Sunday, July 31, 2011

PW Climbing – Death Valley Gets Some Action

The GPS-measured precipitable water at KSLC has been climbing steadily the past two days and is now approaching nearly 4 cm/1.5 inches.

The analysis of precipitable water over the western United States at 0600 UTC from the Global Ensemble Forecast System (sorry this is a bit old, but it's all I've got from home) shows PW > 1 inch across most of the southern Great Basin and > 2 inches in southern Arizona (contours).  The color fill shows the departure of analyzed values from climatology in terms of standard deviations (each color fil level represents one standard deviation).  Across much of the Intermountain West, the precipitable water was 2 standard deviations above the mean, with a peak of about 3 standard deviations around Area 51.  Hmmm...

The National Weather Service has issued flash flood watches for portions of southern Utah and Nevada, with a flood advisory presently in place for Death Valley.  Looks like they are getting some rare rains down there, if it is not all evaporating before it reaches the valley floor.

That's All She Wrote

The Alta-Collins snow-depth sensor finally hit 0 today, July 31.

It took an incredible 100+ inches of snowmelt over the past month to finally bring an end to "winter."  There are still a few patches and lines to ski, but for this meteorologist, it's time to start thinking about 2011–12.

Saturday, July 30, 2011

More on Issues with GPS and Sounding Precipitable Water

We've posted a few times about discrepancies between precipitable water measured by GPS and National Weather Service Soundings (e.g., How Moist Is It?).  Bob Maddox, former director of the National Severe Storms Lab, discusses how these two measurements compared during summer 2010 in today's post on his Madweather blog.

Given that the differences are sometimes quite large, I wonder how much of an impact the sounding precipitable water, which frequently looks to be too low in the afternoon, impacts numerical forecasts and reanalyses.

And, in case you are wondering, the precipitable water in soundings from KSLC has ben substantially lower than that measured by GPS for the last two days.

Source: NOAA/ESRL  

Friday, July 29, 2011

Tropical Storm Don Eyes the Southwest

Tropical Storm Don presently moves toward the south Texas coast at about 22 km/hr.  Landfall is expected this evening.

Image: NOAA/National Hurricane Center
Although tropical cyclones are commonly thought not to impact the southwest United States, they frequently modulate and contribute to monsoon precipitation.  This may be the case with Don, which has been tracking to the west-northwest in the easterly flow on the equatorward side of the monster ridge that has been parked over the central and eastern US for the past few weeks.

48-h loop of 925-mb winds, precipitable water, and IR satelite
imagery ending 1500 UTC 29 July 2011. 
In particular, note the slug of high precipitable water accompanying Don across the Gulf of Mexico and how Don's present track is taking it toward northern Mexico, west Texas, and southern New Mexico and Arizona.

We can track the remnants of Don in the GFS forecast initialized 0600 UTC 29 July.  To do this, I've plotted 925-mb absolute vorticity and precipitable water.  The former is a measure of the circulation accompanying the storm, the latter the moisture.  I've also highlighted the 3.5 cm precipitable water contour.

During this period, precipitable water is generally high over the Gulf of Mexico and west of the Sierra Madre and along the Gulf of California.   Lower precipitable water over the Sierra Madre at least partly reflects the fact that there is no lower troposphere over this high terrain area.  As Don moves inland, it is accompanied by a surge of moisture into west Texas and northern Mexico.  A careful look at the loop shows a slug of higher precipitable water (>3.5 cm) penetrating across the northern Sierra Madre, tracking pretty much over Tucson, and merging with the tongue of high precipitable water on the west side of the Sierra Madre.  After that merger, the moisture associated with the remnants of Don is difficult to track.  

0600 UTC 29 Jul GFS forecast through 1800 UTC 1 Aug.
925-mb absolute vorticity color filled, precipitable water
contoured with 3.5 cm contour highlighted.
We'll have to keep an eye on this event and see how this Atlantic hurricane impacts precipitation over the monsoon region.  Ritchie et al. (2011) examine the influence of eastern Pacific tropical cyclones on monsoon precipitation, but I'm not aware of any papers that specifically investigate the influence of Atlantic cyclones.

Thursday, July 28, 2011

US Skier/Snowboarder Avy Fatalities 1999/2000–2009/2010

The summer doldrums have settled in meteorologically and we're stuck in this persistent pattern of weak flow with some afternoon convection.  Being a snow guy, I've never had a great deal of interest in the nuances of convection, so my attention has turned back to issues related to the white stuff.

