Thursday, September 27, 2012

Typhoons and Potential Connections to Alaska

Source: Joint Typhoon Warning Center
That's super typhoon Jelewat, which presently has maximum sustained winds of 135 mph, with gusts to 165 mph.  The maximum significant wave height is 48 feet.  That will get your attention.

The current track forecast has it turning clockwise, brushing by Taiwan and then moving across Okinawa.  Little wonder why the US Navy and Pacific Air Forces are so concerned about typhoons.
Source: Joint Typhoon Warning Center
I've been looking at the long-range forecasts produced by the GFS and after undergoing what is known as extratropical transition, the remnants of Jelewat move over Nome, Alaska by October 4th.  The weak low center over western Alaska in the image below is it.

It is pretty common for tropical cyclone remnants to move across the North Pacific and undergo reintensification (a process known as extratropical transition).  Many move into the high latitudes.  What role do these events play in the climate of Alaska and the Arctic?  This strikes me as a potentially interesting avenue of research.  Some relevant discussion of major events from weather historian Christopher Burt is available here.  

Wednesday, September 26, 2012

Wild Times in the Gulf of Alaska

As we approach the cool-season, life in the midlatitudes is starting to get interesting.  Today we have an example of an explosively deepening cyclone or bomb over the Gulf of Alaska.  Watch in the loop below how the cyclone deepens from a central pressure of about 1002 mb to less than 960 mb 24 hours.

1200 UTC 25 Sep – 1200 UTC 26 Sep IR imagery and
GFS sea level pressure analysis
The term "bomb" was used in the classic paper by Sanders and Gyakum (1980), which used that label for extratropical cyclones with a central pressure that falls at least 24 mb in 24 hours.

Title and abstract of the seminal paper by
Sanders and Gyakum (1980)
Sanders and Gyakum (1980) showed that bombs in the Northern Hemisphere are most common over oceanic areas, especially along the Gulf Stream in the Atlantic and and from along the Kuroshio Current to the Gulf of Alaska over the Pacific Ocean.  Their analysis was based on a small sample, but more recent climatologies reflect this distribution.

Distribution of bomb events from Sanders and Gyakum (1980)
At the time, bombs were poorly forecast.  Growing up on the east coast, I experienced this first hand as forecasts of nor'easters (cyclones that frequently develop explosively and move up the US east coast) were notoriously bad.  Although annoying for landlubbers, these cyclones are particularly problematic for marine safety and shipping as they can produce hurricane-force winds and high seas.

Major field programs such as GALE (Genesis of Atlantic Lows Experiment) and ERICA (Experiment on Rapidly Intensifying Cyclones over the Atlantic) were held during the 1980s to better understand and forecast this type of cyclogenesis.

Today, bomb forecasts are much better, and this is largely a result of improved numerical model resolution and better assimilation of satellite data over the oceans.  Surprise events are rare and we typically have a good idea of the potential of an explosive cyclogenesis event several days in advance.  Below is the 72 hour GFS forecast valid for 1200 UTC 26 Sep (this morning) and it pretty much nails it.

Such a forecast would have been extremely rare in the 1980s, but is common place today.

Update@1115 MDT:

As shown in the 0000 UTC and recently released 1200 UTC manual surface analysis from the Ocean Prediction Center, the central pressure of this bomb appears to have dropped from about 991 mb to 958 mb in 12 h.  Very impressive!

Tuesday, September 25, 2012

Real Snow

That's more like it.  Today you can believe your eyes.

Monday, September 24, 2012

Is this Snow?

I'm not so sure.  I can't find snow in photos from other cameras (e.g., Snowbird Hidden Peak), I can't see it when I look at Lone Peak from my office, and the temperature at the top of the Collins lift has gotten no lower than 39ºF, which is quite marginal for snow to stick.  Rumor has it that the Sugarloaf camera image often suggests there's snow on the ground.  What do you think?

