Friday, September 30, 2011

One Year of the Wasatch Weather Weenies

Tomorrow marks the first anniversary of the Wasatch Weather Weenies.   I'd like to thank all of you out there in the blogosphere for making the first year a success.

We started as an invited only, password protected blog.  I wanted to be able to discuss weather and climate issues without a lot of political baloney (a problem these days when it comes to climate), but I was quickly inundated with requests for access and eventually realized that my fears were unwarranted.

Readership has grown fairly steadily over the past year, with a slight dip during the summer when the U is in recess and interest in snow is admittedly low.  In September, we've had an average of about 140 pageviews per day.  That's far better than the attendance at my classes!

Despite my innate interest in snow and winter weather, the six most popular posts have nothing to do with snow (what is wrong with you people out there!).  The Wasatch Weather Weenies first year top-10 is:
  1. Tour de France Weather
  2. Catskill Flooding Continued
  3. Republican Presidential Candidates and Global Warming
  4. Utah Legislature to Address Global Warming
  5. Utah Natural Disasters and Hazards
  6. Patience Young Jedi Knight
  7. I Can't Take it Anymore!
  8. La Nina Ski Realities
  9. Updating the Norwegian Cyclone Model
  10. Four Horseman of Yesterday's Weather Apocalypse
Utah Legislature to Address Global Warming was an April Fool's joke. I keep wondering if that one will come back to haunt me...

The biggest challenge of the blog is finding the right level of technical content.  The meteorological background of our readers is quite varied and I've dealt with this by varying the technical level of the posts.  Some are written at the senior undergraduate to graduate atmospheric science student level, but others are aimed at a general audience.  Such unevenness may not be optimal, but it does help me address my desire to contribute to both the education of my students and the general public.

If you have any suggestions or comments about the blog, please add them here.  Feedback greatly appreciated and it helps me keep this thing going!

Thursday, September 29, 2011

Hilary False Alarm

On Tuesday, I violated one of my cardinal rules by digging into the small-scale details of a medium-range forecast.  As usual, that has left egg on my face.

Of concern at that time was the potential recurving of Hurricane Hilary into the southwest United States.  Ahead of Hilary, a surge of tropical moisture was expected into the lower Colorado River Basin.

While that surge of moisture may yet materialize, Hilary has other ideas.  She kept moving westward and her core of moisture is now expected to continue in that direction.

This is not to suggest that the weather in Utah won't get interesting the next few days.  It looks like it is going to be an exciting week.  Stay tuned.  

Red Sox Woes and Weather Forecasting

On September 3, the Boston Red Sox sat 9 games over the Tampa Bay Rays for the American League wildcard.  The FiveThirtyEight blog from the New York Times estimates that they had a 99.6% chance of making the post season on that day.

Yesterday morning, they were tied with the Rays for the wildcard spot.  Later that night, the Rays trailed the Yankees 7–0 in the 8th inning.  The odds of a Rays win (also from FiveThirtyEight)?  0.3%.  As the Rays were chipping away at the lead, the Red Sox were beating the Orioles 3–2 in the 9th inning after a rain delay.  The odds of a 9th inning comeback by the Orioles?  2%.  But it's even worse than that.  The Orioles eventually were eventually down to their last strike.  Two outs and two strikes on the batter, Dan Johnson, who was hitting .108 for the season.  He had two strikes on him, and was 1–45.  The odds of the Rays making the playoffs were long indeed.

So what happened?  The Orioles came back and won the game, the Rays came back, put the game into extra innings, and Evan Longoria hit a walkoff home run that will go down in Red Sox infamy with home runs hit by Bucky Dent and Aaron Boone.

What does any of this have to do with weather forecasting?  Well, it is the low-probability outlier event that causes meteorologists to lose sleep.  Low probability does not mean zero probability. How can we best provide useful information about such events to the public and consumers of weather forecasts?  One doesn't like to sound like chicken little and it is well documented that false alarms erode the effectiveness of weather forecasts and warnings.   

