Friday, December 06, 2019

Threat Of Showers Sunday

Nice colors at sunrise today. Above is view from Atmo on campus, and below is from Santiago Peak, southeast of Los Angeles.

The GEFS ensembles have been getting more aggressive with forecast of rainfall at Tucson - above is QPF plumes for TUS from 06 UTC members. The ensembles indicate main rainfall event from Sunday night into midday Monday. Current NWS morning forecast brings event in a bit sooner faster than the ensemble forecasts.

Above shows total PW for 5:00 am MST (12 UTC) this morning. It is currently very dry over Arizona, but there is a broad plume of higher PW along 120 W extending northward to about 30 N. The model forecasts bring this moisture eastward into Arizona - below is 06 UTC WRF-GFS forecast of PW valid at 5:00 am on Sunday morning the 8th.

The 06 UTC GFS operational forecast for 500 mb winds and heights (above) is valid at 5:00 am Monday morning the 9th. The 500 mb short wave affecting Arizona has a strong positive tilt (trough axis runs from west-southwest toward the east-northeast). Positively tilted troughs are often poor precipitation producers, since they don't force strong southerly moisture intrusions - so there's a fair amount of uncertainty with this developing event. Note that the WRF PW forecast above brings the moisture influx from the west across northern Baja.

The total precipitation forecast below is from the 00 UTC WRF-GFS and is valid from now through midnight Monday night. The forecast event is a minor one, with a few spots with around half an inch, particularly in higher elevations. Note that the amount forecast at TUS is much less than that forecast by the GEFS above. It will definitely be interesting to watch how all of this actually evolves.

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