Climate Variability Paper

University of Utah Atmospheric Sciences Professor
Dr. Thomas Reichler
We interrupt today's blissfully cool and cloudy weather for a quick plug of a recent paper by University of Utah Atmospheric Sciences Professor Dr. Thomas Reichler, Atmospheric Sciences graduate student Junsu Kim, and two colleagues at other institutions, Elisa Manzina and Jürgen Kröger. The paper, which was published yesterday in Nature Geoscience and is available here (possibly paywalled for those outside the U or without a subscription to Nature), explores a possible link between the stratosphere and Atlantic climate variability.

To learn more, see the paper or this press release from the University of Utah.

Sunday, September 23, 2012

A Fly in the Ointment

A high-amplitude upper-level ridge remains parked over western North America, but as shown in yesterday afternoon's 0000 UTC 23 Sep 500-mb height analysis, there is a potential fly in the ointment for Utah weather.

The fly is the weak closed low that is sitting over the coast of Washington, which results from a weather feature that meteorologists call a coherent tropopause disturbance, or CTD.  A CTD is a localized cyclonic vortex, which is associated with counterclockwise rotation in the Northern Hemisphere and is strongest at the tropopause.  The tropopause separates the stratosphere from the troposphere.

Source: NASA
The CTD and closed low move into Nevada by tomorrow morning [1200 UTC (0600 MDT) 24 September].

A different perspective at this time is provided below by a 3-D colored representation of the tropopause, with cool colors indicating a lower tropopause and warm colors indicating a higher tropopause.  Note how the tropopause is locally depressed where the CTD and closed low are found over Nevada.

Next, we have a vertical slice taken through the CTD the line that bisects the image above.  In this slice, the tropopause is indicated by a red line.  Note how the tropopause is locally low just left of center where the vertical slice cuts through the CTD.  

The other lines are temperature contours.  Note how they dip down near and beneath the CTD, which is an indication of colder air aloft.  

Thus, as the CTD approaches Utah, temperatures in the upper-troposphere will fall.  This, combined with large-scale rising motion ahead of the CTD, will act to destabilize the atmosphere and initiate some showers and thunderstorms.  As a result, although we are still under the influence of a large-scale upper-level ridge, we have a chance of some isolated showers and thunderstorms today and scattered showers and thunderstorms tomorrow.  

Friday, September 21, 2012

Arctic Amplification and Midlatitude Weather

In the previous post, we discussed how the summer sea-ice extent is declining in the Arctic.  This sea-ice decline is simultaneously a result of and a contributor to enhanced warming of the Arctic relative to other regions, which is sometimes referred to as Arctic Amplification.  Arctic Amplification is produced by increased greenhouse gas concentrations combined with feedbacks such as increased absorption of solar radiation due to declining snow and ice cover.

An area of growing scientific interest concerns the implications of Arctic Amplification for midlatitude weather.  More rapid warming of the Arctic compared to the lower latitudes leads to a decrease in the average temperature contrast in the midlatitudes.  The strength of the upper-level flow is proportional to this temperature contrast.  Therefore, one would expect upper-level flow to weaken (on average) in response to Arctic Amplification.  

Recently, Francis and Vavrus (2012) proposed that this effect could contribute to more persistent weather patterns in the midlatitudes.  In particular, as the upper-level flow weakens, waves (i.e., troughs and ridges) tend to progress more slowly eastward.  They also suggest that the amplitude of these waves has increased due to Arctic Amplification, with upper-level ridges extending farther into the high latitudes, leading to higher amplitude upper-level waves that tend to move more slowly.

How the midlatitude flow will respond to global warming is a critical issue for projecting future regional climate change.  There is arctic amplification, but also other aspects of the climate system that are changing in ways that could affect the midlatitude flow.  This is an area of growing interest and research and fertile ground for motivated graduate students looking for a good thesis project.