On the other hand, a low-probability outlier event can have huge impacts and we are dealing with an atmosphere that cannot be predicted with precision.  Lake-effect snow is a good example.  The difference between a big event and a few snow squalls is largely dependent on processes that can neither be resolved by our observing system nor predicted with precision at lead times of more than a few hours.  Most meteorologists go to bed on a potential lake-effect night knowing that there's a small chance we're going to get hammered, but that it is most likely that we'll just see some scattered snow showers.  Repeated forecasts of such odds, however, erode public confidence in the forecast, even though it is the state-of-the-science today.  

There are at least two issues here.  The first is to quantify and improve probabalistic forecasts so that they are more reliable.  Current ensemble modeling systems are helpful, but need substantial improvements.  This is a physical science challenge.  The second is improving forecast communications, which I see largely as an interdisciplinary challenge.  I think it is clear, however, that the latter is a challenge that must be addressed if we are to maximize the effectiveness of weather forecasts for the benefit of society.  

Wednesday, September 28, 2011

Patience Young Jedi Knight

"I cannot teach him.  The boy has no patience." - Master Yoda

The time has come to channel your inner Jedi and be patient as the anticipation of the upcoming ski season approaches.

Although Master Yoda trained Jedi Knights for 800 years, I'm not quite that long in the tooth, but I have lived long enough in Utah to recognize that early snow is a blessing and a curse.  It helps us feed our primal urges, but often leads to nasty avalanche problems in subsequent weeks.  Utah Avalanche Center Director Bruce Tremper likes to say that thin snow is weak snow, but an important twist of this phrase is early snow usually becomes weak snow.  

Near the ground, the temperature of the snowpack is nearly always near freezing, but the snow surface can be much colder.  A thin snowpack has a larger temperature gradient than a deep snowpack since the temperature change is concentrated over a smaller distance.  Large temperature gradients favor the development of weak, faceted snow (a.k.a. depth hoar) that is angular, poorly bonded and has a great deal of pore space (e.g., air gaps).  In contrast, small temperature gradients favor the development of strong, rounded, highly bonded snow grains with great density and cohesion.  See for more information.

The bottom line for backcountry skiers and snowboarders is that the best start to a Utah winter involves the rapid accumulation of a deep snowpack so that we don't have to worry about depth hoar. 

This is where your Jedi training is important.  It is very difficult (but not impossible) to accumulate a deep snowpack in October.  Early-mid October storms, in particular, are usually followed by a dry period during which the snowpack rots and facets.  Use the force to ensure that we stay high and dry until late October, and then let the floodgates open.  What we want is a big late October/early November storm cycle to lay down a deep, high-density snowpack that opens up a ton of terrain and precludes the wide-spread development of depth hoar.  

You want to start using the force now.  It's far enough out that we can't read into it too much, but the GFS 180 hour forecast is calling for a major winter storm to move into northern Utah...

then again, maybe you just want your powder fix as soon as possible... 

Tuesday, September 27, 2011

Hilary Possibilities

I am violating one of my cardinal rules today by digging into the small-scale details of a medium-range forecast.  Given the large-scale uncertainty that exists at long lead times, this is always a dangerous thing to do, and could set me up for a real egg-on-my-face moment in a few days.  However, there's little of interest happening right now, so let's go off into what I call "dream-prog-land."

Hurricane Hilary is presently churning away happily at category 3 in the eastern Pacific, with maximum sustained winds near 115 mph.

The contours in the analysis above are of precipitable water and show that while there is a local maximum near Hilary, the entire eastern Pacific off the coast of southern and central Mexico is quite moist.

The GFS forecast shows that in advance of Hilary, there is a very strong Gulf of California surge that taps into this area of high precipitable water and advects it northward into the lower Colorado River Basin.

