Yup, it is going to be a scorcher today in northern Utah. South winds are picking up in advance of an approaching upper-level trough and the GFS pops the 700-mb temperatures up to a remarkable +16ºC over Salt Lake City by late this afternoon.
That means we'll see a maximum temperature in the high 90s at the airport. In fact, I wouldn't be surprised if we hit 100, although that's on the outer edge of what is possible. In any event, it will be our hottest day of the year thus far and we will likely break the record for the day, which is 96ºF. In addition, fire-weather conditions look terrible with strong winds, high temperatures, and low relative humidities on tap. Winds will be strongest to our west, but should pick up in the Salt Lake Valley during the afternoon.
Wildfire conditions can't get much worse in early June. This is not a time to be playing with fire or participating in activities that could throw a spark.
A friend pointed out to me that some remarkably large dewpoint depressions have been reported McCarran International Airport in Las Vegas (KLAS) the past few days. The dewpoint depression is the difference between the temperature and the dewpoint. Yesterday, the dewpoint depression at KLAS reach 101ºF at 1500 PDT when the temperature was 102ºF and the dewpoint only 1ºF. That's a relative humidity of 2.3%.
Impressive, although if you look at the graph above you can see that on May 29 there was an observation with a dewpoint depression of 104ºF when the temperature was 93ºF and the dew point -11ºF.
Inspection of the minimum dewpoints over the past 24-h shows that KLAS is the big winner for the lowest in the Las Vegas area, although there is a site on the slopes of Mount Charleston that bottomed out at 2ºF.
How trustworthy the dewpoint observations are at these very low relative humidities is unclear, but the saying "but it's a dry heat" applies pretty well this week.
After several days of blue skies, some weird white stuff spread over northern Utah last night. The clouds can be seen in this morning's infrared satellite image.
Hoping for rain? It's unlikely. The problem is that it is bone dry in the low levels. This morning's upper-air sounding from the Salt Lake City International Airport shows what meteorologists call an inverted-V in which the temperature (red line) and dewpoint (green line) traces form an upside down V.
Such a sounding indicates well-mixed, but dry conditions in the low levels. In this case, the inverted-V is quite deep, which means any precipitation falling from the high-based clouds will need to fall through over 3000 meters of increasingly dry air before reaching the valley floor. If a strong convective cell pops up today, perhaps it could generate measurable precipitation on the valley floor, but in all likelihood virga (snow and rain that evaporate before reaching the ground) will dominate.
Inverted-V soundings are also favorable for strong downdrafts and gusty winds that are generated as precipitation falling into the dry low levels evaporates and cools, creating locally dense, negatively buoyant air that quickly crashes to the ground. The resulting winds, however, can be locally strong.
Finally, any thunderstorms that form under these conditions are potentially life threatening. People have a habit of not seeking shelter during so-called "dry thunderstorms." As the saying goes, when thunder roars, head indoors, even if it is not raining. Now is a good time to review our past post on lightning safety.
As noted in the previous post, a wildfire was sparked in Utah County yesterday. The latest from the Salt Lake Tribune suggests that it was sparked during target shooting yesterday afternoon in the hills west of Utah Lake and south of Saratoga Springs. See utafireinfo.gov and http://inciweb.org/incident/2884/ for updates on the now named AR Fire. Fortunately, no structures are currently threatened.
I've been on the road so much lately that I missed an important snow-cover lowlight. The snowpack SWE at the Snowbird SNOTEL has now officially bottomed out, bringing a pathetically early end to the snow-cover duration at that site.
In the graph above, you can compare this year to last year. There's about a 5-6 week difference, depending on which minimum you use for the end of snow-cover duration during 2011/12.
Most of the SNOTEL sites in the southwest are now snow free. The sites with white squares and nearly all of the red squares below have no remaining snow cover.
Source: NWS/CBRFC
Those hoping to do turns-all-year will be road tripping to the Pac Northwest or working hard to find scraps in Utah and environs this summer.