Thursday, September 20, 2012

Whither the Arctic Sea Ice Extent

This is the worst kept secret in science as you have probably read about it in the past week in the news or earlier on this blog, but records for the arctic sea ice minimum extent were smashed this summer.  The National Snow and Ice Data Center suggests that a minimum of 3.41 million square kilometers was reached on September 16, breaking the old record set on September 18, 2007 of 4.17 million square kilometers.  
Source: NSIDC
The image below provides a comparison of the sea ice extent on September 16 (white) with the median extent on that day from 1979–2000 and helps illustrate just how much sea-ice loss there has been during summer in recent years.

Source: NSIDC
Many sea-ice extent records begin with the satellite record in 1979.  Efforts have been made, however, to use proxy data to extend a sea ice record into the more distant past.  For example, recent work by Kinard et al. (2011) suggests that the recent decline may be unprecedented in at least the past 1,450 years (see red line, with 95% confidence interval indicated with shading).

Source: Kinnard et al. (2011)
Perhaps we'll have a short-term rebound in the next year or two as happened after 2007, but in the long run, declines are expected to continue as a consequence of anthropogenic global warming.  We will discuss possible implications of this sea-ice loss for weather in the midlatitudes in a future post.  

Wednesday, September 19, 2012

The Smoke Filled Valleys

I'm on the road and have missed out on the smoke in the Salt Lake Valley, but I assure you it is far worse in other areas.

Check out this MODIS image of the central Idaho Mountains from yesterday.

The valleys are choked with smoke.  There may be a bit of fog in there as well, but for the most part, its smoke.

Stable cold pools that form in these valleys at night prevent the vertical transport and mixing of the smoke until late in the day this time of year.  This is very similar to what happens in Salt Lake during an inversion.  As a result, the smoke remains trapped in the valleys.  

One can really see just how strong these nocturnal cold pools are by looking at the weather the past few days at Stanley, Idaho.  Yesterday they observed a high of 75ºF, but that was preceded by a morning minimum temperature of 26ºF.  It got down that low again last night.  

A few years ago I was on a camping trip in the Sawtooths in September and it was in the 20s at night and near 80 during the day.  We started hiking with every layer we had on, but finished the day in shorts and t-shirts.  Such is the fall climate of the central Idaho mountains.  

Tuesday, September 18, 2012

Yeah, It's Coming Eventually

Source: Loveland Ski Area
Source: Unknown
Portions of the Colorado Rockies got a dusting of snow yesterday (Monday) morning as a weak front traversed the region.

Fall is the one time of year when Colorado can sometimes get the goods over Utah.  Owing to altitude and the somewhat colder, continental climate, they often see more early season snow than the Wasatch.  In addition, the snowpack hasn't decomposed yet into nasty avalanche-prone depth hoar, as it usually does during the Colorado winter.  

Have no fear.  We always catch up eventually.  

Monday, September 17, 2012

A Western Blocking Pattern

I can sum up the forecast for the next 10+ days in one word.  Dry.  An upper-level ridge is going to be parked over the western United States, with our only hope of any precipitation being the possibility of a closed low swinging underneath it and tapping into a last gasp of monsoon moisture.

We had a weak upper-level trough brush by last night.  That trough is presently moving across the upper midwest and eastern Rockies, with nothing upstream except a high amplitude ridge.  

IR satellite imagery and 500-mb geopotential height analysis for
1200 UTC 17 September 2012.
The medium range ensembles show nothing but ridging from now until the end of time (that's an exaggeration as they only go out 384 hours in the future, but that is the end of time as far as they are concerned).  Here are a few snippets at forecast hours 120 (valid 0000 UTC Sep 22), 240 (valid 0000 UTC 27 Sep), and 384 (valid 0000 UTC 3 Oct).

Source: Penn State E-Wall
All of the ensemble members keep the jet well to our north throughout the 384 hour forecast period.  A few bring the occasional rogue closed low into the area. 

Forecast skill degrades with time, so one needs to be cautious about surprises as the forecast lead time increases, but it appears we are in for an extended spell of clear fall weather.  Meteorologists call this a blocking pattern because the ridge acts to block or redirect troughs and cyclones away from our area, leaving us relatively high and dry.   

