Tuesday, November 30, 2010

Avenues snow

The urban-ski terrain interface, 29 Nov 2010
Over the years I've enjoyed a number of tours in the Avenues Foothills.  Most of the aspects that you see from town are baked quickly by the sun, but there are a few short shots that drop into City Creek canyon that can be quite good.

Stoked by the recent snowfall, we did a quick tour yesterday after work.  I debated which skis to grab, but fortuitously went with the old tele gear.  Although this latest snowstorm was impressive (16" in the upper aves), most of the snow was low density and fell on a meager base.  As a result, one could "feel the bottom" on many turns.  It is always odd to feel the powder waffing up your thighs while your skis feel like they are dragging across sandpaper.

Mother nature never ceases to surprise and remarkably just a short distance from the cul-de-sac at the top of Terrace Hills we found this small natural wet-slab avalanche on a wind-loaded southeast exposure.

A good reminder that proximity does not equal safety.

Monday, November 29, 2010

Orographic precipitation enhancement, interrupted

Yesterday's storm is now in the record books and it was an impressive one.  It's been quite a while since I can remember this much snow in my yard.  Perhaps the last time was Thanksgiving 2001 after the famed 100 inch storm (named for snowfall at Alta, although the valley got pounded too).

Storm-total snowfalls reported on the NWS web site for locations in the Salt Lake and Tooele Valleys:

And in the Wasatch Mountain south of I-80:

Bottom line: There is remarkably little difference between the lowland and mountain snowfall in this case, as discussed in an earlier post.  Even overnight, when I thought we'd see more orographic precipitation enhancement, there was little.  This case provides a great example of why the use of a climatological precipitation-elevation relationship to downscale or redistribute coarse-resolution numerical guidance can be dangerous.  At issue is whether or not one could reliably anticipate a similar event in the future.  

Sunday, November 28, 2010

Where is the orographic precipitation enhancement?

This (Sunday) morning was interesting with regards to the lack of orographic precipitation enhancement.  Let's have a look:
  • Snow-water equivalent at CLN from 0000-1400 MST: 0.31 inches
  • Snow-water equivalent at KSLC from 0000-1400 MST: 0.27 inches
Thus, the ratio of mountain to lowland precipitation, known as the orographic ratio, was only 1.1 (Actually, it may have been <1 for the period during which precipitation was falling at KSLC).  In contrast, the climatological Alta/KSLC orographic ratio is ~3.5.  Further, driving down Big Cottonwood Canyon this afternoon, I couldn't help but notice that more snow fell in the lower canyon and along the east bench than in the upper canyon.  How deflating for skiers!  What gives?

The orographic ratio varies considerably from storm to storm and in some cases during storms.  Steenburgh (2003, 2004) shows that the orographic ratio can be low in northern Utah when the precipitation dynamics are governed by strong synoptic forcing, such as during the passage of a cold front.  Blocking, which occurs when the static stability is high, can also can enhance lowland precipitation rates (and hence reduce the orographic ratio) by shifting the vertical motion upstream of the initial slope of the mountain barrier.  Neiman et al. (2002) present a nice conceptual model showing this process (below), and Cox et al. (2005) describe a similar effect upstream of the northern Wasatch Front near Ogden.  

Neiman et al. (2002) conceptual model of orographic rainfall distribution in California's coastal mountains, including the impact of terrain-blocked flow.  
I think this morning's situation was a bit more complex.  Large-scale forcing did contribute as radar reflectivities were relatively uniform and widespread

KMTX 1500 UTC radar reflectivity
and the inverted-S profile in the radar velocity analysis is consistent with low-level warm advection.  

KMTX 1500 UTC Doppler Velocity Analysis
The morning sounding also illustrates that we were dealing with a situation characterized by high static stability below about 775 mb.  Even above this level, the lapse rate was absolutely stable.   

KSLC 1200 UTC Sounding
Although MesoWest data suggests were weren't dealing with a blocking-front scenario similar to that illustrated above by Neiman et al. (2002), the strong static stability and lack of a substantial cross-barrier flow component in the morning sounding is consistent with limited orographic lift.  Often in these situations, the valley is filled with a dry airmass and sub-cloud evaporation helps create a positive orographic ratio, but that is not the case today as we were saturated throughout the valley atmosphere.  Further, anyone skiing in the upper cottonwoods this morning likely noticed that the storm was quite shallow as the sun was visible through the snow for long stretches.

Solitude ski area ~1000 MST (1700 UTC)
All in all, it appears that low-level warm advection, a lack of substantial orographic lift, and the shallow nature of the storm may have conspired to give us the unusually low orographic ratio this morning.  Perhaps others have other hypotheses worth sharing.  Those wishing to see the mountains get back in the action will be pleased to know that the orographic ratio should be increasing now that the upper-level trough has passed, the northwesterly flow is increasing, and the stability is decreasing.  

The Greatest Snow on Earth is back

Any good meteorologist or true snow geek wears black pants to make it easier to examine snow crystals while skiing.  The photo below shows that we're not dealing with accretional growth and graupel anymore and instead diffusional growth is once again dominating precipitation microphysics.  Click for a bigger image.