The GFS forecast for 1200 UTC 2 October (Sunday morning) includes a tongue of moist air that extends into southwest Arizona with precipitable water values exceeding 35 mm.

Ah, but it's a dry heat!  Time will tell if things come together to pull this moisture into Utah.  I've already played up the Hilary potential a bit more than I should at such long lead times, but desperate times call for desperate measures.

Monday, September 26, 2011

Here Comes Hilary?

No, not her, this Hilary,

a category three hurricane with sustained winds of about 125 miles per hour and one powerful eye.  

The latest (0600 UTC) GFS forecasts Hillary to make a hard right turn, saunter up the Baja Peninsula, and then move into the southwest United States.

Even if this track verifies, Hilary will weaken substantially before reaching the southwest United States, but that doesn't mean she won't pack a punch.  Of primary concern is the potential for heavy precipitation and related flash flooding.  The GFS brings abundant moisture into northern Baja by 132 hours, with the remnants of Hilary moving into southern Arizona by 150 hours.  This occurs as a mid-latitude trough moves onto the Pacific Coast, serving as the so-called "kicker trough" that initiates the eastward movement of Hillary.

Whether Hilary has any impact on Utah will depend greatly on how the large-scale pattern plays out over the next several days, as well as how quickly she moves northward.  The GEFS ensemble shows a relatively small standard deviation of 500-mb height in the forecast area by 1 October, which might suggest we can have some confidence in Hilary's track.

I think, however, that might simply reflect the fact that the 500-mb height gradient over the region is relatively small, so the standard deviations are not large.  I believe this remains a difficult forecast that is quite sensitive to small changes in the flow in mid-latitudes and the subtropics.  Keep an eye on things as even if Hilary's moisture doesn't make it to Utah, the midlatitude trough has the potential to finally bring some changes in the weather.

Sunday, September 25, 2011

Lessons from Joplin Tornado

The 22 May Joplin tornado killed 160 people, the most people killed by a single tornado in the United States.

Source: Army Corps of Engineers
The NWS recently issued a report on the tornado, and there is also a nice article on Andrew Revkin's Dot Earth blog.  Have a look.  Preventing such large losses of life is more than just a meteorological challenge.  Societal, economic, and engineering factors are important contributors and need to be addressed for society to become more weather and climate resilient.

Friday, September 23, 2011

Weird Radar Loop

Not a cloud in the sky, but there are some decent radar returns in the Ogden area.

Might these be birds?  If so, what species?

Even the more diffuse clear-air returns in the south part of the domain above are impressive by northern Utah standards.

Remarkable Rush Valley Temperature Range

The temperature evolution at the Pony Express Marker site in the Rush Valley this past week is quite remarkable.  Note how the diurnal cycle amplitude has generally increased through the period.  

This morning, the minimum temperature was 27F, the coldest this week, and as of 3:15 PM they are sitting at 84F the warmest this week.  That's a daily temperature range of 57F, and we might still squeeze a couple of degrees out of it!

Sea Ice Minimum

Based on the National Snow and Ice Data Center analysis, it appears we have passed the sea-ice extent minimum for the 2011 calendar year, as the extent has increased over the past several days.

This year's minimum was just slightly higher than the record minimum for the satellite era, which was set in 2007.  Nevertheless, model estimates of total sea-ice volume from the Polar Science Center at the University of Washington, show this was a record low year when the total ice volume is considered (the analysis below removes the monthly mean, which helps take out the seasonal cycle).

If you are wondering how this compares with the pre-satellite era, there is a discussion of longer records of ice coverage in the IPCC fourth assessment report.  They concluded that there was a "clear indication of sustained decline in arctic ice extent since about the early 1970s, especially in summer" as can be seen in the time series of northern hemisphere sea-ice extent for September (red line below).

