Monday, August 13, 2018

Deep Convection and Haboobs

Over the past few days I've travelled to and from Tempe, Arizona where we dropped my daughter off for her freshman year at Arizona State.

It was a pretty exciting period meteorologically.  The first night, we experienced 60+ mph outflow winds from thunderstorms approaching Phoenix from the Mogollon Rim to the northeast, which then provided a great lightning show.  The next night, incredibly, I slept through 3+ inches of rain that fell in Tempe in a huge deluge.

My daughter texted me late yesterday saying they had just had a haboob, that the power was out, and she was holed up somewhere on campus until the storm passed.  A haboob is a dust storm that is frequently initiated by the outflow from thunderstorms and typically found in desert regions of the world.

For yesterday's haboob in Phoenix, a quick look around YouTube unearthed the video below that is available here and I hope is accurate for the date and time.  Note in particular the deep convection on the right of the frame, with the outflow and concomitant dust running well away from the storm.

Imagery from the KIWA radar operated by the National Weather Service shows the evolution very well.  The leading edge of the outflow and haboob, known as a gust front, is indicated by an arc shaped "fine line" that moves southwestward across the Phoenix Airport (red square) well in advance of the thunderstorm that produced it.


Passage of the gust front is easy to spot in the meteogram from the Phoenix airport.  Note the abrupt 20˚F drop in temperature and increase in sustained winds to 45+ mph with gusts to 58 mph.


The fine line enables meteorologists to track features like this and provide very good short-range forecasts of hazardous wether in conditions like this.  The fine line is produced by radar returns not from precipitation, but instead from what meteorologists refer to as "nonmeteorological" scatterers or targets, such as bugs, dust, sand, and the like.

Modern radars collect statistics on radar pulses that allow meteorologists to identify non-meteorological scatterers.  Non-meteorological scatterers tend to produce complex radar scattering signals that show lower correlations from radar pulse to radar pulse compared to precipitation.  As a result, a product known as correlation coefficient is produced by National Weather Service radars that can be used to discriminate between non-meteorological scatters and precipitation.  Below is the correlation coefficient analysis for the radar scan collected right after the passage of the fine-line past the Phoenix Area.  Values accompanying the thunderstorm are near 1 (red), whereas those accompanying the fine line are lower (.8 to .9, light blue, green, and yellow).


Haboobs provide an excellent example of how thunderstorm impacts can occur at a great distance from the storms themselves.  Thanks to modern technology, I found warnings from the National Weather Service, which appeared in a timely manner on my phone, were extremely helpful while we were traveling across Arizona. 

Wednesday, August 8, 2018

My Seasonal Outlook

The Silly Season is upon us when prognosticators spouting an alphabet soup of climate phenomena (e.g., ENSO, PDO, NAO, etc.) make claims for what this ski season will be like.

As far as northern Utah is concerned, ignore all this crap.  The correlations are low.  The useful skill is non-existent.  Yeah, I know there is a 70% chance of El Nino.  Who cares.  Nobody knows what kind of ski season the Wasatch are going to have.  Nobody.  This is why I call it the silly season

Here's my official forecast, previewed on twitter a couple of weeks ago.


There, I feel better now.

Sunday, August 5, 2018

Smoke Filled Weekend

Yesterday's imagary from the Suomi NPP satellite shows how northern Utah has been in the crosshairs of plumes from fires further upstream, especially California. 


PM2.5 concentrations at Hawthorne Elementary have been in the moderate to unhealthy for sensitive groups categories for about 48 hours, and in the moderate category at times for several day before that. 


This is one of the worst stretches of smoke that I can remember and I've developed a persistent cough in the last couple of days. 

Dry, clean, cool air is desperately needed!

Friday, August 3, 2018

Contrasting Heat Waves: July 1960 vs. 2018

Motivated by an intriguing tweet by University of East Anglia Professor Tim Osborn, I thought we would compare global temperatures for July of 1960, which was the warmest 20th century July at the Salt Lake City International Airport to July of 2018, which was the 5th warmest on record and also feature remarkable temperatures in many regions across the Northern Hemisphere. 

The graph below illustrates the anomalous warmth of July 1960 as well as the recent string of very warm Julys in the 21st century. 

Source: NOAA Regional Climate Centers
For convenience, I am going to examine the surface temperature departure (or anomaly) from the 1981-2010 average from the NCEP/NCAR reanalysis for the two Julys.  There are better datasets for doing a surface temperature comparison, but the differences are not going to be all that significant.

