03z M 9 May 2011 Forecast Discussion

After hanging in from the east most of the day, the wind here in Boulder finally turned into the W < 25mph about 01z. M2 is still not available, but the Siemans turbine is barely turning, so I presume winds are lighter on the flats than they are here in Boulder this evening.

This evening there is rather low pressure at the surface over much of the West. The boundary between the north Pacific air that has come in over the Pacific Northwest and much of California at 02z lies from near the WY-SD-NE triple point to just south of Casper to the northwest corner of CO across UT to northwest of Desert Rock NV. It appears to be slowly edging its way eastward. Aloft, the associated cold trough has its axis north-south across eastern WA southward across extreme western NV and thence offshore near Santa Barbara. It appears to me that there is a well-marked embedded short-wave feature aloft that is coming across central and southern CA attm. It is this feature that offers the next possibility for appreciable wind on the flats.

The models seem to be coalescing about the evolution suggested by the Europeans yda, namely, that this prominent short-wave feature now approaching central and southern CA will rotate around the bottom of the trough and approach us from the SW, passing near us in the 00-03z time frame. It will be accompanied by development of a surface low in northeast CO / southwest NE Monday afternoon. This argues for probability of W wind beginning on the flats, say, early to mid afternoon on Monday and continuing for a time Monday night and possibly into Tuesday. The chief concern I have attm is that the overall flow may be a bit too southwesterly for optimum conditions on the flats, but I consider this to be a low enough risk that this opportunity should not be shunned.

Following this, it is pretty certain that the main trough will remain to the west, and that another high-plains surface development will occur on W or Th. Timing of this is uncertain, but I anticipate that there will likely be a gap of 24h or so between the end of the first wind event and the start of the second, assuming the latter materializes.

As the westerlies develop with the frontal passage M afternoon or early evening, light pcpn is likely from a highly convective Foehn cloud over the Divide that contains weak, glaciated Cbs that dissipate as they move off the Divide toward the plains. If the wind really cranks up, I think this precipitation will be fairly short lived, and be toward the beginning of the wind event. Nevertheless, I put the probability of measurable pcpn in Boulder between 18z M and 00z Tu as 70%, and between 00 and 06z Tu as 50%, this latter most likely early in the 00-06z period. The pcpn is not likely to be continuous if wind is blowing hard from the W-NW, but to come in occasional bursts associated with individual convective elements coming off the Divide.

Between the first and second wind event, assuming both occur, I think a period of lighter winds, probably mainly from the NE, is likely, and pcpn is likely as well during the wind remission. There is some possibility of pcpn with a deep convective Foehn cloud on the Divide with the second wind event as well as with the first.

Probability of M2 max gusts exceeding ... 15m/s .......... 25m/s

00-06z M 9 May ..................................... 20% ............ near 0%

06-12z M 9 May ......................................20% ............. near 0%

12 - 18z M 9 May ..................................... 40% ............ near 0%

18z M 9 - 00z Tu 10 May .........................70% ............. 20%

00-06z Tu 10 May ..................................70% ............. 20%