Sunday, September 16, 2012

Big Diurnal Temperature Ranges

If you want to see huge swings between maximum and minimum temperatures, September is probably your month.  There's still a decent amount of incoming solar radiation during the day, the soils are dry so much of this energy goes into heating rather than evaporation and transpiration, and the nights are moderately long.

This enables us all to enjoy warm days and cool nights, but where is the difference between the minimum and maximum temperatures, which meteorologists call the diurnal temperature range, the largest in northern Utah?

The best place to look are in basins that are either fully enclosed or only allow cold air to leak out through very constricted channels.  One such place is the Rush Valley, which is located southwest of Salt Lake City and the Oquirrh Mountains.

Overnight minimum temperatures on 16 Sep 2012.  Ignore misleading
caption at upper left!
Despite its name, the Rush Valley is really an enclosed basin.  Passes to the north to the Tooele Valley, east to the Utah Valley, and to the south are all higher than the valley floor, where temperatures plummet on clear nights.  Last night, the minimum temperature at the Clover observing site plummeted to 36ºF, much cooler than sites to the south that are 100–200 m higher (the slightest variations in elevation matter in conditions like these).  Over the past two days, the diurnal temperature range at Clover has been almost 50ºF.

That's impressive, but we usually get data from another site just to the south called Pony Express Marker that gets a few degrees colder than Clover at night.  Unfortunately, that data is not coming in today.

Those with a keen eye might notice in the top image that a site in the Heber Valley bottomed out at 33ºF.  The Heber Valley is not a basin, but it is fairly large in volume with cold air only able to exit via Provo Canyon, which is fairly narrow.  The diurnal temperature range at the Heber airport the past two days has also been around 50ºF.

Finally, we have the Peter Sinks, limestone sinkholes located in the Bear River Range east of Logan, which holds Utah's all-time minimum temperature record of -69ºF.  As can be seen in the image below, the temperature at the bottom of the sink last night bottomed out at about 22ºF, compared to only 48ºF on the rim of the sink just to the north.

Overnight minimum temperatures on 16 Sep 2012.  Ignore misleading
caption at upper left!
These sites are less than a kilometer apart, with a difference in elevation of only 100 m!  We did a field program in the Peter Sinks several years ago and I froze my butt off but good on a mountain bike descent to the bottom of the sinkhole one morning.  From shorts to hypothermia in less than a minute!

It looks like the diurnal temperature range in the Peter Sinks has actually been somewhat smaller (~40ºF) than found in the Rush and Heber Valleys the past two days.  I'm a bit surprised it isn't larger as 50+ºF temperature ranges have been documented under similar conditions.  It could be the observing site is not quite in the sweet spot.

Friday, September 14, 2012

West Desert Temperature Variability

It's that time of year where in the morning there are some remarkable contrasts in surface temperature due to topographic and land surface contrasts.

Nowhere is this more apparent than in Utah's West Desert.  Check out the observations from 1347 UTC (0747 MDT) this morning.  It was an incredible 55ºF over the salt playa along I-80, but as cold as 38ºF over the sagebrush region of Dugway Proving Ground to the southeast.

Zooming into the Dugway Proving Ground you can really see the intensity of the inversions that formed overnight.  Temperatures on the valley floor (~1310 m) range from 38–52ºF.  On the summit of Camel Back Mountain (just below the center of the image, 1547 m), it was 65ºF.

This variability is related to the formation of nocturnal cold pools due to radiational cooling at night.  The coldest temperatures are often found in those low elevation areas where the flow is weakest as the lack of turbulence and mixing enables the radiational energy loss to be concentrated in a shallow layer.  Areas with stronger flows, such as near gaps in the terrain (e.g., the 52ºF observation above is just west of a gap and the flow was 15 knots compared with 5 nots at the other sites), see more mixing and higher temperatures.  The coldest temperatures on nights like this are typically in basins where the cold air is trapped and the air becomes calm.