In July 1960, anomalous warmth covered much of the western continental U.S. and southwest Canada, with the most anomalous temperatures along the US-Canadian border.  Elswhere, one sees a mixture of both cold and warm anomalies with Most of southern and central Europe and central Asia being cold and far northern Europe warm.  By eye, it appears that overall the Northern Hemisphere is probably a bit cooler than the 1981-2010 climo, which is what we would expect given the global scale warming that has occurred since 1960.

Source: NOAA/ESRL
In July 2018, we see a different story.  Not surprisingly, anomalous warmth exists over the western U.S., but not the dominance of warm anomalies elsewhere, including maxima in Scandanavia, North Africa, and central Asia.  Cold anomalies are more localized.  

Source: NOAA/ESRL
This is consistent with the warming that has occurred in recent decades, which has stacked the deck for warmth, so that longer, more severe heat waves are more likely.  

Tim Osborne's tweet went a step further than my simple analysis, however.  He compared the 1976 and 2018 heatwaves in the UK (relative to the 1960-1990 average, which is 20 years sooner than the plots above), but then examined what would happen based on climate projections for 2070 and scenarios in which greenhouse gas emissions peak in 2080 (RCP6.0) or are aggressively reduced to limit the global average temperatures to less than 2˚C above preindustrial.  In either case, we have more warming in the pipeline and are facing a hotter future with longer, more severe heat waves.  How much hotter depends on future greenhouse gas emissions.  

Source: @TimOsbornClim (https://twitter.com/TimOsbornClim/status/1023950044234502144)
This is why "Normal Is Gone Forever".  The future climate is dramatically different from the one you grew up with.  

It should be noted, however, that the emergence of climate shifts depends on the variable.  Detectable and significant shifts in temperature are early "emergers".  Shifts in other variables, like snowfall and snowpack are occurring, but may emerge more slowly in some regions compared to others (e.g., declines in snowfall and snowpack may occur later or more slowly in colder regions than warmer regions).  For more discussion of this, see our August 2013 posts Western Snow Trends and Global Warming Part I and Part II.

Wednesday, August 1, 2018

July in the Rear View Mirror, Thankfully

A little something to get you going on this August 1st as we put July in the rear view mirror and begin to look forward to cooler times.


The weather this July can best be described using words like ugly and heinous.  

At the Salt Lake City International Airport, the mean temperature for the month was 83.1˚F, tied for 5th warmest all time.  
Source: NOAA Regional Climate Centers
And here's a factoid for you.  The 11 warmest Julys have all happened in the 21st century.  People in Salt Lake simply didn't suffer like this in the 20th century.  If you go back to Y2K, the hottest July on record was 81.2˚F (1960).  Now, it is 85.3˚F (2017). 

Source: NOAA Regional Climate Centers
Digging in beyond the mean monthly temperature, the mean maximum and mean minimum for the month were 95.5˚F and 70.7˚F, respectively.  The former only rates as the 14th warmest on record, but the latter is the 3rd warmest.  Minimum temperatures are a big driver of comfort as they allow for cooling of the home and good overnight sleeping weather.  Sadly, this July saw a very high mean minimum temperature, 21 days with a minimum temperature 70˚F or higher, and a run of 13 consecutive days (5-17 July) above that threshold.

It is interesting to compare July 2018 to the hottest July in the 20th century, 1960.  1960 featured a mean temperature of 81.2˚F, 0.9˚F cooler than this July, but the average daily temperature range was much larger with maximum and minimum temperatures of 98.2˚F and 64.2˚F, respectively.  I suspect 1960 would have been far more comfortable as a whole due to the lower minimum temperatures.  

How about precipitation?  It was scant.  The total for the month was 0.19 inches.  That rates as the 34th driest out of 145 years, so not exceptionally dry, but dry nonetheless.  If you are wondering, there have been four Julys with only a trace, so it is possible to get "shut out" or at least nearly shut out.

For the summer to date (i.e., 1 June - 31 July), we've had 0.24" of precipitation, which rates as the 11th driest.  I've noticed that virtually nothing is growing in my gardens and I've mowed my grass about once in the past 6 weeks.  I try to water at what I consider to be a minimum plant survival level and little rain and the heat, it appears I'm right on that level. 

July is the month with the smallest amount of year-to-year variability in temperature in Salt Lake City.  If one looks at the chart above and does an eyaball, there's about a 5˚F range in mean temperature for the month, with a few outliers.  One of those is 1993 when the mean temperature was 69.9˚F (and the mean minimum temperature was and incredible 56.1˚F).  I have a dream that July 2019 is the new July 1993.