Tuesday, November 22, 2011

Very Difficult Forecast Situation For Thanksgiving

A very difficult forecast situation has developed for Thanksgiving through Friday morning. The models have been consistent in forecasting a plume of moisture from low-latitudes (apparently triggered by now Cat. 4 hurricane Kenneth) to advect into the Southwest. This occurs ahead of the Pacific trough that will move into the Southwest on Thursday, with resultant (in the forecasts) heavy precipitation. Shown above is NAM 24-hour precipitation for period ending 5 am MST on Friday morning (forecast from 18 UTC data). The Tucson area is in a strong gradient of precipitation, with most significant amounts off in the southeast corner of the state. The corresponding GFS forecast is shown below, with the significant precipitation forecast to avoid Arizona and impact northern Mexico and southern New Mexico. This morning's early run of the Atmo WRF-GFS model forecast was similar to the NAM - with a strong gradient of QPF, ranging from 2.5" over the far corner of Arizona to zero at TUS. All of this is difficult to evaluate, since the forecated moisture plume has yet to become apparent in blended PW products. So, will it materialize? How much of Arizona, if any, will it eventually impact? All of this at less than three days out, indicating that the models still have much uncertainty at large scales for some synoptic situations.

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