Wednesday, December 09, 2015

More On NWS GFS Ensembles

First all the model forecasts shown here are from the initial conditions at 00 UTC on 9 December 2015.

The GFS spaghetti plots for the 21 versions of the GFS forecast model are shown above at 84-hours and below at 168-hours. There is very little spread or uncertainty among the models at 84-hours (valid at 12 UTC on Saturday the 12th). There is a bit of uncertainty regarding the exact positioning of the Southwest US trough. There is much greater spread over the West by 168-hours, as would be expected.

I have looked at the plumes again this morning, and show them for three weather parameters here. the plot above is of the ensemble forecasts of 10-m wind speed (kt). While the graphic below shows 2-m temperature (F).  There is very little spread in these the forecasts out through next Sunday. Basically the model forecasts a strong cold frontal passage at TUS next Friday afternoon, preceded by strong winds. The front brings in much colder temperatures for the following five days.

As for accumulated precipitation (above in inches), there is much more spread, beginning after the frontal passage. For Friday night the model consensus continues to be essentially 100% POPs for precipitation at TUS on Friday night (20 of 21 members). There is much more uncertainty for the first part of next week. These graphics illustrate the inherent difficulties of forecasting precipitation and amounts (QPF) within the models. The forecasts of accumulations of rain at TUS at 84-hours range from 0.0 to 0.25" - so the amounts have become more clustered as the event approaches, with the average of the forecasts being less than 0.1". So, not a substantial precipitation amount, but a distinct shift to colder and unsettled weather.

This morning's forecast from the NWS continues at 40% POPs for TUS during Friday night, and I note that POPs for the same period up on Mt. Lemmon are only at 50%, which seems quite conservative with such a strong front coming by. Final caution is that the precipitation event forecast by the GFS is almost three days away, and things still occasionally go awry at short time and space scales - so we'll just watch as this change to unsettled weather unfolds.

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