I was curious about skier and snowboarder avalanche fatalities, especially incidents in which lifts are used to access the backcountry.  The American Avalanche Association provides a summary of US avalanche accidents on their web site, which allows one to get at least a cursory look at the circumstances contributing to these accidents.

Skiers and snowboarders today often use terms like backcountry, frontcountry, slackcountry, and sidecountry.  Steve Romeo has some rational definitions of these terms, but for my interests, I simply broke things down based on whether or not the skier or snowboarder accessed the terrain with or without lift assist.  I consider any terrain outside of ski area boundaries as backcountry.  Mother Nature doesn't really care what you call it.  

Nationally, 72 avalanche fatalities from 1999/2000–2009/2010 involved backcountry travel with no lift assist, 38 involved backcountry travel with lift assist, 7 involved public skiers in open ski-area terrain, 4 involved public skiers in permanently or temporarily closed terrain inside ski-area boundaries, and 2 were ski patrollers conducting avalanche control operations.  

So, nationally, there are roughly 2 pure earn-your-turns backcountry avalanche fatalities for every fatality involving lift assist.

However in Utah during this period, there were 9 fatalities involving backcountry travel with no lift assist and 10 with lift assist.  Thus, we have a relatively high fraction of lift-assist backcountry fatalities compared to the nation as a whole.  I have some ideas why, but perhaps others would like to speculate...

Wednesday, July 27, 2011

Record Paradise SWE?

With the snow at Alta-Collins nearly gone, I'm looking for a new station to keep an eye on and am zeroing on the SNOTEL station near Paradise Ranger Station on Mount Rainier.

As discussed previously, there was an incredible snowpack on Mount Rainier this year.  Although it didn't peak out at record levels, thanks to cool weather, the snowpack SWE is now running the same as it did during its biggest snow year (presumably 1971-72).

Source: Pacific Northwest River Forecast Center
I suspect that puts the current snow depth at record levels, although some sleuthing is needed to confirm that a year other than the record max SWE year didn't sneak in.

Any bets on the last day of measurable snowpack SWE at the Paradise SNOTEL?

Update @ 2:13 PM:

Mark Albright pulled data and reports that snow was on the ground 6 times out of the past 28 summers (since 1984) on July 26.  This year's 50.9 inches is eclipsed only by 53.1" in 1999.  So, we're not at a record, but this is certainly a BIG year.

Reanalyses are retrospective analyses of past conditions that typically concentrate on the atmosphere, ocean, and/or land surface.  They are widely used in atmospheric sciences for everything from climate research to weather forecasting to commercial applications.  Unfortunately, it is surprisingly difficult to find information about reanalyses, including ways to obtain and analyze the data.  Sure, "power users" know how to do this already, but we need reanalysis democracy where everyone has equal access.  Being and educator, I like anything that knocks down the barriers to my students performing science at the speed of thought.

Enter, a wiki to "facilitate comparison between ranalysis and observational datasets." It contains a nice summary of atmospheric and oceanic reanalyses, as well as plotting and data manipulation tools.  This looks like a great start to at least lower the barriers to access.

Tuesday, July 26, 2011

CLN @ 15 inches

The snowdepth at the Collins snow stake has fallen to 15".  Over the past month, the snowdepth has decreased over 100 inches, or a bit more than 3 inches a day.

I have been hoping that we would make it to August with measurable snow at the gauge.  That would be a great psych point and a nice end to a huge season, but with 5 days to go, we might not make it.  Of course, the gauge could get flaky, or an adventurous student could hike up there and make some "adjustments" to make it happen...

Just kidding.

How Moist Is It?

We've already discussed some of the differences in precipitable water measured by soundings and ground-based GPS, but today provides a particularly aggravating example.

A two day loop of IR satellite imagery, Dynamic Tropopause pressure, and precipitable water shows quite nicely the monsoon surge that brought us showers last night, but also the advection of drier air from the west early this morning.

