01Z W 4 May 2011 Forecast Discussion

The cold front finally made it through here about 9am MDT this morning, followed by brisk E - SE flow locally, part of a strong Longmont Anticyclone.

The forecast for Thursday has turned out to be more difficult than I thought it would be, particularly on the timing of the onset of winds from the westerly quadrant. I think that by early to mid-afternoon there are likely to be west winds, driven mainly by vertical mixing of WNW flow aloft, mainly above 700mb. It is possible but unlikely that westerly winds will begin as early as 8am MDT; best estimate attm is for a noon west-wind start. If the onset of the west wind is of interest, then starting observations before noon is advised. Wind gusts at 10m appear to me unlikely to get above 15m/s, so this will not be a high-wind event.

Ambient wind at mountain-top level on the Divide looks to not be strong enough to maintain a vertically-propagating mountain wave that would have the proper configuration to favor continuously windy conditions Th night. However, there may be occasional breezy periods from the west overnight, as shear in the nocturnal boundary layer temporarily breaks down to allow some westerly momentum to penetrate close to the surface.

If there is W-quadrant wind on F, it will again most likely be due to vertical mixing of momentum, as I expect the mixed layer will grow deep enough to entrain westerly momentum aloft.

Probability of max gusts on M2 .......... >15m/s .................. > 25m/s

00-06z Th 5 May ................................ near 0% ................. near 0%

06-12z Th 5 May ................................ near 0% ................. near 0%

12-18z Th 5 May ................................. 20% ....................... near 0% most likely late

18z Th 5 May - 00z F 6 May ................ 40% ....................... near 0%

00 - 06z F 6 May ................................ 20% ....................... near 0% most likely early