Monsoon Breaks and Monsoon Surges

A monsoon is a seasonal shift in the winds that produce pronounced changes in precipitation.  In North America, the North American Monsoon System is a circulation system characterized by mid-and-upper level high pressure centered over the southwest United States that brings easterly flow across much of northern Mexico and showers and thunderstorms to that region and portions of the southwest United States.  

Precipitation associated with the North American Monsoon waxes and wanes.  These variations are called monsoon breaks (little to no precipitation) and monsoon surges (more frequent precipitation).  

Precipitation associated with the North American Monsoon is heaviest and most frequent in northwest Mexico and decreases to the west and north.  In northwest Mexico, monsoon precipitation is more frequent and may occur daily except during monsoon breaks.  

Source: https://www.climate.gov/news-features/blogs/enso/north-american-monsoon

In contrast, northern Utah is on the periphery of the monsoon region.  Monsoon precipitation is less frequent in our region and occurs during and following monsoon surges when moisture is drawn into our area from the monsoon region.  

We've seen pronounced monsoon surges and breaks in the past week.  A major surge on Friday produce a round of severe convection, including the tornado near Fruit Heights.  Then we moved into a break, followed by another surge on Tuesday.  Each of the surges was associated with an upper-level trough in the midlatitude westerlies that helped to draw monsoon moisture northward and provided some enhanced flow and wind shear to help with convective storm dynamics.  

We are currently in a monsoon break and experiencing splendid weather over northern Utah.  The next possible monsoon surge for northern Utah occurs Sunday into Monday, but this one has a lot of moving parts and how it plays out is quite uncertain for our area.  Below is the ECMWF AI Forecast System (AIFS) forecast for 0000 UTC 28 September (1800 MDT Sunday).   An upper-level trough in the westerlies lies along the Pacific Coast (dashed line).  Ahead of this system, the southwesterly flow draws moisture from the remnants of Hurricane Odalys (westernmost red circle) into Utah.  Also contributing to the monsoon surge is a plume of moisture transport from Hurricane Polo (easternmost red circle) that moves up the Gulf of California and into southeast Utah. 

That's a lot of moving parts and contributors.  Whether or not this surge produces some showers and thunderstorms in northenr Utah will depend a lot of the paths of Odalys and Polo and the strength and speeed of the upper-level trough.  

Regardless, if you are planning outdoor activities in the southeast US, it's worth monitoring forecasts over the next several days.  

The Mt. Timpanogos Lightning Fatality

Friday was a remarkably active thunderstorm day in northern Utah, including a verified tornado in the Wasatch Mountains east of Fruit Heights.  

Sadly, a 19 year old man was killed by lightning on the summit of Mt. Timpanogos Friday morning.  Media reports suggest the victim was taking shelter during the storm in the shack on the summit, with authorities called around 6 AM.  Lightningmaps.org provides access to strike data for 24-hour periods.  The map below is for 0600 UTC 18 September to 0600 UTC 19 September (Midnight to Midnight MDT Friday) showing the high density of strikes in the Mt. Timpanogos area and surrounding region.  The prevailing flow was such that the storms on Friday were moving from southwest to northeast and you can see a strong clustering extending from southwest of Utah Lake over Mt. Timpanogos.  

Source: https://www.lightningmaps.org/

It's unclear precisely when the lightning strike occurred, but at the time of the reported call to authorities, a strong cell with maximum reflectivities > 50 dBZ was located just to the southwest of Mt. Timpanogos with lower reflectivities extending downstream of the summit.  Lightning can occur many miles away from the strongest cells, including as much as 25 miles from rainfall and beneath the anvil that spreads downstream from the storm, so this could be the cell that produced the lightning, although this requires confirmation with a better estimate of the time of the strike and analysis of the lightning data.  The earliest reported strike I could find on Mt. Timpanogos was at 1223 UTC (0623 MDT) and there was one later on the summit at 1415 UTC (0815 UTC).  These are a bit later than the reported call and produced by cells that developed later than the image below. Lightning detection is also not perfect so it is possible that the strike was missed.  More work is needed to piece together the timeline.  

I don't know anything about the circumstances leading up to Friday's fatality and as a long-time hiker, skier, and backpacker, I've certainly been in some less-than-desirable situations when it comes to thunderstorms (and other weather events).  In fact, I decided to become a meteorologist after experiencing a thunderstorm on a backpacking trip with my father.  I never wanted that to happen again!  

