A comparison of soundings from yesterday morning and this morning shows further strengthening of the inversion over the past 24 hours. Yesterday morning, the surface temperature was 27ºF (-2.8ºC), with temperatures aloft maxing out at 1.4ºC just below 700 mb (10,000 ft).
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| Source: SPC |
This morning, the surface temperature is once again 27ºF (-2.8ºC), but temperatures aloft have increased at all levels up to about 650 mb, with a maximum of 3.6ºC at 773 mb (8000 ft).
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| Source: SPC |
Above the valley floor, the freezing level isn't hit until just over 12,000 feet. You'll find some below freezing air in cold mountain spots this morning, but with the exception of shady north-facing spots, temperatures throughout the Wasatch Range will be above freezing today.
Air quality in the Salt Lake Valley continues to deteriorate. Observations from DAQ sensors and from Trax-mounted sensors operated by the University of Utah are generally between 35 and 60 ug/m3, which is in the unhealthy for sensitive groups category.
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| Source: MesoWest |
Observations from Hawthorne Elementary along 700 East reached 55.2 ug/m3 overnight, just shy of the dreaded red "unhealthy" threshold. They have relaxed some overnight, but remain in the unhealthy for sensitive groups category.
Although there are short term ups and downs in PM2.5 concentrations, with values teaching their highest values late in the day through midnight followed by a decline, the long-term trend is upward. That upward trend is actually not due to the inversion getting stronger (once the valley is capped off, further strengthening doesn't matter). Instead, we're in a situation where emissions continue to accumulate in the valley. Further worsening of the air quality will occur through
at least the end of the week.
The models are flirting with a trough passage this weekend. I say flirting because much depends on what model and ensemble member you look at. Some have a brush by, others a more direct hit. At this point, it's too soon to say if it will crack this thing, give us at least a partial mixout, or leave the air pollution fully intact. The Euro provides a brush-by to the north, but perhaps enough to stir us up a bit.
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| Source: Penn State e-wall |
That being said, there's no guarantee we'll get much relief yet and we'll have to see how the forecasts evolve in the coming days. Steel yourself for a long ordeal, reducing driving, and hope for relief this weekend.
How high in elevation do you generally have to go to get above the worst pollution levels?
ReplyDeleteThe answer to this question depends on the depth of the cold pool in the Salt Lake Valley, depth of pollution, and time of day. If you are near canyon mouths and the cold pool is shallow, there can be a flush of clean air overnight. On the other hand, sometimes the pollution is too deep for this to occur.
DeleteUsually 5500-6000 feet is the top of the gunk layer in most events, although it can be lower or higher depending on the situation.
Incredible - temperature didn't even drop below freezing last night at Snowbird or Mill-D North. But other mountain locations (Brighton) did see a night freeze in the twenties. Any ideas why the nighttime temperature stayed warm in some mountain locations and dropped below freezing in others? Does this have to do with the exact elevation of the inversion layer, or with shape of the terrain?
ReplyDeleteNighttime temperatures under clear skies are very sensitive to the shape of the local terrain and exposure to wind (even light breezes). There typically is a lot of variability in temperature on clear, generally calm nights in the Wasatch. In addition, stations near the canyon floors are often colder than those at mid elevations. That's because local cold pools (topped by an inversion) form in the canyons. However, those cold pools and inversions burn off during the day and are not the same animal as what we have in the Salt Lake Valley.
DeleteIs there a measurement to quantify the seasonal extent (or std dev) of the polar cell? For example, a season with cold troughs on average extending further south will result in a colder season etc. Curious if there's much change in the average or std dev annually. Thanks!
ReplyDeleteJim, have you seen the Purple Air website? I'm sure you have but I haven't seen it mentioned anywhere on your blog yet, and it's a resource for air quality data (from a network of laser particle counters around the Wasatch Front) that you would be interested in, I think:
ReplyDeletehttp://www.purpleair.org/
I am aware of the site. I think that data will be useful, but it probably needs bias correction (values are too high relative not only to DAQ sensors but also U of U sensors). Hopefully once that's done, we'll have another valuable tool for examining the inversions.
DeleteDo you know why Cache County's PM2.5 readings at air.utah.gov have disappeared?
ReplyDeleteI don't. That's a serious problem. Hoping the sensor has simply had a problem and that it will be fixed soon.
DeleteIt looks like fog (not smog, but the dense ground fog and low stratus) has formed out over the west desert and is spreading even during the daytime hours today rather than burning off. That area currently has the least amount of snow cover and the highest dew points, around 30F. It will be interesting to see what the fog does and how much it spreads in this scenario.
ReplyDeleteI just moved to SLC this past fall and have been told that inversions don't occur outside of winter. With that in mind, at what temperature does an inversion usually clear out (or not form), in the absence of a storm to push it out?
ReplyDeleteInversion season is Nov to Feb when the amount of energy provided by the sun isn't enough to heat up and burn off cold pools in the valley.
DeleteDuring that period, when a cold pool is present, you either need to scour it out through increasing winds or mix it out by bringing in cooler air aloft (or both). A strong cold front or cold upper-level trough can do it. Determining whether or not a weaker cold front or upper-level trough can do the job is more difficult and one of the more challenging forecast problems.