Source: Lemke et al. (2007)

Thursday, September 22, 2011

Stable Layer Smoke and Brown Cloud

Yesterday provided a few examples of mixing (or lack thereof) in a stable atmosphere.  First, let's have a look at the morning sounding from Salt Lake City, which showed a shallow surface-based inversion below 850 mb, surmounted by a stable layer that extended to about 775 mb, or 1 km above the valley floor.  In both of these layers, one would expect limited vertical transport and mixing.  

Indeed that morning there was copious evidence of this limited vertical transport and mixing.  First, there was a structural fire at about 9:30 am near Cottonwood Heights that produced a fascinating smoke plume with a zig-zagged or perhaps spiraled layered pattern (click to enlarge).

Photo: Sebastian Hoch
The layering of the smoke provides visual evidence confirming the atmospheric stability in the lower km of the atmosphere.  Meteorologists call such flows with limited mixing between the layers, laminar. The real mystery here, however, is explaining the zig-zagging of the smoke, which I interpret as the result of either a reversal of the cross-valley wind component with height or an oscillation in wind direction over time, but can't really confirm or deny either of these hypotheses with the observations available.  

Second, was the hideous brown cloud over the Great Salt Lake and northern Salt Lake Valley.  

This is a common sight when looking west from the University of Utah following a clear night.  I've long thought this was a reflection of the advection of all our urban pollutants out over the lake by the nocturnal southeasterly drainage flow over the Salt Lake Valley, although optical effects may contribute to its apparent positioning as well.  Typically one can see the layer deepen and spread into the valley during the day as the surface heats, mixing ensues, and the up-valley flow and lake-breeze produce north-northwest flow.  

Fortunately for us, we still get enough heating from the sun to develop a decent convective boundary layer during the day.  Yesterday, the boundary layer reached to about 725 mb, which means we're not seeing a buildup of high pollutant concentrations.

This same scenario in January, with snow on the ground, would certainly produce hazardous air quality.

Wednesday, September 21, 2011

Basin Cold Pools of the Rush Valley

As mentioned yesterday, we are in a pattern that lends itself to the development of strong cold pool, especially in topographic basins.  The Peter Sinks are the king of all Utah cold spots, but the Rush Valley is a close second and, more importantly, is well instrumented.

The Rush Valley is poorly named as it is really a closed hydrologic basin located south of Tooele and East of Provo.

Map Background Source: ACME Mapper
It is surrounded by a jumbled mess mountain ranges including the Stansbury and Onaqui Mountains to the west, the Sheeprock and Tintic Mountains to the south, the Oquirrh Mountains and Thorpe Hills to the east, and South mountain to the north.  Much of the valley floor is below 5100 feet, but the low point is at Rush Lake, just south of the Stockton Gap, at about 4960 feet.  The topography surrounding the Rush Valley has two major weaknesses.  One at what I call Stockton Gap (~5120 ft), the other at Five Mile Pass (~5300 ft).  Between the West and East Tintic Mountains, Boulter Summit, sits at about 6000 feet.

On a clear night like last night, the Rush Valley gets cold.  Really cold.  This morning, the minimum temperature at stations near the floor of the valley ranged from 29–34F, with the coldest temperature at the Pony Express Marker.

Minimum 24-h temperatures reported to MesoWest.  Winds
plotted are the most recent observation within the hour
ending at 1427 UTC 21 Sep 2011.
The time series from the Pony Express Marker shows a diurnal temperature range of about 50F.  As is often the case in basins, the temperature fell most rapidly during the first part of the night, then fell more slowly, reaching a minimum near sunrise.

Contrast this with Ophir Station (5561 ft) on the east bench or Vernon Hill (5761 ft), which sticks up in the southern part of the Rush Valley, where the minimum temperatures were only 53 F and 58F, respectively.   At Vernon Hill, temperatures remained between 58 and 63 F for much of the night.

