Thursday, December 15, 2016

Tomorrow's Weather Event

Models forecasts are still a bit of a mixed bag for late tomorrow and tomorrow night. Windy and cooler tomorrow afternoon - but will showers develop before the front comes by after dark? An end to the very warm weather is certain and chances for showers by Saturday morning are also high, but rain amounts at low elevations continue uncertain.

The MIMIC PW analysis above is for 1100 UTC this morning. While all the weather chatter is emphasizing an atmospheric river (AR) event, it appears that what is happening is more complex than a distinct AR. There is a very broad plume of tropical moisture jumping northward - centered at about 130 W. This plume is merging with weakening ARs from east of Hawaii (max around 140 W) and a second AR to the north from around 140 W into northern California. These all merge, in the forecasts, into a broad influx of high moisture into the West.

The 06 UTC GEFS plumes for QPF at Tucson (TUS - below) have come into pretty good agreement and indicate 100 % POPs of at least 0.01" at the airport (NWS POPs continue at 70% for tomorrow night). The operational version of the model (blue) is very close to the ensemble mean (black). Most of the members forecast showers during the day tomorrow, followed by heavier showers with the front during the night. The GEFS amounts forecast at the airport range from a bit less than 0.20" to a bit more than 0.40" - not a big event, but it would be first rain here at house for December and would also impact the Winterhaven Christmas event which began last weekend.

These two precipitation forecasts are from the 06 UTC WRF-GFS last night - above is on the 5.4 km grid and below is from the 1.8 km version. The really big event stretches from California into Colorado. The high-resolution forecast below continues to forecast little at the airport, but with better amounts over the north and east portions of the metro area, while the Catalinas wring out heaviest precipitation over southeastern arizona.

The forecast sounding at bottom is valid at 7:00 pm MST tomorrow evening and is quite favorable for showers at lower elevations. In fact, the sounding forecasts appear to me to be quite favorable for showers from tomorrow afternoon into the early morning hours Saturday - so I suspect that the WRF may again be low on its QPF forecasts.

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