Friday, February 17, 2017

Overview Of Developing Weather Situation

The global models have performed amazing well during past 4 or 5 days, and the strong trough they have been predicting is off California, digging toward northern Baja. Above is 00 UTC GFS forecast for 250 mb valid at 12 UTC this morning - note the intense jetstream on northwest side of the trough, which is causing the southeastward movement. The detailed, upper-level structure is complex, as indicated by 1330 UTC water vapor image just below. A surface plot from about 07:00 am MST (second below) shows moderate to heavy rains over much of California, with snowfall already into northwest Arizona.

The ensemble plumes shown here are for TUS QPF - above is from the 00 UTC GEFS, while below is from the SREF (available at SPC web page). Both sets of ensembles indicate 100 % POPs for tomorrow night, with some spill-over before and after. Amounts, however, are much more problematic. The GEFS has two clusterted sets of forecasts - one around 0.60", while the other is around 0.30" - this makes the average (black) somewhat of an artifact. The operational GFS is in the drier cluster.

The SREF models are mostly clustered around 0.25", but the 5 forecasts of more than 0.60" pull the mean toward higher amounts. The WRF variants from Atmo at 06 UTC have airport amounts close to the SREF average (black below).

The WRF continues to forecast very strong southerly winds this afternoon and tomorrow. The 06 UTC WRF-GFS forecasts here are of 10-m winds valid at 03:00 pm this afternoon (above) and at noon tomorrow (below). The forecast for tomorrow indicates very strong winds around the northern flanks of the Santa Ritas.

I also looked at the WRF-GFS forecast for total snowfall during the event (below through 11:00 am on Monday). While the amounts are likely overdone, the forecast indicates that snow will generally stay above the elevations of I-10. However, the high spots along Highway 83 from I-10 south to Sonoita may experience some snowfall. The model forecasts snow at Kitt Peak, although NWS grids only forecast rain there. So, many interesting details to watch this weekend.

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