One of the "cool" (pun intended) things about cold pools is that they tend to be most intense below the lowest gap, and this is supported by the evidence above.  In the case of the Rush Valley, that's Stockton Gap, where all sorts of wild things happen.  At the Stockton Bar MesoWest site, the maximum temperature yesterday was about 75 F.  There was a modest drop in temperature around sunset, followed by relatively steady temperatures until about 0430 UTC.  At this time, the temperature dropped from 65 to 58 F.

This temperature drop coincided with a wind shift from northerly to southerly, which likely occurred when the cold pool in the Rush Valley reached sufficient depth to pour through Stockton Gap.

Following this wind shift, the wind remained from the south for the remainder of the night, although you can see some warming and cooling cycles that are somewhat regular.  These could be due to sloshing of the cold pool (sometimes referred to as a seiche).

Curiously, the temperature at Five Mile Pass, which is about 180 feet higher than Stockton Gap, only dropped to 44F.  The shallow, intense cold pool near the valley floor does not extend as high as Five Mile Pass.  Whether or not this is due simply to nocturnal radiative and turbulent processes, which perhaps are not sufficient to deepen the cold pool that much, or due to a loss of mass through Stockton Gap, remains for further investigation.

Tuesday, September 20, 2011

Basin Cold Pools of the Peter Sinks

The high-amplitude ridge is settling in over Utah and we are in for several days of clear, dry weather with large daily temperature ranges.

Large diurnal temperature ranges will be observed during this period, especially in basins, closed topographic depressions with no drainage outlet.  Such basins include the Rush Valley south of Tooele (which is misleadingly named and is really a basin) and the meteorological king of the diurnal temperature range, the Peter Sinks of the Bear River Mountains east of Logan.

We'll have a look at the Rush Valley tomorrow, but let's talk about the Peter Sinks today.  The Peter Sinks are a series of limestone sinkholes known for remarkably cold temperatures.  As described by wikipedia, the lowest temperature ever recorded in Utah -69.3F, was measured in the Peter Sinks on 1 Feb 1985.

The Peter Sinks aren't much to look at.  They aren't even very deep.  They are modest depressions, but do feature a pronounced vegetation inversion with trees growing only on the upper slopes and rim.  This may be due to the extreme temperatures that occur within the sinks.

A view of the Peter Sinks.  Photo: John Horel
Craig Clements, Dave Whiteman, and John Horel ran a field program examining the cold pools that form in the Peter Sinks during September 1999 (Clements et al. 2003).  They instrumented the northern portion of the Sinks with a series of meteorological instruments and ran a few tethersondes (i.e., tethered balloons that can be lowered and raised) near the bottom and along the sidewall of the Sinks.

Peter Sinks field experiment design
(Clements et al. 2003)
The remarkable cold pool that forms in the valley floor is well described by data from the meteorological stations (T1–T5, with T1 the lowest) during EXP1, a clear, calm night.  The meteorological stations have similar temperatures at sunset (SS), but the diurnal temperature range is more than 10C larger on the floor of the Sink (T1) than on the rim (T5).  Considerable variability in the temperature at T5 during the night reflects the occasionally sloshing of cold air from the sink across the rim.  Temperatures quickly become similar again shortly after sunrise (SR).

Temperature traces from meteorological stations T1–T5
(Clements et al. 2003)
Basins become extremely cold at night for two major reasons.  First, they are more protected from the large-scale flow and see less turbulence, which tends to mix the atmosphere and prevent such intense cold pools from forming.  Second, the cold air that forms has no place to go.  It sits there and continues to cool via radiative and sensible heat fluxes during the night.  Typically the cold pool in a basin grows to a depth of the lowest gap on the rim, which serves as a conduit for the cold air.

Tomorrow we'll have a look at another great cold pool basin, the Rush Valley.

Sunday, September 18, 2011

First Freshies

I was quite pleased to see a dusting of the white stuff on the American Fork Twin this afternoon.

It won't last, but the deep powder days will get here eventually.