The warm nighttime temperatures over the salt playa are a bit of an anomaly.  Based on topography alone we would expect to find the coldest temperatures along I-80, but it turns out that the thermal characteristics of the playa are markedly different than that of the surrounding sagebrush region.  We will talk about this in a future post.

Thursday, September 13, 2012

Rivalry Week Weather Thoughts

Why are a tornado and a BYU divorce similar? 
You know someone is going to lose a house trailer.

Why doesn't BYU have an atmospheric sciences program?
It requires math beyond counting on your fingers and toes.

Go Utes!

What an Airmass!

Addendum @ 1150 AM:

It's a few hours after writing about the unbelievable airmass in place over northern Utah and it appears that smoke has moved in yet again!  Will it ever end?  Try and enjoy anyway....


The weather doesn't get much better than yesterday and today.  Yesterday at the U we saw a maximum temperature of 73ºF, with an overnight minimum of 57ºF.  Dewpoints are in the teens to low 20s.

The morning temperature at the Salt Lake Airport appears to have dipped to 50ºF.  The last day we saw a temperature that low was June 11th, three months ago.  Open up the windows and feel the chill!

It appears that the incredible weather will be with us for quite a while.  Nearly all the models keep the jet to our north and east and a ridge over the western United States for at least the next week, as illustrated by the 144 hour ensemble mean 500-mb height forecast by the GEFS, which is valid for 0000 UTC Sep 19.

It will be a bit warmer than it was the past couple of days, but the weather should nevertheless be great.  The bottom line is that we have waited all summer for weather like this.  Get out and enjoy.

Wednesday, September 12, 2012

El Nino Update

The Climate Prediction Center (CPC) has issued an El Nino Watch as conditions are favorable for the development of a weak El Nino this fall.

In August, sea-surface temperatures were weakly above average across much of the equatorial Pacific Ocean (for example, see the so-called Nino3.4 region, which lies between 90–150ºW and 5ºS–5ºN), although the far eastern equatorial Pacific saw some areas of below average sea-surface temperature.

The development of above average sea-surface temperatures in the central and eastern equatorial Pacific is a hallmark of El Nino.  Nevertheless, the departures from average are small and atmospheric conditions in the tropics are not yet consistent with El Nino.  Convection remains primarily in the western equatorial Pacific (it typically shifts eastward during El Nino), and the easterly trade winds were near average (they typically weaken during El Nino).

For these reasons, the CPC classified August as ENSO-neutral (meaning no clear EL Nino or La Nina signal), but expects a weak El Nino to develop this month and persist through December–February.

As things stand now, I wouldn't change my winter ski plans based on this forecast.  The presence of a moderate or strong El Nino stacks the deck for a drier than average winter in the northwest and a wetter than average winter in the southwest (northern Utah is a toss up), but it appears this will be a borderline event.  The unpredictable random draw of weather and climate variability will probably play a dominant role in what happens this winter.  In other words, I haven't a clue what is going to happen this winter.

Tuesday, September 11, 2012

Timp Snowfield Status

A few of our readers sent me photos of the Timp snowfield.  The first, from Carolyn Stwertka, was taken on July 29th and shows some skiers giving their skis a good stone grind.

There are easier ways to add texture to your bases!

The second was taken on Sunday by Beth Blattenberger and shows a few snow patches remain.

The long range forecasts call for ridging and warm weather to return.  In other words, there's more melt to come.  Can the Timp Snowfield hold on?

Monday, September 10, 2012

The Need for Speed

Like Maverik and Goose in Top Gun, meteorologists feel the need for speed.

The political conventions are over and predictably, both major-party candidates talked a good game about innovation, education, and the economy.  Well, I have something that either of them could do to immediately help in these areas: Infuse the National Centers for Environmental Prediction with the funding needed to put the United States back on top again in numerical weather prediction (NWP).

NWP involves the use of mathematic models of the atmosphere and other Earth-system components to predict the weather (this is why meteorologists call them models).  First envisioned by Vilhelm Bjerknes, a Norwegian physicist and meteorologist in the early 20th century, NWP became a reality in 1950 when a group formed by John von Neuman and led by Jule Charney produced the first one-day forecast on ENIAC, the first electronic computer (see Platzman 1979 for a historical review).

