Friday, January 29, 2016

GEFS Now Forecasting Bit More Precipitation At Airport

The Sunday night/Monday morning weather event is now getting within the forecast range of the WRF models run at Atmo, and model forecasts shown here are all from the 6 UTC runs this morning. The WRF-GFS forecasts a strong band of showers, and perhaps thunderstorms, moving across eastern Pima County after midnight on Sunday night (above shows forecast of composite radar echoes valid at 5 am). All the models forecast a strong cold front accompanied by gusty winds. Graphic below shows the WRF-GFS forecast for 10-m wind speed valid at 7 am on Monday morning. The WRF forecasts speeds often over 30 mph at the airport during the period from Sunday midnight through the end of the forecast period at 11 am MST on Monday. The WRF precipitation amount forecast for TUS during the event is 0.2", but this may again be conservative.

The current NWS morning forecast for the TUS (airport) grid point is shown above. The forecast is essentially the 06 UTC GEFS forecast verbalized (see below), with the POPs and winds speeds damped down a bit. The accompanying text indicates that winds at the airport could gust as high as 37 mph on Sunday afternoon. 

This leads to the question: How does NWS differentiate between "breezy" and "windy"? I checked the NWS Glossary and found that breezy means winds of 15 to 25 mph and that windy means winds of 20 to 30 mph. Like much that is available on NWS websites this is a bit vague and ambiguous (this is by design since perceptions vary with location - "windy" in Cheyenne, Wyoming, is much different than "windy" in the Tucson metro area). My personal definition for here is: when winds gust over 25 mph it is windy, which sort of fits the glossary, if speeds used there are for gusts (but they probably are not).

The GEFS plumes for QPF above forecast 100% chance of precipitation at the airport between midnight and noon on Monday. The amounts in the models' forecasts now range from around 1/10 to more than 7/10 of an inch. The higher resolution WRF-GFS is at the lower end of this range, but has tended to be conservative during recent months. The 10-m wind speeds (below) definitely forecast a windy event for the airport.

The GEFS forecasts a dramatic change to cold conditions after the front goes by, with the difference in 2-m temperature (above) at the airport dropping from near 70 F on Sunday afternoon to around freezing on Tuesday morning, while temperatures on Monday linger around the mid-40s during the day. The model forecasts of precipitation type (below) also indicate some small chances that there could be snow showers (blue) observed at the airport. All-in-all a very interesting forecast from the models of significant change as February gets underway.

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