So let's spend some time on lightning safety review.  For those of us who travel in the backcountry, well away from substantial buildings, the pamphlet below provides useful guidance for lightning safety from the National Weather Service and National Outdoor Leadership School.  

Source: https://www.weather.gov/media/owlie/backcountry_lightning.pdf

It sounds like the victim and maybe others were taking shelter in the shack on Mt. Timpanogos.  Perhaps this was to get out of the rain or possibly hail that was reported in other storms.  Unfortunately a shelter like that does not provide lightning protection.  This is true not only of the shack on top of Mt. Timpanogos, but many beach, golf, picnic, and other shelters if they are not equipped with a lightning protection system. Such shelters are often designed for sun or rain protection, but often do not provide any lightning protection.   More substantial buildings like most homes in the United States are not equipped with a lightning protection system, but the wiring and plumbing provide some protection from the lightning electromagnetic pulse.  This is why it is best to move indoors into a house or substantial building when you hear thunder.  The biggest threat from lightning in such buildings is fire, but there's usually time to move to safety if that occurs.  Metal-topped vehicles like cars also provide protection, but soft-topped vehicles, convertibles, golf carts, and side-by-sides do not. Hence the National Weather Service recommendation below.

If you are interested in the details of what buildings and shelters do and do not provide lightning protection, see Use Caution with the Word 'Shelter' in Lightning Safety by Daile Zhang and coauthors.  

Northern Utah Mesocyclone

The NWS issued a tornado warning this afternoon for an area including portions of Weber, Davis, and Morgan Counties.

Of concern is one of the best developed mesocyclones (a rotating vortex about 10 km across) I've seen in northern Utah.  Radar reflectivity showed a classic "hook echo" associated with the storm as it impinged on the northern Wasatch near Farmington. 

NWS Doppler radars measure the radial velocity toward or away from the radar, so mesocyclones produce a very clear signature of rotation with a couplet of inbound (green and blue) and outbound (red) velocities colocated with the hook.  This is indicative of the small scale rotation associated with the storm. 

The mesoscale rotation does not necessarily mean that there is a tornado.  About 25% of mesocyclones produce tornadoes, but that's certainly enough to warrant a warning in this case.  

Let's hope this was just a well-developed mesocyclone that didn't produce a tornado.  

Latest CPC Seasonal Outlooks

CPC released their latest seasonal outlooks yesterday.  These are based on overlapping 3-month periods.  For November to January and December to February, all or nearly all of Utah is in "equal chances" for precipitation, which means there is no significant shift in the odds for below normal, near normal, or above normal compared to climatology.  

Source: https://www.cpc.ncep.noaa.gov/products/predictions/90day/

In other words, that period could go any way.  There's no useful skill.  

Later in the winter, during January to March and February to April, most of Utah is in the first level of "above normal" or, in the case of southern Utah, especially later in the winter, in the second level.  For southern Utah, this means there is an increase in the odds of above normal precipitation to 40-50%.  For northern Utah, that shift is much smaller and perhaps to around 35% or so for the Wasatch Range.  

Source: https://www.cpc.ncep.noaa.gov/products/predictions/90day/

In other words, something like a 36% chance of above normal, 33% of normal, and 31% of below normal.  

The long and short of this is that the far southwest has the highest likelihoods of above normal.  These odds decline and northern Utah is pretty much in a spot where there is not a lot of weighting of the dice.  The outlook this year largely reflects historical influences of El Nino and strong El Ninos, although predictions by climate forecast systems (numerical models that forecast several months in advance), and other tools are also used.

So, whether we see an above average snow season in the Wasatch this winter is still pretty much up in the air.  My advice is to just ignore all these predictions for the Wasatch and spend that time getting into shape and enjoying the outdoors.  

Status of weather.utah.edu

Some of you have asked about the status of weather.utah.edu as it has been down for off campus access now for at least 24 hours.  

I was told that "the campus default deny seems to be blocking weather.utah.edu from outside of campus."  I really have no idea what this means other than you can't get to it from off campus.  The people that help me with the web site told me they have contacted the powers that be, but it remains down.  

Those of you who are affiliated with the University of Utah, including my students, can access weather.utah.edu if you fire up the campus VPN from an off campus location.  That's the best I can do for now other than hoping we'll be back in business soon. 

I'll just say that the struggle at the U is real.  