From left to right, Harry Wexler, John von Neuman, M. H. Frankel,
Jerome Namias, John Freeman, Ragnar Fjortoft, Francis Reichelderfer and
Jule Charney in front of ENIAC in 1950. © MIT Museum. Source:
In 1954, the US formed the Joint Numerical Weather Prediction Unit, which began producing twice daily NWP predictions on an IBM 701 in 1955.  Since then, as computer power has increased, atmospheric scientists have refined their mathematical models and forecasts have improved markedly.  In 2006, the 72-hour NWP forecast produced by the National Centers for Environmental Prediction is as good as the 36-hour forecast was around 1985.  This relentless improvement has continued in the past few years as well.

Source: Harper et al. (2007)
Evident in the graph above are regular supercomputer upgrades that, along with advances in model formulations and observational technologies, have contributed to this advancement.  The first supercomputer that I used in graduate school (circa 1990) was a Cray Y-MP, which had a speed of 2.3 gigaflops (think of this as 2.3 billion calculations per second).  The iPad2 clocks in at about 1.5 gigaflops, so you have about that much power in your hand today.

Currently, the National Centers for Environmental Prediction has two IBM Power 575 compute clusters (as is the case at most forecast centers, one is for operational use, the other for backups and development), each with 4,992 cores and a peak output of about 73 teraflops (~73 trillion calculations per second).  These computers are a few years old, however, and are being replaced by two newer IBM compute clusters that are each expected to be rated at 149 teraflops and will become operational next summer.

For comparison, Environment Canada has two IBM Power 775 computer clusters, each with 8,192 cores and a peak output of 185 teraflops.  The European Center for Medium Range Weather Forecasting (ECMWF) has two IBM Power 575 computer clusters, each with 8,320 cores and a peak output of 116 teraflops.

However, what really caught my attention is the latest (June 2012) list of the world's 500 fastest supercomputers at  In that ranking the ECMWF has two supercomputers listed, each with 24,576 cores and a maximum output of 549 teraflops.  I haven't been able to ascertain if these are their future operational computers, but if they are, they will be taking a quantum leap forward in operational weather prediction.

This is the kind of investment that the US can and should be making in operational NWP.  Recent studies indicate that variations in the US economy that are attributable to weather variability amount to 3.4%  of the gross domestic product (Lazo et al. 2011).  That's $485 billion.  Weather forecasts are used for everything from commodities trading to picnic planning.  Evacuating coastline for hurricane landfall costs $1 million per mile.  Better forecasts add up to real money.  They help protect lives and property.  They help container ships that transport hundreds of millions of dollars of goods cross the high seas as safely and quickly as possible.  They help you plan for your morning commute.  Simply put, ensuring that the US is at the cutting edge for computer horsepower and NWP modeling is a good investment for the nation.

Addendum@2:50 PM:

Bob Maddox informs me that Kristine Harper's book Weather by the Numbers provides a detailed history of the early NWP efforts and says that Carl Rossby, who is not mentioned above, played a pivotal role in assembling the group that ultimately built the first successful NWP model.  Further, the ENIAC they used was at Aberdeen Proving Grounds.  

Sunday, September 9, 2012

State of the Snow

In late July I speculated that we might lose all snow in the Wasatch Mountains this year.  A poor snow year followed up by a hot spring and summer is a recipe for snowpack oblivion.

As we approach the end of the melt season, all that remains is a few patches of snow clinging to the high north aspects.  Yesterday, all that was left on the American Fork Twin were three narrow patches of snow that are probably doomed.

Looking for turns?  How about this patch at the base of the northeast face of White Baldy.

It might just survive the summer.  Chances are, this patch is probably neve (compacted, granular snow) from the great winter of 2010–11.  If I remember correctly, there was also a large avalanche on the northeast face of White Baldy that year that probably collected in this area. 

