On September 9, 1942, the US Coastguard Cutter Muskeget was torpedoed by a German U-boat killing all on board. At the time, the Muskeget was collecting weather observations in the North Atlantic. Included in the crew were four civilian National Weather Service Meteorologists: Lester S. Fordor of Cleveland, Ohio; George F. Kubach of Sandusky, Ohio; Edward Weber of New York, New York, and Luther H. Brady of Atlanta, Georgia.
Today we remember these four meteorologists and all who died while serving the United States, including more than 400,000 members of the Greatest Generation who made the ultimate sacrifice during World War II.
Pileus is a latin word for a felt cap. In modern English, it refers to the cap of a mushroom. In meteorology, it refers to a cap or lenticular cloud that forms on top of a cumulus or cumulonimbus cloud. There were a plethora of them forming over the cumulus clouds above the Wasatch Range this morning. Below is an example.
The basic formation mechanism involves the pushing up of air aloft as air within the cumulus cloud ascends. If the air is close to saturation, this leads to the formation of a layered cloud that resembles the cap or lenticular clouds that are generated by mountain waves as the air that flows through the ascending layer aloft follows a path similar to that generated by topography. I have tried to illustrate this below, although the perspective was such that I've probably oversimplified things.
There are some good examples in the time lapse below. The cumulus cloud can punch through the pileus layer if it has enough buoyancy.
The pileus has always been one of my favorite clouds.
At around 1700-1800 MDT yesterday afternoon, a brief but violent dust storm or haboob swept through the Salt Lake Valley, degrading air quality and uprooting trees.
As described in the Glossary of Meteorology, the term haboob was first used to describe sand and dust storms in Sudan. It is now commonly used to describe such storms in arid and semiarid regions around the world. It is often used for dust storms in the American southwest, especially Arizona. Below is an example from storm chaser Reed Timmer.
While there are many types of dust storms, haboob is typically applied to those with sharp leading edges caused by a density current or front. It is often clear ahead of the density current, with opaque dust loading behind it.
I first noticed yesterday's haboob at about 1730 MDT when it had spread into the northwest Salt Lake Valley. Looking toward the southwest at that time, a shallow layer of dust was evident at low levels and clearly confined to elevations below the crest of the Oquirrh Mountains.
At that time, strong northwesterly flow was observed in the northwest valley as high as 40 knots at the Kennecott tailings pile just south of I-80 and as high as 25 knots at the Trans Jordan Landfill in the southwest valley near the base of Kennecott's open-pit mine (labeled Bingham Canyon Mine on the plot below).
Source: Mesowest
Time series from the tailings pile show steady northwesterly to northerly flow if 5-10 knots from approximately 1000 MDT to 1500 MDT. From 1500-1700 MDT, that flow strengthened and the temperature fell about 4˚F. At about 1700 MDT, the haboob hit, with sustained winds increasing rapidly to 50 mph and gusts reaching over 60 mph. The temperature also fell more dramatically.
Source: MesoWest
Another perspective is provided below from the Neil Armstrong Academy just a bit further east and where there is a particulate matter sampler. Here, one can see the Haboob passage isn't quite as abrupt, but still led to a peak in wind gusts to about 26 mph and apike in PM2.5 to 60 ug/m3, which is in the unhealthy for sensitive groups category, but not exceptional for such storms in Utah. I've seen worse for sure.
Source: Mesowest
A curious aspect of the Haboob is that it spread southward in the western Salt Lake Valley and then eastward across the southern Salt Lake Valley while the northeastern Salt Lake Valley around the University of Utah remained temporarily clear. Below is a photo looking south through the Salt Lake Valley showing the dust blanketing the southern Salt Lake Valley.
And below is a video showing the haboob surging across the southern Salt Lake Valley.
And, given Blogger's inability to adequately process video in some instances, here's a still.
Source: University of Utah
Eventually dust came into the northeast Salt Lake Valley from the west. I'm not sure why we were temporarily spared, but here are a couple of possibilities. One is that the dust source, which may have been Salt Lake Playa, was such that the northeast Salt Lake Valley simply wasn't immediately downstream for northwest flow. The other is that the haboob density current was blocked initially by the transverse mountains that extend westward between the northeast Salt Lake Valley and Bountiful (i.e., the terrain surrounding City Creek Canyon, including the Avenues Foothills). Some evidence is provided below for 1800 MDT which shows weaker winds in the northeast Salt Lake Valley.
Source: Mesowest
Further analysis is needed to evaluate those hypotheses.
Further analysis is also needed to evaluate the origins of the haboob, but the analysis below shows that it wasn't a microburst. Strong northwesterly flow is evident from the west Desert to the Wasatch Front.
Source: Mesowest
Thus, this was a fairly large-scale feature, possibly enhanced by cooling beneath the frontal precipitation band that lagged the wind shift.
There were reports of wind damage with the haboob, such as the downed trees below in the Avenues.
I also noticed a report of downed power lines in West Jordan.
This morning we awake to a different scene with much colder temperatures and snow blanketed mountains. What a relief to have rain! However, if you don't like this weather, check out the NWS forecast below. Climbing temperatures all week and a forecast high of 94 on Friday!
Forecasts are looking good for a few tenths of an inch of soaking rain tonight.
The latest 12Z NAM forecast valid 6 PM MDT (0000 UTC) this afternoon shows the surface cold front pushing into the Salt Lake Valley. If this forecast holds, the surface front, marked by the abrupt wind shift in the image below, will precede the precipitation band, so the frontal passage will be dry.