An Out-on-the-Limb Maximum Temperature

Yesterday's high of 93°F at the Salt Lake International Airport was a record for the day but not really all that exceptional when you consider that the record high on the previous day (September 11) is 99°F and a couple days later (September 15) is 97°F.  Somehow September 13 has never gone out-on-the-limb for maximum temperature compared to the surrounding days.  Such are the realities of climate statistics. 

That said, yesterday's maximum temperature was out-on-the-limb in terms of what was forecast.  Several days ago, we posted that the forecast for tomorrow (Sunday) was uncertain.  Some models, like the GFS, were bringing in a cold front on Saturday, resulting in much cooler weather for Sunday.  Others, like the ECMWF AIFS brought the front in during the day on Sunday.  I'm not sure if any called for the cold front to come in Sunday evening.    

These days one can process the forecast from well over 100 ensemble forecasts to produce a range of possible forecast outcomes.  This is what the National Weather Service does for their National Blend of Models (NBM) forecast.  On Tuesday we posted about the enormous spread for the maximum temperature forecast for yesterday (see An Uncertain Sunday) with the middle 50% of high temperature forecasts lying between 68° and 84°F (see the tallest red box in the left-hand graphic below).  

That middle 50% is sometimes referred to as the interquartile range.  However, half the forecasts lie outside of that box.  The whiskers surrounding the box extend through the middle 80% of forecasts (or from what is called the 10th to 90th percentile).  In this case, the top whisker extended to 92°F.  That means that 90% of the forecasts called for a maximum temperature at or below 92°F.  

I went back to that NBM forecast distribution and it turns out it ended at 92°F.  In other words, not a single forecast member called for a maximum at or above 93°F.  The observed maximum temperature on Sunday was outside of the entire predicted ensemble range.  

Ideally that would not happen except in the most exceptional circumstances, but our ensembles are not perfect.  Forecasts do sometimes verify outside of the ensemble range and that's a sign that we still have work to do.  

Part of what made the forecast for Sunday challenging was the occurrence of something called anticyclonic wave breaking (AWB).  If you have been to the beach, you have experienced wave breaking.  As water waves approach the shore, they decelerate and steepen.  Eventually the waves steepen to a point where they overturn and break down into turbulence, or what scientists call wave breaking.

Breaking wave. Source: NOAA.

The waves you experience at the beach are gravity waves that occur at the interface between the high-density water and the lower-density atmosphere.  Gravity waves also occur in the atmosphere where gravity wave breaking is one important cause of aircraft turbulence. 

In AWB the waves that are breaking are not gravity waves but very large-scale atmospheric waves known as Rossby waves.  Rossby waves are named after Carl-Gustaf Rossby, who did pioneering work on the jet stream and the dynamics of upper-level waves in the middle 20th century.  He was even featured on the cover of Time Magazine in December 1956.

Source: https://content.time.com/time/covers/0,16641,19561217,00.html 

Rossby waves are the large-scale upper-level troughs and ridges that we observe at upper-levels (i.e., jet-stream level) in the atmosphere.  These waves are so big (1000 km or more across) that the rotation of the Earth strongly affects their behavior.  

Like waves at the beach, Rossby waves can also "steepen."  In contrast to water waves that steepen in the vertical, Rossby waves "steepen" in the north-south direction, which is what happened last week over the north Pacific. At 1200 UTC 10 September, there was a ridge over western Alaska and a trough over the northeast Pacific.  This ridge-trough system amplified over the next 24 hours, with the 500-mb height contours "steepening" and turning clockwise (anticyclonically) on the downstream side of the ridge.  Eventually, the 500-mb height contour "broke" in the sense that higher 500-mb heights were north of lower 500-mb heights.  This is consistent with the red contour crossing the purple meridian three times at the end of the plot sequence below.  Think of the ridge pushing over that meridian as an overturning wave with the surfer in the the tube or trough at lower elevations.  

AWB is associated with the amplification of a strong upper-level ridge, which is often related to the release of heat by condensation in precipitation systems just upstream of the ridge.  Although AWB events are often predicted reasonably well in terms of their occurrence, the details (i.e., the "turbulence") can be hard to get right.  Sunday was a hard forecast because the strength and position of the trough and cold front was challenging to anticipate.  The 93°F we hit on Sunday reflects the fact that the AWB produced a stronger trough than anticipated, delaying its progression across the Pacific Northwest.  

Surfers can be surprised by breaking wave behavior.  So can meteorologists.