Coming Soon: Ridge

Next to a good mid-winter storm cycle, there's nothing better in Utah than a high-amplitude ridge in September, and Mother Nature is about to deliver.  Check out the GEFS ensemble-mean forecast for the coming week.

That is one monster ridge, and it should lead to a period of "severe-clear" bone-dry weather for northern Utah.  We should see some large diurnal temperature ranges, especially in noted nighttime cold spots like Rush Valley and the Peter Sinks.  Looking for something geeky to do?  Head to the Peter Sinks on Wednesday or Thursday and ride your mountain bike from the rim to the basin floor first thing in the morning.  I did this during the Peter Sinks experiment, described by Clements et al. (2003), and froze my butt off as the temperature fell from something like the 40s on the rim to the teens at the bottom.

And, I should add that I'm still savoring the Ute beatdown of BYU last night...

Saturday, September 17, 2011

Is This Lake Effect?

It is not uncommon to see convection, sometimes in the form of cells, other times in the form of bands, initiating downstream of the Great Salt Lake in westerly flow.  At issue is if the lake plays any role at all in initiating these bands, if they are a reflection of orographic lift, or whether it is pure happenstance that the convection triggers downstream of the lake.

Case in point was this morning when there was some fairly strong convection downstream of the Great Salt Lake near Ogden.

An hour or two later, I took this photo looking north toward Ogden from the Bonneville Shoreline Trail, showing a wimpy cloud band extending eastward into an apparent orographic cloud over the Wasatch Range.

What are the roles of the lake, orographic, and large-scale processes in a case like this?  I think these westerly flow cases are amongst the most perplexing we have in northern Utah.

BTW, the view to the west was simply spectacular and largely cloud free, although that could be do to large-scale processes as much as lake influences (it's Saturday and I'm just doing a quick post and not looking in depth at this case).

 If you have never visited this part of the Bonneville Shoreline Trail, do it.  It is one of my favorite places to ride because of the views, and there is continuing pressure to develop this area.

Friday, September 16, 2011

Great North Atlantic Loop

IR satellite imagery, sea level pressure (contours), dynamic tropopause
(color fill), and 925-mb winds
The loop above illustrates several key large-scale atmospheric processes that are important for weather prediction.  In fact, I could probably give an entire exam for synoptic-dynamic meteorology from this loop alone.  Can you more advanced Wasatch Weather Weenies identify them?

The Holy War

Today is going to be a very interesting weather day, but with the Holy War tomorrow, I can't help but take a few cheap shots at that so-called university down the road.

Now I have a number of friends and colleagues at BYU, and they have even had my back scientifically.  But, for the next 48-hours, we will simply refer to those at BYU as down south scum.

Being that this is a weather and snow blog, and that the Holy War inevitably involves debate about which school is superior, I now announce the official Wasach Weather Weenies Top-10 reasons why the University of Utah is better than BYU:

10. We have the Great Salt Lake effect, they have the Utah Lake effect
9. We chase hurricanes, they chase honor-code violators
8. We have Alta-Snowbird, they have Sundance
7. The Cottonwoods get blower-pow in northwesterly flow, Timpanogos gets nothing
6. The Utah ski team has 10 NCAA championships and 63 individual national champions,  BYU races division II.
5. We ski on Sundays, they go to church.
4. We have, they have, well, nothing anywhere near as cool as
3. We are one of the best colleges for skiers and snowboarders, they are the most stone-cold sober school
2. We have Mountain Meteorology by Prof. Dave Whiteman, they have the Book of Mormon
1. Our faculty ski deep powder and grace the cover of major scientific journals


Planning for Storm Chasing

Photo: Josh Wurman, Center for Severe Weather Research
University of Utah students who are interested in learning more about winter storms and getting hands on experience with polarimetric radar are invited to attend a meeting at 2PM today (Friday 16 Sep) in 711 WBB.  As announced earlier this month, the famed Doppler on Wheels research radar will be deployed to the University of Utah from about 21 Oct – 21 Nov and available for student-directed research projects.  At today's meeting, we will be identifying some key phenomena and research issues to examine, and forming teams for developing deployment strategies, including siting and scanning sequences.

