Why does this output irradiance forecasted values to be somewhat the same? I am using PVLIB's GFS model. What am I doing wrong here?

Question

#Initialize the mode here

model = GFS(resolution='half', set_type='latest')

#the location I want to forecast the irradiance, and also the timezone

latitude, longitude, tz = 15.134677754177943, 120.63806622424912, 'Asia/Manila'

start = pd.Timestamp(datetime.date.today(), tz=tz)

end = start + pd.Timedelta(days=7)

#pulling the data from the GFS

raw_data = model.get_processed_data(latitude, longitude, start, end)

raw_data = pd.DataFrame(raw_data)

data = raw_data

#Description of the PV system we are using

system = PVSystem(surface_tilt=10, surface_azimuth=180, albedo=0.2,
                                 module_type = 'glass_polymer',
                                 module=module, module_parameters=module,
                                 temperature_model_parameters=temperature_model_parameters,
                                 modules_per_string=24, strings_per_inverter=32,
                                 inverter=inverter, inverter_parameters=inverter, 
                                 racking_model='insulated_back')

#Using the ModelChain

mc = ModelChain(system, model.location, orientation_strategy=None,
                aoi_model='no_loss', spectral_model='no_loss',
                temp_model='sapm', losses_model='no_loss')

mc.run_model(data);

mc.total_irrad.plot()

plt.ylabel('Plane of array irradiance ($W/m^2$)')

plt.legend(loc='best')

Here is the picture of it

I am actually getting the same values for irradiance for days now. So I believe there is something wrong. I think there should somewhat be of different values for everyday at the least

Forecasting Irradiance

Answer 1

I think the reason the days all look the same is that the forecast data predicts those days to be consistently overcast, so there's not necessarily anything "wrong" with the values being very similar across days -- it's just several cloudy days in a row. Take a look at raw_data['total_clouds'] and see how little variation there is for this forecast (nearly always 100% cloud cover). Also note that if you print the actual values of mc.total_irrad , you'll see that there is some minor variation day-to-day that is too small to appear on the plot.