Two day loop (1400 UTC 24 Jul – 1400 UTC 26 Jul 2011) of
dynamic tropopause pressure (shaded), precipitable
water (contours), 925-mb wind, and IR imagery.
These changes in moisture are important for assessing the potential for convection today.  Knowing the importance of looking at real data, I decided to see what the ground-based GPS shows.  Consistent with the loop above, it shows a pronounced peak in precipitable water at over 3 cm just after 0000 UTC yesterday afternoon, followed by a gradual decrease to about 2.6 cm today.
The NWS sounding, however, gives an entirely different impression.  In particular, note the massive dry bias in the 0000 UTC sounding yesterday afternoon (I am assuming the GPS is closer to ground truth).

Comparing yesterdays 0000 UTC sounding with this mornings 1200 UTC sounding shows substantial moistening of the lower troposphere.  

0000 UTC (purple) and 1200 UTC (red, green) 26 Jul 2011
soundings from KSLC.
In fact, from 0000–1200 UTC the measured precipitable water increases  from 2.31 to 2.75 cm.  Although it is unreasonable to expect the 12-h soundings to pick up on the peak after 0000 UTC, the dry bias at 0000 UTC gives a false impression that the PW has increased during this 12-h period, whereas it has actually declined slightly (see GPS meteogram above).

In 2007, Bob Maddox discussed some of the problems with the RRS Sippican Sondes used by the National Weather Service at that time.   I don't know if these sondes are still being used, but it certainly appears that problems persist, with significant implications for weather forecasting and long-term climate records.  

Some things are a shame, but the decline in the quality of our upper-air observing system is a damn shame.   

Monday, July 25, 2011

Deja Vu All Over Again

We have another surge of monsoon moisture into Utah today.  Altostratus has overspread northern Utah and the PW has risen sharply to about 2.25 cm at KSLC.
Similar to the event from last monday, the approach of a mid-lattitude trough in the mid-latitude westerlies may be playing a role in the surge.

Two day loop of PV (color fill), 925 mb wind, and precipitable
water (contours) ending 1600 UTC 25 July 2011.
At issue in these events is to what degree the cyclonic circulation associated with the upper-level trough and associated PV anomaly induces southerly flow at low levels and the advection of monsoon moisture into the Intermountain West.  Is this an important player, or are changes in the large-scale circulation associated with the ridge over the central United States and perhaps embedded disturbances in the easterly flow over Mexico and the Southwest US important?

This issue has been explored to some degree by Stensrud et al. (1997), Fuller and Stensrud (2000), Adams and Stensrud (2007), and Ladwig et al. (2009).  These papers show that easterly waves play an important role in modulating monsoon precipitation and in surges of moisture over the Gulf of California.  Stensrud et al. (1997) suggest that troughs in the midlatitude westerlies may also play a role in surges from the Gulf of California into Arizona.  For our part of the world, troughs in the midlatitude westerlies may be of fundamental importance and could be an important consideration for seasonal and subseasonal forecasts, as well as understanding interannual variability of monsoon precipitation in the Intermountain West.  There could be a good research project here for a motivated student.

Saturday, July 23, 2011

It's Not the Heat It's the Humidity

Yesterday's 108F high in Newark, New Jersey was even higher than Salt Lake's all time record high set on 13 July 2002 of 107F.  Remarkable.

Truly oppressive, however, is the combination of heat and humidity.  Let's do a quick comparison.  When Salt Lake hit 107, the dewpoint was about 40F and the relative humidity 10%.

That's a heat index of "only" 101F.

In contrast, when it hit 108F yesterday in Newark, the dewpoint was 69F and the relative humidity 29%.

 That works out to a heat index of 117F.  Unbearably uncomfortable, although I suspect there were locations with a higher heat index in the region.

Friday, July 22, 2011

The Race of Truth

We take a brief break from the weather to talk about how incredible this years Tour de France is.  Three riders sit within 57 seconds of each other heading into tomorrow's 42 km time trial (a.k.a. the Race of Truth).  Several riders today gave it all and finished the race fully depleted.  Alberto Contador with his aggressive attack early in the ride, Andy Schleck who was still feeling his stage 18 victory in his leg, Cadel Evans who had to fight back from mechanical problems, Thomas Voeckler who tried desperately to keep the yellow jersey, and Pierre Rolland who rode an inspired race and in a real upset won the stage on top of the famous Alpe d'Huez.  Kudos to them all.

Twenty-one switchbacks faced by the riders on
the climb up the Alpe d'Huez.  Source: Wikipedia Commons
Tomorrow every rider will be by themselves, riding against the clock.  No hiding behind team members.  What a race it is going to be.  Will Evans make up the time, or will one of the Schlecks take it?  I don't know, but I expect all three riders to cross the finish line fully depleted.