I was on a river trip the past couple of days and it was a great lesson in the power of evaporative cooling. Temperatures in Moab reached the high 80s yesterday, and floating on a raft with a personal floatation device on was hot, if you were dry. If you were wet, however, it was absolutely frigid and any breeze or gust of wind provided a great lesson in the power of evaporative cooling.
The temperature commonly cited in weather reports is called the dry-bulb temperature by meteorologists. It is the temperature measured by a thermometer that is not exposed to the sun or moisture.
If wet, however, the evaporation of water can affect the temperature of an object. The lowest temperature that an object can reach through the evaporation of water is called the wet-bulb temperature. The wet-bulb temperature is a function of the dry-bulb temperature and the relative humidity. In a dry airmass, the wet-bulb temperature can be much lower than dry-bulb temperature. In a humid airmass, however, they may not differ by much (in fact, they are the same when the relative humidity is 100%).
Back in the day, the wet-bulb temperature (as well as dewpoint and relative humidity) was measured using something called a sling psychrometer. The sling psychrometer was a thermometer with piece of fabric on it that you would wet. You'd then "sling" the psychrometer around and the evaporation of water from the fabric would lower the temperature measured by the thermometer to the wet-bulb temperature. This is a common experiment done in Earth science classes.
Contrary to conventional wisdom, meteorologists don't measure wet-bulb temperature (or dewpoint and relative humidity) manually anymore. Meteorological observations are fully automated.
Nevertheless, I was the human wet-bulb thermometer the past couple of days on the Colorado River, and the observations from the USGS/National Park Service Building in Moab pretty much show what I observed. Yesterday afternoon (29 May), for example, the dry-bulb temperature was in the high 80s yesterday (red line), but the wet-bulb temperature was in the low 50s (green line).
This difference of more than 40F reflects the remarkably dry airmass resident over the upper Colorado basin yesterday. The dewpoint was below 20F and the relative humidity less than 10%.
Thus, there was tremendous potential for evaporative cooling. When you were dry, it was hot in the sun, but when you were wet, your liver would quiver, especially when the wind blew hard and the energy loss to evaporation increased.
This is a common situation in Utah during the warm season and a reason why evaporative (a.k.a. swamp) coolers are quite effective for home cooling. Of course, that effectiveness goes down quickly during monsoon surges when the dewpoint, relative humidity, and wet-bulb temperature are higher.
Not satisfied with its brief appearance during the ski season, winter has returned this Memorial Day Weekend. The observations are a bit squirrelly, but the snow depth at the Alta-Collins site increased about 7 inches in the past 24 hours, with about 1.09" of snow water equivalent.
And the view from the top of Snowbird looks quite inviting.
Pity that we couldn't have gotten this snow in December when it was desperately needed.
Those of you looking for sunny day have no fear. Tomorrow looks beautiful.
Following up on the previous post, check out the coverage of dust (pink colors) over the four corners area in the Aqua-Modis image from this afternoon.
It is nuking in the four corners area where there is an impressive dust storm underway, not to mention a smoke plume from fires in New Mexico. The latter was clearly evident in the 17:55 UTC Terra/MODIS overpass.
Source: http://rapidfire.sci.gsfc.nasa.gov
It's tough to see the dust in the image above, perhaps because the event was just getting going at 1755 UTC (1155 MDT), as shown by the observations from Blanding (K4BL). The latest report from 2055 UTC shos wind gusts to 52 mph with blowing "sand" (probably dust) reducing visibilities to 1 mile.
One can also see wide-spread dust in the four corners area in the visible satellite loop below. Click to enlarge.
Must be fun camping in the 4 corners area today....
Addendum 5:37 PM MDT:
The Naval Research Lab dust algorithm picks up on the dust in the MODIS image above quite nicely. The pink areas below are likely dust and clearly shows the dust that is affecting the blanding area.