I'm still waiting for someone to send me a recent photo of the Timpanogos snowfield. 

Saturday, September 8, 2012

The Weatherman Is Not a Moron

"The problem with weather is that our knowledge of its initial conditions is highly imperfect"
-Nate Silver

The Weatherman Is Not a Moron is an excerpt published in yesterday's New York Times from a forthcoming book by Nate Silver entitled "The Signal and the Noise: Why So Many Predictions Fail – But Some Don't." 

I nearly bought a copy of the book in advance last week, but I was worried it was going to be a boring treatise of digital signal processing, but that is clearly not the case.  The Weather Is Not a Moron is a pretty good read and provides insights into how uncertainty impacts weather forecasting.  Hopefully the book delves into ensemble weather forecasting and efforts by meteorologists in the past two decades to quantify uncertainty, as that has become a critically important component of our profession.  

The author covers numerical weather prediction in the excerpt, and how forecasters deal with uncertainty, but never states that many recent advances in weather forecasting are due to the development of ensemble modeling systems that attempt to quantify uncertainty.  Rather than running a single model with the highest resolution (meaning detail) possible, it is common today to run an ensemble of many models with somewhat lower resolution (meaning less detail) in order to see the range of possibilities in the future.  It is somewhat counter intuitive that a suite of less detailed model forecasts can be more valuable than a very detailed model forecast, but this is the case in weather prediction because the atmosphere is so chaotic and sensitive to the initial conditions.  This ensemble modeling of uncertainty has helped decrease the number of meteorological predictions that "fail" in recent years.  Hopefully this is covered in the book.  If not, I look forward to reading it anyway.  

Friday, September 7, 2012

Subtleties and Data Assimilation Matter

Yesterday provided a great example of why it is important to remain vigilant as a forecaster and keep an eye on the most subtle weather features.

The flow was fairly weak yesterday, but inspection of the 700-mb winds in the Rapid Refresh analysis showed a weak trough draped across northern Utah and central Nevada with a tongue of higher precipitable water air extending from southern Nevada into western Utah.  This moisture tongue became the locus for the development of convective rain showers yesterday afternoon, which proceeded to move across central Utah and produce a few showers as far north as Little Cottonwood Canyon.

The moisture tongue was entirely unresolved by the conventional upper-air observing network, which provides observations only at Elko, Salt Lake, and Las Vegas.  The fact that the Rapid Refresh picked up on such a feature is a testament to the tremendous advance in numerical weather prediction and data assimilation that has occurred in recent years.  The Rapid Refresh assimilates everything from radar reflectivities to aircraft moisture observations, making it a powerful tool for weather forecasting.

If you really want to blow your mind, check out the experimental High-Resolution Rapid Refresh (a.k.a., the "HRRR"), which is presently under development at NOAA's Earth System Research Laboratory.  Running at a grid spacing of 3-km (compared to 13 km for the Rapid Refresh) and incorporating state-of-the-art assimilation of radar data, the HRRR will likely revolutionize short-range weather analysis and forecasting over the western United States.

Thursday, September 6, 2012

The Heat of Summer

The National Weather Service reports that this August was the hottest in the records at the Salt Lake City International Airport, and this summer (June–August) was the second hottest.  We obliterated the previous August record (1994) by 0.9ºF, while we just missed the summer record (2007) by only 0.1ºF.

1994 is now the third warmest year on record, although I think it carries on as the summer with the most 100 degree days (21).  This year we had 11.

The heat was associated with an upper-level ridge that setup over the central United States and dominated the weather for most of the summer, as shown in the mean and anomaly 500-mb geopotential height analyses below.

In August, this ridge was located a bit west of its mean position for the summer, which contributed to the anomalously high temperatures.

Note also that the ridge had a positive negative tilt, meaning the ridge axis was aligned from southeast to northwest.  This pattern helped keep monsoon moisture from penetrating into northern Utah.  

This was an exceptionally warm summer by historical standards, but we will see more like it in the coming decades.