I don't have a favorite among those three riders, but will be pulling for Levi Leipheimer to take the stage as he has had a very difficult tour this year.

Thursday, July 21, 2011

The End of an Era

Source: NASA
The Atlantis touched down this morning at 5:57 EDT, bringing to an end the shuttle-era at NASA.

Born in 1967, I was a bit to young to remember the moon landing and have no recollection of the later Apollo moon missions, which concluded with mission 17 in 1972.  I did, however, grow up in the shuttle era.  I recall the promises of regular space flight (at 2 week intervals), the testing of Enterprise, the launch of Columbia in 1981, and the catastrophic explosion of Challenger in January 1986.

Source: Wikipedia Commons
The Challenger disaster is one of a handful of moments in my life that I remember vividly where and when I was when I heard the news.  I was in a barber shop, and, upon the completion of my haircut, spent the rest of the day in my dorm following the news.

The shuttle program never lived up to early promise, and proved to cost about $1 billion per launch...far costlier than advertised early in the program.  Nevertheless, I think we will miss it.  There is something inspiring about space flight.  Even though I didn't experience Apollo, I was moved to tears when I visited Kennedy Space Center and stood next to a Saturn-V rocket.

Although manned space flight is at a crossroads, it is worth remembering that NASA does much more, including the development and operation of polar orbiting and low inclination satellites for observing the Earth System.  Such efforts have greatly advanced our understanding of the atmosphere, biosphere, cryosphere, lithosphere, and hydrosphere (that's alot of spheres!).

Source: NASA
Let's hope we will find a way to do big things as we move forward from here.

Arctic Sea Ice Update

The latest (20 Jul 2011) updated from the National Snow and Ice Data Center (NSIDC) shows that the Arctic sea ice extent is currently running lower than it did in 2007, when the September minimum set a record.

The difference is fairly small, so time will tell if this year's minimum will be a new record.  See NSIDC's Arctic Sea Ice News & Analysis web site for more information and updates.

And, further to the south...

Congrats to Andy Schleck for winning todays Tour de France stage in an incredibly daring 60-km breakaway, and to Thomas Voeckler and Cadel Evans for champion-like performances in pursuit.  The three of them, along with Andy's brother Frank, sit within 1 min 12 sec of each other in what is shaping up to be one of the most unbelievable tour finishes.  

Wednesday, July 20, 2011

Altitude, Inspired Oxygen, and the Tour de France

Tomorrow, during a grueling stage 18 of the Tour de France, riders will be making a 2409 m (7902 ft) climb from Revello to the Col Agnel.  This is the first of three major climbs on the day (classified as Haute Category, or Beyond Category), culminating in the highest finish in tour history at Galibier Serre-Chevalier (2645 m/8676 ft).

Tour de France riders are amongst the fittest humans on the planet.  They are the elite of the elite.  Five time winner Miguel Indurain had a resting heart rate of 28 beats per minute and a cardiac output of 50 liters/minute.  In the case of the latter, a fit amateur cyclist has a cardiac output of about 25 liters.

Nevertheless, even at at this elite level, athletic performance is affected by altitude.  Oxygen comprises 21% of the atmosphere.  At sea level, where the mean pressure is about 1000 mb, the partial pressure of oxygen is about 210 mb.  At Revello, it is about 203 mb, and at the Col Agnel it is 152 mb.  That's about a 25.1% reduction during the climb.

But, the partial pressure of inspired air in the lungs is somewhat lower than it is in the atmosphere.  This is because the lungs are an extremely moist place and saturated with water vapor at body temperature (37C).  At that temperature, the saturation vapor pressure for water vapor is 63 mb, and this affects the partial pressure of oxygen in inspired air.

What this means is that 63 mb of the the inspired air will always be associated with water vapor in the lungs.  So, to estimate the partial pressure of oxygen in the lungs, you must first deduct 63 mb from the ambient air pressure and then multiply by 21%.  So, at Revello, the partial pressure of oxygen in inspired air is:

(966.5 mb - 63 mb)*.21 = 190 mb

whereas at Col Agnel it is 

(725.6 mb - 63 mb)*.21 = 139 mb

Because of the effects of water vapor in the lungs, the relative reduction of oxygen in inspired air during the climb is about 26.8%, slightly larger than the relative drop in ambient oxygen.