Thursday, September 15, 2011

A Pathetic Cold Front

Recent work has shown that the Intermountain West is a fertile breeding ground for strong cold fronts, many of which develop over eastern Nevada and northwest Utah (e.g., Shafer and Steenburgh 2008; Steenburgh et al. 2009).  In particular, Shafer and Steenburgh (2008) show that the number of strong cold frontal passages, defined based on a temperature fall of at least 7ºC and pressure rise of 3 mb in 2–3 h in conjunction with a 700-mb temperature gradient of at least 6ºC/500 km, increases as from the Pacific Coast to Salt Lake City where there is a local maximum.

Total number of strong cold-frontal passages 1979–2003 (Shafer and Steenburgh 2008).
There's a minimum at Wendover, but some weird stuff happens there (meteorological and other) because of the mountains to the immediate north and northwest.

However, climate is what you expect and weather is what you get.  Some fronts that move across the Intermountain West do not intensify.

For example, the NAM model predicts that a fairly healthy cold front will develop tonight over the Pacific Northwest.  The cross-front temperature contrast is not large, but the gradient becomes quite concentrated by 1200 UTC (0600 MDT) tomorrow (Friday) morning.

1200 UTC 15 Sep Initialized NAM model forecast of sea level pressure,
10-m wind, and 3-h accumulated precipitation (top) and 700-mb temperature,
wind, and relative humidity (bottom) valid 1200 UTC 16 Sep 2011.
I suspect some of the frontal development is related to sub-cloud diabatic cooling from precipitation in the post-frontal environment.

Although many of these fronts intensify as they move into Utah, the NAM model forecast calls for the front to weaken by tomorrow afternoon.

1200 UTC 15 Sep Initialized NAM model forecast of sea level pressure,
10-m wind, and 3-h accumulated precipitation (top) and 700-mb temperature,
wind, and relative humidity (bottom) valid 1200 UTC 16 Sep 2011.
That doesn't mean that we won't have an interesting weather day tomorrow.  Whenever a mid-latitude trough plays with monsoonal moisture, I tend to stay alert.  Perhaps we'll get some decent outflow boundaries as well with a rapid temperature fall.  But this won't be an event where we're in shorts one day and parkas the next.  

There are probably a few reasons why this event does not intensify over the Intermountain West.  First, the large-scale flow and cold air are well to the north.  They are not interacting at all with the Sierra Nevada and southern Cascade range.  Second, diabatic processes appear quite important in the frontal sharpening tonight, but also perhaps the decay tomorrow (note the well organized frontal precipitation band forecast for tomorrow, but the more scattered nature of the precipitation forecast for tomorrow afternoon).  There is abundant moisture ahead of the front, which contrasts with our big spring cold fronts, which usually have a dry pre-frontal environment.  Finally, I suspect the large-scale flow in this case is simply not favorable for strong frontal development, even in the absence of topography.  

I've always thought an interesting extension of Shafer and Steenburgh (2008) would be to look at landfalling Pacific cold fronts with and without intensification over the Intermountain West.  Tomorrow may provide an example of the latter.  We'll see if the NAM forecast verifies.

And for you skiers out there, don't bother waxing the skis yet.   

Wednesday, September 14, 2011

Something Out of Nothing

The upper-level trough sliding across the southwest United States has organized the monsoonal clouds and precipitation over the past two days, with a nice comma cloud developing over southern Utah.

Beneath the comma cloud is a weak, somewhat organized rain band with a few embedded convective cells.

We often think of monsoon precipitation as airmass convection, but large-scale systems can be important.  This is especially true in September when midlatitude troughs interact with monsoon moisture.