Flirting with June Gloom

In the far Pacific Northwest they call it the June Gloom, the seemingly never ending extension of the rainy season into June.  Places like Portland and Seattle are typically heavenly (dry and mild) in July and August, but that gorgeous weather always seems to take forever to arrive.  Over the next week, I suspect that northwesterners will be pulling their hair out.  The GFS pretty much keeps them in an active storm track, resulting in frequent periods of rain and forecast accumulations approaching four inches in places.

Source: NCEP

Those are healthy accumulations when you consider that the GFS doesn't fully resolve the mountains and their influence on precipitation.  Unfortunately, Utah remains on the edge of the storm track, with no precipitation forecast.  We only flirt with the June gloom, which is too bad as a good soaker would be appreciated here.  Let's hope the storm track dips a bit further south than progged.

We Are in Outlier Mode

There's some wild weather to be found in western North America thanks to a jet stream that is in outlier mode.

The dynamic tropopause (jet level) analysis for 6 PM MDT yesterday afternoon shows a high amplitude upper-level ridge over Alaska, a short-wave trough over the Yukon, and a closed upper-level low off of the Pacific Northwest coast.

The impact of the Alaska–Yukon ridge-trough system on the weather of parts of Alaska is astounding.  How about a maximum temperature of 90ºF in Valdez yesterday.  How crazy is that?  Their previous record high for the day was 75ºF.  It also broke their all-time record high of 87ºF.  Have a read of the rather enthusiastic record event summary from the National Weather Service.  

Meanwhile, ahead of the closed low off the Pacific Northwest coast, Utah is mired in fire-weather hell. How about these observations from Milford yesterday where at 3:52 PM MDT they reported a temperature of 90ºF and a dewpoint of -18ºF for a dewpoint depression of 108ºF and a relative humidity of 1%. 

It will be even hotter today, although we are due for a cool down beginning tomorrow.  The cause of that cool down is the migration of the closed low off the Pacific Northwest coast into the northwest interior.  Yeah, it was hot in Alaska, but how about some freshies for Glacier National Park?  Looks like a good possibility for tomorrow night and Thursday.

Yup, Mother Nature giveth, and Mother Nature taketh away.  Weird weather happens when the atmosphere is in outlier mode.  

Lessons in Buoyancy at the Spiral Jetty

If you have never taken a trip to the northern Great Salt Lake (a.k.a. Gunnison Bay), do it.  It is a spectacular place, far removed from the hustle and bustle of Salt Lake City.  We made a trip to the Spiral Jetty today and had a great day enjoying the swimming and the sites.

The Spiral Jetty was built by Robert Smithson in 1970

Yeah, that's right, the swimming.  Or, better put, floating as we forgot goggles and the last thing you want to do is get that water in your eyes.  Many people have floated around at Saltair and Antelope Island, which is all fine and dandy, but the salinity in that half of the lake (a.k.a. Gilbert Bay) is "only" about 10–12%, whereas in the north half is usually near 28%.  Ocean water is about 3.5%.

Source: USGS

So, if you really want to bob like a cork, the north arm is the place to do it.  In fact, a float in the north arm is a great lesson in buoyancy, as I discovered today.

I can't hold a pose like that in freshwater, but it's a piece of cake in the northern Great Salt Lake.  This is an example of Archimedes's principle.  The upward force exerted on a body, even one as decayed as mine, equals the weight of the fluid displaced.  Salty water is denser than freshwater, so you don't have to displace as much fluid to balance your weight and you bob higher relative to the water line.  You could do an experiment by taking a day trip to float in Utah Lake, the southern Great Salt Lake, and the northern Great Salt Lake and you'd surely find the worst buoyancy in Utah Lake and the best buoyancy in the northern Great Salt Lake.  Use a toy boat and mark the water line at each site and you'll have clear evidence of Achimedes's principle in action.

Why is the northern Great Salt Lake so salty?  An earthen railroad causeway was built across the center of the Great Salt Lake circa 1960.  There are only a couple of small gaps in this causeway, so there is very little mixing of water between the two halves.  Most of the freshwater inflow comes into the southern half, which dilutes the water.  In contrast, the northern half has little freshwater inflow and is heavily enriched with minerals.