The graph below illustrates pressure of partial pressure of inspired air versus atmospheric pressure.  

Source: Dill and Evans (1970).  Scales are mmHg, with mb scale
for atmospheric pressure on left.
There are also a host of physiological effects that influence rider performance with increasing altitude, although I'll skip that as it is outside of my area.  The bottom line, however, is that riders who excel at climbing at high altitude will be at an advantage tomorrow.  With eight riders within 4 minutes of the overall lead, it is going to be a very interesting stage. 

Tuesday, July 19, 2011

TDF Stage 16 Weather Impacts

As we anticipated yesterday, it was a wet stage today in the Tour de France, with the weather contributing to some important shuffling in the race for Le Maillot Jaune.

On wet roads, Alberto Contador laid down a surprise attack on the final climb of the day, with Cadel Evans and Samuel Sanchez following in pursuit while the Schleck brothers and Thomas Voeckler fell behind.  On the subsequent descent to the finish, superb bike handling by Evans, a former professional mountain biker, enabled him to open up a small gap on Contador and Sanchez.  The gap was probably 10 seconds at one point, but closed to 3 seconds as the road flattened and Contador and Sanchez were able to team together to make up ground before the finish.

Frank and Andy Schleck lost more substantial ground, with Andy criticizing the descent as dangerous.  Such is bike racing and those who made up ground today took advantage of an opportunity, got out in front to avoid crashes, and utilized their bike handling skills to their benefit on the descent.

The image below shows the upper-level cyclonic potential vorticity anomaly and comma cloud that impacted the race today.

Looking at the GFS forecast, we may be dealing with cool, unsettled weather for the next couple of days, including Thursday's stage when the riders go over three major passes, including the Col Agnel at 2744 m (9000 ft) and a finish at Galibier Serre-Chevalier at 2645 m (8675 ft), the highest finish ever in the tour.

In case you are wondering, the vertical from Revello to Col Agnel is almost 2500m/8000ft!  

Forecasting Challenges of Deep Convection

If you want to be meteorologically humbled, try forecasting deep convection in the Intermountain West.  I thought yesterday we would see some scattered deep convection in the Salt Lake area.  Although it came close, most of the action remained to our south and east and we were left with a few sprinkles from "cumulus pancakus."

Source: NCAR/RAL
As discussed by Doswell (1987) and Johns and Doswell (1992), there are three necessary ingredients for deep convection:
  1. a moist layer of sufficient depth in the low-mid troposphere
  2. steep enough lapse rates to allow for "positive area" (i.e., CAPE)
  3. sufficient lifting for a parcel from the moist layer to reach it's level of free convection (LFC)
In short, this comes down to instability, moisture, and lift.  Instability and moisture are commonly called "thermodynamic factors" and are typically assessed using observed and model soundings, whereas lift can be assessed using model vertical velocities, quasi-geostrophic thinking (e.g., the omega equation), and conceptual models of processes such as thermally driven flows, etc. 

In the midwest, there is often plenty of thermodynamics, and the issue is whether or not there will be sufficient lift (i.e., dynamics) for parcels to reach their level of free convection (often referred to as "breaking the lid").  For example, in yesterday afternoon's sounding from Davenport, IA, there was a whopping 5048 J/kg of surface-based convective available potential energy (CAPE).  

Source: NOAA/NWS/SPC  
That's plenty of thermodynamics!  The problem, however, was that they were sitting right underneath the upper-level ridge.  There was no large-scale lift, and surface-based parcels could not break through the shallow stable layer at the top of the boundary layer and get to their level of free convection.  In other words, the convection was capped, and there was nothing going on in the area around KDVN.
Source: NCAR/RAL
In contrast, the KSLC sounding features limited CAPE, but also no capping layer.  Instead, there's an inverted-V with a deep convective boundary layer, which is surmounted in the middle troposphere by a near-saturated layer.  Some weak stable layers are evident above 500 mb.   Instead of being capped, as was the case at KDVN, parcels were able to reach their level of free convection, but the combination of a marginally stable midlevels and (presumably) entrainment led to the "cumulus pancakus" that we saw yesterday.  

Assessing moisture is a major challenge in our part of the world, and, unfortunately, there is sometimes great uncertainty in its observation.  Take yesterday afternoon for an example.  The precipitable water in the KSLC sounding above is 1.01 inches (26 mm).  However, precipitable water at KSLC is also measured quasi-continuously by ground-based GPS.  That system shows that the PW at the time of the sounding was 35 mm!  

