Thusfar, the downslope windstorm forecast issued by the National Weather Service has verified quite well. They called for east winds of 35–45 mph, with gusts to 75 mph, in the high-wind watch they issued at 2:55 AM yesterday morning. Peak gusts over 60 mph reported thusfar to MesoWest include:
UTPR2: 73 mph (Portable weather station installed by UDOT)
Centerville (CEN): 70 mph
Centerville (CENWWS): 65 mph
Centerville (CENEWS): 64 mph
Farmington (AP611): 61 mph
As we discussed yesterday, high-amplitude mountain waves of the type producing the current windstorm feature a descending layer of high momentum air, but also a hydraulic jump across which there is often a very rapid decrease in wind speed and, in some cases, a rotor in which there is a circular motion of air about a horizontal axis that leads to a complete reversal of the flow at the ground.
 |
| Source: Whiteman 2000 |
There is strong evidence of the hydraulic jump and rotor this morning. Note in the surface map below the strong easterly and northeasterly winds at sites near the base of the Wasatch Mountains from the Bountiful Bench northward through Farmington and Fruit Heights, which contrasts with the much weaker
westerly flow observed along the Legacy Highway and just west of I-15 near Farmington.
 |
| Source: MesoWest |
Meanwhile, in the central Wasatch Mountains,
we discussed on Sunday morning how the model forecasts for this period were producing dramatically different amounts of precipitation at Alta with the GFS calling for less than a half inch of snow, the 12-km NAM 11.5" of snow, and the 4-km NAM 36" of snow for the period from 12 AM Monday through noon today. This forecast spread was in part related to the position of the precipitation band, which did end up far enough north to give a good dump to the central Wasatch. It looks like 11 inches thusfar at Alta-Collins (the resort reports 12 inches), with about an inch of water. The big winners, however, are the Park City resorts with about 18" of snow thanks to the predominant easterly flow making them the windward side of the Wasatch during this period.
If you look at the forecast model time series, it's interesting how a good chunk of artic air from Canada got entrained into the system from the north and east. I see that Laramie and Cheyenne were a cozy 8 F this afternoon, in April! A lot of cold air seems to have been channeled westward along I-80 in between the Wind River Range (and adjoining ranges to the north in western Wyoming), and the Uintas. I wonder if it was the main catalyst for the downslope wind event. Overall, a pretty complex storm.
ReplyDelete