Here are a few time series from overnight. They illustrate easterly downslope winds reaching 60 mph at Farmington, 55 mph at Logan, and 51 mph at the University of Utah.
We really whiffed on these forecasts. Here are the NWS forecasts issued 3:39 PM yesterday afternoon. The zone forecast for the Salt Lake Valley has no mention of downslope winds:
TONIGHT...PARTLY CLOUDY. ISOLATED SHOWERS AND THUNDERSTORMS IN
THE EVENING. LOWS IN THE UPPER 40S. CHANCE OF PRECIPITATION
20 PERCENT.
THE EVENING. LOWS IN THE UPPER 40S. CHANCE OF PRECIPITATION
20 PERCENT.
And, for the Northern Wasatch Front, they have a mention of east winds, but the forecast ultimately underestimates the strength of the event by a large degree:
THE EVENING. LOWS IN THE MID 40S. EAST WINDS 10 TO 20 MPH. NEAR
CANYONS...EAST WINDS 10 TO 20 MPH INCREASING TO SOUTHEAST 25 TO
35 MPH LATE IN THE EVENING...THEN BECOMING EAST 15 TO 25 MPH
AFTER MIDNIGHT. CHANCE OF PRECIPITATION 30 PERCENT.
Having forecast for a field program Saturday morning, I know I probably would not have called for such a strong downslope wind event either.
Thus, this is an event worthy of further study to ascertain the ingredients of the strong downslope winds in this event. At issue is whether or not such strong winds could have been better anticipated and what improvements might be needed in numerical modeling to help better anticipate such events in the future.
Just an FYI for future reference: While it's really your only option for going back and looking at previous forecast issuances, the ZFP is probably the least accurate product for knowing what's actually going to happen. First, the formatter somehow averages all the grid points within the zone, so the valley floor would mute out the benches. Then, I'm not sure of SLC's local/regional policy (it does appear they are allowed to hand edit since it mentions "near canyons"), but with the de-emphasis of some legacy text products, many offices publish the zones automatically via a script and the forecaster never sees them ("focus on making accurate grids, don't worry about what words fall out of them"). The website point and click is usually more precise (sometimes too much so), but again you're at the mercy of the formatters. The best place to look is probably in the Hazardous Weather Outlook or Area Forecast Discussion, assuming the forecaster takes the time to write a detailed summary of their thoughts on potential hazards. (note: I am not making any commentary about NWS performance and/or this particular forecast scenario)
ReplyDeleteThe hazardous weather outlook issued early Saturday morning for Saturday and Saturday night is below and contains no suggestion of hazardous winds. I examined these prior to my blog post, as well as the forecast discussions. The first hazardous weather outlook that I could find that mentioned the winds was issued 5:06 PM Saturday. Clearly they were onto the possibility of stronger winds by then, but the lead time was short. The point here is simply that this was not a well anticipated event (by everyone).
DeleteThe lack of consistency between the NWS products has been and remains an issue for the NWS to address. Perhaps the time has come for them to axe the zones altogether.
Jim
THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE WESTERN TWO THIRDS OF
UTAH AND SOUTHWEST WYOMING.
.DAY ONE...TODAY AND TONIGHT
NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. ISOLATED TO
SCATTERED SHOWERS AND THUNDERSTORMS ARE FORECAST TO DEVELOP ACROSS
NORTHERN UTAH AND SOUTHWEST WYOMING TODAY AND TONIGHT.
Jim,
DeleteThanks for posting the HWO. I had no doubt it was a "sneaker" event based on your commentary. I just wanted to post as an aside... when I see someone "citing" the zones, it makes me cringe a little knowing the process (or lack thereof) which goes into them. NOAA Weather Radio may be the biggest "user" of the zones... I'd be curious to know if many other entities actually still use them.
Just a couple of ideas... I think that gusty easterly winds observed at the surface sites along I-80 in WY (such as Rawlins, Rock Springs, Evanston), in combination with cold air advection may be a huge red flag for downslope winds here. If their elevation-adjusted temp is also colder than the Salt Lake Basin (adjustment of about 12-15 degrees F for these sites) that would be a clear case of near-surface CAA. This was the case yesterday, with gusty easterly winds and at least weak CAA through the I-80 corridor. Might be interesting to look closely at these sites in future cases.
ReplyDelete