That is a remarkable difference.  In fact, if you look at the time series above, it is the largest error over the past 5 days, although there is clearly a systematic bias for the KSLC sounding to measure a lower precipitable water than the GPS system. 

What to believe?  It's hard to assess the moisture part of the deep-convection menu when dealing with such large uncertainties.  

Monday, July 18, 2011

Tour de France Deluge?

Today is a rest day in the Tour de France, but tomorrows Stage 16 takes the riders from Saint-Paul-Trois (red pin) to Gap (purple pin) in the western Alps.

The stage could be wet.  A strong cyclonic potential vorticity anomaly will be swinging across France tomorrow, with rain sweeping into the western Alps during the race.

GFS dynamic tropopause pressure (red contours), surface wind
(grey vectors) and 6-h accumulated precipitation (color filled
every 5 mm after initial .25 mm contour) at 1200 UTC 19 July 2011.
Source: Don Murray/ESRL.
Velonews reports that up to an inch of rain is expected for the finish in Gap, with temperatures in the 50s or low 60s.  The riders are fortunate that this is a relatively low Alpine stage with a maximum altitude of 1268 m in the Col de Manse.

Source: Velonews
The forecast available on the official Tour de France web site calls for "Persistent and heavy rain, locally thundery, in the morning....mostly cloudy during the stage with outbreaks of rain, thundery at times...fog in places, especially in Manse Pass."

This stage was not expected to tilt the balance of the race, but with the competition for the yellow jersey so tight this year, perhaps the weather will provide an opportunity for someone to make a move.

Monsoon Surge

The IR, precipitable water (contours) and wind vector loop below shows quite nicely the "surge" of moisture from the south and east into Utah over the last couple of days.

As seen in the 0100 UTC (0700 PM MST) regional radar composite below, precipitating convection was confined primarily to eastern Utah yesterday evening, but should be statewide (but scattered) today.

Source: NCAR/RAL
The GFS dynamic tropopause loop (top image below) shows nicely the large-scale evolution responsible for the surge.  We have the persistent long-wave troughing along the west coast and monster ridge over the midwest, but note also the cyclonic potential vorticity (PV) anomaly digging down the back side of the upper-level trough and another cyclonic PV anomaly (a.k.a., easterly wave) moving across northern Mexico in the easterly flow on the subtropical side of the ridge.

The net impact of this large-scale pattern is strong confluence between relatively dry air from over the eastern Pacific Ocean and relatively moist air originating over the Gulf of Mexico.  This produce a strong gradient in precipitable water over our region, which shifted northward and westward across Utah late yesterday and overnight.  This shift partly reflects the amplifying trough on the west coast and apparent retrogression of the large-scale ridge, which could be related to diabatic heating associated with convection over eastern Utah and Colorado yesterday.  The easterly wave helps to enhance moisture transport into the four corners area.   

Thus, after a nice weekend, we should see scattered showers and thunderstorms this afternoon in northern Utah.  One can never complain, however, when it is showery in Salt Lake in July.  

Sunday, July 17, 2011

Great Sunset

There's nothing I like better at the end of a summer weekend than sitting on the porch as the sun sets and reflects of the Wasatch and some high based "cumulus patheticus."

Those of you searching for your pot of gold should climb Lone Peak tomorrow.

Saturday, July 16, 2011

Nuclear Summer

I've seen some e-mails from friends at the University of Oklahoma describing the "nuclear summer" they have been having there.

The time series below, which covers the last 2 weeks, makes me sweat just looking at it.  

As I write this, the temperature in Norman is 104F, but at least the dewpoint is only 63!

Friday, July 15, 2011

Tour de France Weather

Weather has always played a role in the Tour de France, and this year is no exception.  This year's tour has featured a large number of crashes, of which there are probably multiple causes, including weather.

Stage 10 pileup.  Photo: Graham Watson/VeloNews
For crashes, rain can be an issue, especially a light rain, which isn't sufficient to wash all the oil off the asphalt.  Further, the traffic lines in France are often made of plastic rather than paint, and can be quite slick when wet.  This contributed to a crash by Levi Leipheimer during stage 6.  Rain even influences the choice and tuning of equipment used by the riders.

