Wednesday, October 19, 2016

Winds Tomorrow At Mt. Hopkins RAWS Site

I have been studying (with some inputs from John Brown in Boulder) the high winds that occur occasionally, from the east, at the Mt. Hopkins RAWS site (as well as at other weather stations on the F. L. Whipple Observatory Complex). These appear to be caused by the terrain configurations of the Santa Rita Mountains, which lead to high speed gap winds because of the sky island's large massif blocking the synoptic winds. Mike Leuthold has added a sounding point to the WRF model output that is just upstream of the Santa Ritas - the skewT forecasts labelled "Sonoita" are located along Highway 83 about a mile north of the Sonoita junction. The models have been forecasting an east wind event tomorrow for almost a week now, and I want to examine the situation in this post using the 06 UTC WRF forecasts from last night.

The view above is looking west toward the Santa Ritas from about the point used in the WRF for the Sonoita sounding. Mt. Wrightson is the highest peak of this sky island at 9452 ft MSL. The very large and deep saddle (Riley Saddle) to the south (left) of Mt. Wrightson is immediately upwind from Mt. Hopkins and provides a large gap that probably accelerates blocked flow. Mt. Hopkins is physically separated from the main massif and principal ridge-line of the Santa Ritas, adding to the complexity of the situation when winds are from the east.

These two graphics show the Whipple Complex with the main features identified - satellite view above and terrain map below (thanks to Carl Hergenrother for these figures). The RAWS station is considerably lower than both the Observatory and the MMT telescope at the summit. The terrain at the Observatory is obviously very complex, and there appears to be another gap just to the east of the RAWS site.

The two forecasts here are for 10-m winds from the 06 UTC WRF runs at Atmo. Above is from the WRF-NAM valid at 10:00 am MST tomorrow morrning, while the WRF-GFS forecast below is valid at 9:00 am. Both forecasts show blocked flow accelerating around the sky islands of southeast Arizona with light winds at the highest elevations, as per Mt. Lemmon and Mt. Wrightson.

The two WRF-GFS forecast soundings for the Sonoita point shown here are valid at 6:00 am (above) and at 7:00 pm (below), both on the 20th (tomorrow). The strong easterlies don't reach above 750 mb, leaving my forecast somewhat uncertain, particularly since I have not looked at many of these situations to date. There is definitely a zone of strong vertical wind shear at the top of the layer of easterlies, and I am not sure what we will find in the RAWS observations for Mt. Hopkins tomorrow

Finally, I looked at the hour-by-hour NWS grid point forecasts for Mt. Hopkins (above) and for TUS (below). Whatever algorithm is used to generate these forecasts (probably driven by the operational GFS) ends up forecasting max wind gusts of 31 mph at both sites, which is probably not very likely - but we will see what tomorrow brings.

No comments:

Post a Comment