The impact of the construction of the causeway can clearly be seen in the USGS salinity data graphed below.  Note how there was a temporary decrease in salinity in the northern half of the Great Salt Lake (Gunnison Bay) during the huge snow and runoff years of the 1980s, but it still remained substantially higher than that found in the southern half (Gilbert Bay).

Source: USGS

If you want to visit Spiral Jetty and have a look and float for yourself, there is a nice description here.  The road is in excellent condition and is now passable by passenger vehicles around Rozell Point all the way to the Jetty.

Breath Deep

We have reached that time of year when you have to take a few moments to appreciate any cool airmass that pushes into the state.  With a forecast high of only 77ºF at the Salt Lake City Airport, sip and enjoy every breath.  If you can get out for a hike or ride, do it.  The models hint that we might get another cool down later next week, but July is coming and the dog days of summer will be here soon enough.

The hot weather the past several weeks has helped us catch up on where we stand thusfar this year relative to the average climate for the 20th century.  Recall that the statewide annually averaged temperature for Utah has not been below the 20th century average since 1993, and that last year was the hottest in the instrumented record.

Utah's statewide average temperature departure from
the 20th century average.  Source: NCDC

I frequently comment that it is unlikely that we will ever again see a year with a statewide average temperature below the 20th century average, but I was getting pretty nervous in March when we were running cold.  Well, we've made up quite a bit of ground in recent weeks.  We're still 1.5ºF below average for the January–May period, but that's close enough that it can be made up in the coming months.

Source: NCDC

Time will tell where we end up, but I think it's likely than not we will come out in the red again.

More Wild Wind

A cold frontal passage has ushered in some cooler air, but has been accompanied by some very strong winds.  Here are a few of the peak gusts thusfar from around the area:

• Flight Park (near point of the mountain): 72 mph
• Saratoga Springs: 68 mph
• Lehi: 59 mph
• Point of the Mountain: 57 mph
• Highland: 54 mph
• Antelope Island: 53 mph
• Pleasant Grove: 51 mph
• Orem: 50 mph

The Microburst

Schematic of the Traveling Microburst (Fujita 1981)

Yesterday evening I was working on my computer at home when the Emergency Broadcast System alarm went off on the TV in the adjoining room.  It really got my attention since I wasn't expecting much in the way of severe weather, so I thought it was either an Amber alert or perhaps something more serious.  I walked over to where I could see the TV and noticed it was a severe thunderstorm warning issued for Salt Lake and Davis County.

The culprit was a microburst, a localized downburst that produces strong straight line winds at the ground as depicted above.  The one we had last night was an example of a dry microburst as most of the precipitation that generated it evaporated before reaching the ground.  Some meteorologists differentiate between microbursts and macrobursts depending on the size of the area affected.  Indeed, this event may have had macroburst scale, but we will stick with microburst for this discussion.  

Dry microbursts are typically generated in Utah by high based storms.  Precipitation falling from these storms falls into the dry low-level airmass and evaporates, with the resulting cooling generating an area of locally cool, dense air that sinks very rapidly towards the ground.  Environmental conditions were ripe for microburst generation last night with an extremely deep, dry boundary layer extending from the surface to near 500 mb.

Meteorologists use a variable known as Downward Convective Available Potential Energy (DCAPE) to assess the potential strength of downdrafts and downbursts.  Last night, the DCAPE was 1684 joules/kg, which is a very high value indicating the potential for downburst-related strong winds.  What was needed was some precipitation to get things going. 

The curious thing about last nights storm is that it barely showed up on the lowest elevation radar scan that we normally use to examine precipitation (especially in the winter).  It was best picked up by the so-called "third tilt", which is at 1.3º relative to the local horizon.  Even still, the storm was pretty unimpressive with relatively weak radar reflectivities.  

But, the key here was the evaporation of that light precipitation into the dry airmass.  And, it led to very strong winds.  Here are  some storm reports issued by the NWS, which include a 75 mph wind gust.  I saw one house on the news with a very large tree on it.  The residents of that home were very fortunate that it was well constructed!

Finally, we have a video, courtesy George Wilkerson and Steve Krueger, of the microburst rampaging through the Salt Lake Valley.  Keep an eye on the lower left hand corner.   And it will eventually appear.