Weather also affects strategy during the race.  Wind, for example, plays an important role in peloton dynamics.  Cross winds make drafting more difficult and can blow the peloton apart.  Special strategies are needed for dealing with strong cross winds, and astute riders can take advantage of the chaos they impose, including Lance Armstrong during the 2009 tour.   Riders can also take advantage of the wind to either launch or reel in attacks, depending on the circumstances.

Tour de France riders are at the limit of human endurance, and temperature and sun exposure play an important role.  The stages yesterday (12) and today (13) were the first mountain stages in the Pyrenees, but temperatures were cool.  Versus announcers suggested that temperatures at the Col du Tourmalet yesterday were about 45F.  Heat tolerance was not an issue for the riders, and the bigger concern was probably staying warm on the descent.  Riders donned windbreakers or stuffed newspapers in their jerseys for the descent.

Profile of Stage 12 of the 2011 Tour de France, including the
Col du Tourmalet.  Source:

On a hot day, however, heat has caused some riders to "crack" during mountain stages, as happened to Bradley Wiggins during the 2010 tour.

Although I've been involved in weather support for the Winter Olympics, I've never had the opportunity to observe the efforts for the Tour de France.  I suspect it is a major undertaking.  The official Tour de France website contains weather forecasts for each of the stages about a day in advance.  The forecast for stage 14 suggest that pass temperatures will be cool tomorrow and that there will winds of as high as 45 km/h, certainly strong enough that they could play a role.

Such forecasts provide some basic guidance for spectators, but the teams are certainly receiving more information than this, including weather observations from along the course during stages.

Thursday, July 14, 2011

Lightning Safety

Photo: John R. Southern/Wikipedia Commons
As reported in today's Salt Lake Tribune, there have been two tragic lightning deaths in Utah this week.  On Monday, a man was struck and killed near the Wedge Overlook in the San Rafael Swell, and yesterday a Boy Scout was killed while camping at Scofield Reservoir.  Apparently, the Boy Scout was moving to shelter when he was struck.  What a tragedy.

Perhaps today is a good day to discuss lightning safety and what you can do to minimize lightning hazard.  The NWS has a brochure on lightning safety available here.  The bottom line is that there is no outdoor place that is safe during a thunderstorm.  Head indoors or get in your car.  When car camping, move from your tent into your car.

Many readers of this blog spend considerable time in the backcountry where one can take steps to minimize (but not eliminate) lightning hazard.  Plan ahead.  Know the forecast.  The rule of thumb to be off high ridges in the afternoons is just that...a rule of thumb.  There are certain situations when this applies and when it doesn't.   Keep an eye to the sky.  When thunderstorms are near, stay away from open areas where you are the highest object and away from tall isolated obstacles.

Keep in mind that many people are struck either before or after it rains.   Don't wait to find shelter or minimize exposure, and linger a bit longer as the storm moves out.

Although there is a tendency for lightning-strike frequency to be higher on ridges and mountains, this does not mean that lightning cannot strike in lowlands or valleys.  For example, a few years ago after a thunderstorm in the Oregon Cascades, we observed a lightning-induced spot-fire at Crater Lake, well below the surrounding rim and nearby mountain.

You can minimize the odds of being struck by going low, but not eliminate it.  If you can get indoors or in a car, do so.

Wednesday, July 13, 2011

Vacation Recalibration

After 2 weeks in the Pac NW, I returned to a remarkably humid, green, and lush Utah late yesterday.  The drive through Sweetzer Summit at sunset was incredible as it looked more like mid-May than mid-July.

Indeed, in trying to recalibrate myself to Utah, I started checking out recent observations for KSLC.  Dewpoints primarily above 50 for the past week with a few peaks in the low 60s!

Remarkable stuff.  No wonder the grass is so green.  Average dewpoints this time of year at KSLC are in the 40s.  Not only do I need to recalibrate for Utah, I need to recalibrate for an early monsoon surge.

We had a brief burst of heavy rain this morning, so I thought I'd look at the radar.  Not an echo in sight!  WTH?

Source: NCAR/RAL
Surprised, I took a look at the visible satellite imagery, which did show that most of the Great Salt Lake basin is presently quiet, but there are some decent cells to our southeast.

Source: NCAR/RAL
Turns out the radar is down for a system upgrade.

Ugh! Full recalibration may need to wait until tomorrow.