However, the lagging frontal precipitation band slides in over the next few hours. The period from 2100-0000 MDT (0300-0600 UTC) looks to feature a good soaking rain, with the NAM producing more than 0.25 inches at the airport for the three hour period. Gardens rejoice!
Rain will then taper off to showers by morning. In total, the 12Z NAM pumps out 0.69 inches of precipitation for the airport, and what a godsend that would be for our crusty soils. The 6Z GFS (the 12Z isn't in yet) is a bit less bullish on precipitatoin, but still produces 0.42". A look at the SREF shows a range of about 0.17 inches to 0.7 inches, with a median of just over 0.35 inches, so we will certainly get something.
Precipitation in the upper elevations will fall as snow (snow levels could drop down to about 6000 feet by tomorrow morning). On average, precipitation increases with elevation and under the right conditions, perhaps one would expect this to produce a big dump, but I'm not too excited about that happening in this one for a few reasons. First, the primary mechanism for precipitation development is the frontal lifting and under those conditions, we don't usually see as strong of an increase of precipitation with elevation. Second, the flow direction at crest level never really comes around to northwesterly. Third, it's a fairly stable event during the frontal passage.
These storm characteristics can be seen in the time-height section below. There's only a brief period of shallow northwesterly flow at 6Z (0000 MDT) and the increase with height in a variable known as equivalent potential temperature (black contours) above 650 mb (about 11,000 feet) is indicative of the stability.
Thus, the mountains will get something, but perhaps not a lot. Even the NAM generates less precipitation in the mountains than in the valley, with 0.27 inches at Alta. The GFS is a bit more bullish with 0.45 inches. I'll call it 3-6 inches, with an upper end of 8 inches if things come together and a bottom of 2 inches if it falls apart. If you feel the need to scratch the itch, the best option will probably be low-angle terrain with a smooth underlying surface. That might actually ski quite well in the creamy snow if we can get 6 inches or more.
Pete Townshend's birthday was earlier this week, so we conclude today with Love Reign O'er Me as a prelude to tonight's rain.
After 33 consecutive days without measurable precipitation, the Salt Lake City International Airport recorded 0.05" of rainfall yesterday.
Hooray!
"Yesterday", however, is defined as the standard-time calendar day. The reality is that the airport received no measurable precipitation during the day yesterday and the .05" actually fell at just before midnight last night standard time.
It appears another .02" fell after midnight and will count toward the total for May 21.
Two days in a row of measurable rain. Will miracles never cease?
Some preliminary precipitation reports through yesterday evening are below and show up to 0.19" in the Salt Lake Valley through 7 PM. That observation was from our mountain meteorology lab facility at the mouth of Red Butte Canyon on the University of Utah campus.
Source: NWS
Overnight rain brought the total at that site to 0.25". I wish it was more, but that's better than nothing.
For the Salt Lake Valley, it looks like this event is over. We may see a stray shower this morning, but skies should clear this afternoon.
The good news is that we have another shot at rain Friday evening and night as a bonafide cold front pushes through. The NAM forecast below is valid at 3 AM MDT (0900 UTC) Saturday and shows widespread precipitation along the Wasatch Front with a sharp temperature contrast indicative of the front between southeast and northwest Utah.
Yesterday's southerly winds wreaked havoc in the West Desert with peak winds along I-80 topping 70 mph. The public intormation statement below from the National Weather Service shows numerous stations with peak wind gusts above 50 mph in the Great Salt Lake Desert and Mountains, including 78 mph at I-80 milepost 29 and 73 mph at another I-80 site.
Source: NWS
A look at the meteograms for I-80 milepost 29 shows that yesterday morning at 0800 MDT the flow was north-northwesterly. This is consistent with the passage of the weak front discussed in yesterday's post. The flow then veered (turned clockwise) over the next four hours, becoming persistently southerly and sustained between 25 and 40 mph from 1400–1900 MDT when the winds began to veer to southwesterly in advance of the front, which passed at just after 2000 MDT and was marked by an abrupt drop in temperature and dewpoint.
Source: MesoWest
The time of peak gust was 1710 MDT. A look at the sustained (red line) and gust (plus signs) speeds from 1400–1900 MDT suggests this was an unusual period. In particular, the gusts were much stronger than the sustained winds at 1710 and the prior observation time 1700 MDT. In other words, the ratio of gust speed to sustained wind speed maximized at this time.
It's unclear why this happened. One possibility is that the stronger gusts reflects the influence of increased turbulent downdrafts associated with precipitation falling aloft. Radar imagery at 1700 MDT (2300 UTC) shows some echoes in the West Desert area. The surface dewpoint at this time was below 20˚F, so the potential for downdrafts as virga fell into this dry near surface air was high.
This is, however, just a hypothesis. In any event, the strong winds and accompanying blowing dust led to the closure of I-80. KSL reported that 4 semis were toppled. Below is a photo provided to them by the Utah Highway Patrol of two tipped semis.
Source: Utah Highway Patrol via KSL
Given this wind-driven event, now is a good time for me to promote yesterday's PBS American Experience feature on Dr. Ted Fujita, better known as "Mr. Tornado." Fujita was known for his meticulous and detailed storm analyses and pioneering research on tornadoes and other phenomenon. He developed the Fujita scale for tornado intensity, which today has been updated to the enhanced Fujita scale (EF-scale). I haven't seen it yet, but it was well received by colleagues. The trailer is below and the full show is available at https://www.pbs.org/video/trailer-mr-tornado-american-experience/.
