Line7-9: ETA model forecasts
Line10-12: NGM model forecasts
Line13-15: AVN model forecasts
Line16-18: MRF model forecasts
Blank windspeed indicates model forecast found but wind data missing
"---" windspeed indicates model forecast not found
Wave Predictions based upon criterion developed by Doug Armstrong of Reno NWS
Asterisk (*) or Plus (+) precedes windspeed when two mt. wave forecast indicators are met
(1) windspeed >= 25 kts at ridgetop level
(2) windspeed >= 65 kts at 300mb
where both criteria must be met simultaneously for a predicted mt. wave
(300mb height is roughly 31,000ft)
Asterisks (*) indicate ridge-prependicular components greater than 18 & 46 kt at rigetop & 300mb
(i.e. when criteria windspeeds are within 45° of the ridge perpendicular)
Pluses (+) are used when the perpendicular criteria are not met.
Notes:
Predictions for the same forecast period are provided
from several models so that
forecast variability at a given time will be apparent (a educational aspect of the WINDIP
is to demonstrate the variability inherent in meteorological forecasting to those who have
been used to relying
on a single model!). The forecast
uncertainly is greatest when only a single model predicts wave and
least when all models predict wave to occur on a certain
day/hour. Model variability results from differences in the
models' resolution, physical equations, and initial conditions.
The fact that there can on occasion be relatively large differences
between different model predictions should be noted by those who are
used to seeing, and accepting, a single weather prediction.
The ETA model is considered to be the most accurate
predictor at a given forecast time, since it has higher resolution
than the other models, but it is known to underpredict windspeeds near
300mb and thus will tend to underpredict mt. wave formation.
Models which forecast further ahead in time do so by
having coarser resolution and thus lesser accuracy. I personally feel
that forecasts beyond 5 days (120 hrs) are of little value for wave
forecasting, as "teaser" waves often appear and disappear.
Trends in windspeed are expected to be more accurate than are absolute
magnitudes of the predicted speeds.
The empirical criteria used to predict mt. wave
occurrence are necessarily simplistic and omit several factors which
affect wave formation, notably atmospheric stability effects
which can produce trapped waves. (It should also be noted that
the empirical mt. wave criteria were developed for the Reno area and
their use in locations with significantly different ridge heights
would be questionable.) The actual behaviour of mt. waves is
more complex than can be predicted on a simple "yes-no" basis from
winds at two heights, so the intent of the WINDIP is (1) to provide a
quantiative overview allowing quick comparison between wind
forecasts at different times or for different models and (2) to
provide an "early warning" alerting pilots to the likelihood of wave
occurrence.
Additional analysis would be required to obtain more
details, such jet stream analysis maps which indicate the strength and
horizontal location of the observed/forecast wind jet maximum and thus
provide a spatial perspective of the "big picture" at 300 mb; jet
stream analyses are available for the AVN model at
http://virga.sfsu.edu/crws/jetstream.html and for the ETA model at
http://weather.unisys.com/eta/4panel/eta_300_4panel.html.
Also useful is real time data, such as the 1 km
resolution satellite photos at
http://www.wrh.noaa.gov/satellite/1km/vis1.html, which can confirm
the existence of wave prior to a flight.
Link to the latest LWIP mt. wave forecasts
Link to the latest BLIPMAP thermal forecasts
Link to DrJack's home page