Saturday, November 27, 2010

Stingy inversion

I'm quite impressed with the intensity of the inversion that is presently in place over the Salt Lake Valley.  This morning's sounding shows a roughly 10C inversion extending form the surface to about 775 mb where the temperature was about +2C.  

MesoWest obs this afternoon (~3:30 PM MST) show that fully inverted conditions persist over the valley.  At KSLC, it's only 27F, whereas is it in the 30s in the Park City area and at a few locations in the Wasatch Mountains.  I watched the car thermometer drop from 35F to 29F descending from the base of Mineral Fork to the bottom of Big Cottonwood Canyon this afternoon.  

The flow aloft is certainly quite strong as Alta-Mt. Baldy has had several hours with gusts >60 mph and a peak gust of 72 mph.  My experience, however, is that strong flow at the ridge tops is pretty ineffective at removing an inversion/cold pool.  Instead, one needs some sort of large-scale pressure gradient to drive strong southerly flow within the valleys and we don't have that today.  In fact, there is a north wind presently at Point of the Mountain.  

PCAPers may want to keep a close eye on this one as we could be dealing with a frontal passage late tonight or early tomorrow morning with minimal pre-frontal southerlies, which contrasts with many cold-pool events.  Thus, cold pool removal may not occur ahead of the front, but may depend instead on post-frontal northwesterly flow and cooling aloft.  The NAM brings us down to -16C at 700 mb by mid day Monday, which is not quite cold enough to mix out the current cold pool thermodynamically, so at issue is whether or not there will be sufficient flow to do it mechanically, or if a lens of cold air might persist over the lake and along the valley floor.  

Friday, November 26, 2010

Today's mountain weather: A perfect 10

No discussion today.  I just want to grin and enjoy what was an absolutely perfect day in the mountains.

Thursday, November 25, 2010

Minimum temperature oddities

A fascinating night last night with regards to minimum temperatures.  I figured we we were primed to get into the negative numbers at KSLC when it was 10F at 9 PM last night, but alas, it wasn't to be.  Instead, temperatures declined little, with 6F being the lowest temperature reported thus far in the 5-min obs.

In fact, nearly all of the Salt Lake Valley is in the positive numbers this morning, as is the northern Wasatch Front.  If you want cold, -17 at Pony Express Marker and -15 in Logan seem to be the big winners at valley locations (note that these are not official lows, which may fall between observation times).

Some of the minimum temperature variability is due to topography.  For example, it gets very cold at the Pony Express Marker and at Logan because they are in isolated basins.  In contrast, slope-valley flows persist all night long in the Salt Lake and Tooele Valleys, providing a mechanism for turbulence and mixing that weakens the intensity of the nocturnal inversion.

On the otherhand, most of the Tooele Valley was much colder than the Salt Lake Valley last night.  I suspect the urban heat island is contributing here and that the signal is quite large today given how cold it is and the fact that there is a modest amount of new snow on the ground, which means there is alot of patchiness in the city, but you have full coverage in rural areas (note also that the warmest location in the Tooele Valley last night was the city of Tooele).

BTW, if you are headed to the mountains for some pre-turkey laps, there is some good news.  It is -11F at the base of Alta, but overnight temperatures at the top of Collins warmed from -11F to -4F.  The rebound has begun...

Wednesday, November 24, 2010

A glimpse of things to come?

Inversion: 22 Jan 2005
The next few days should prove quite interesting for inversion aficionados.  We've just had one of the strongest cold surges in years, have a nice coat of fresh snow, and, beginning tonight, an upper-level ridge will be building over Utah and temperatures aloft will rebound rapidly.  NAM 700-mb temperatures are forecast to reach -12C by tomorrow afternoon, which doesn't sound very warm, until you think about how cold it is down here in the valley.  Bottom line is we'll be inverted tomorrow.

By Friday, we're pushing -2C at 700 mb and a whopper of an inversion should be in place.  The saving grace, however, will be reduced emissions during the holiday break and the fact that the next upper-level trough swings in later in the weekend, so air quality shouldn't suffer too much.  For PCAPers, this case should offer big "signal to noise" for examining the thermodynamic budget of inversion buildup, and then an interesting erosion of the cold pool over the weekend.

Record cold and lessons for DLE forecasting

The 12Z KSLC sounding is now available and it came in with a 700 mb temperature of -23.3C, which is the lowest observed in November since at least 1956.

It is presently 14F at KSLC, -3 at Alta Base, and -17 on top of Mount Baldy.  Don't be fooled by the warm temperatures downstream of the GSL and in the northern Salt Lake Valley where there's some cloud cover.  It is much colder in the southern Salt Lake Valley, Tooele Valley, Rush Valley, and West Desert.  Especially cold spots in the valleys include Bluffdale (1F), and Clover (-13F).

With regards to the Dreaded Lake Effect, the wimpy stuff in the radar above has developed in the last hour or two and represents all that we got last night.  Yeah, we had a HUGE lake-700-mb temperature difference, but check out how dry the boundary layer is in the KSLC sounding above.  No matter how big the lake-700-mb temperature difference is, you still need additional ingredients to bake the lake-effect cake.  Without those ingredients, increasing the lake-700-mb temperature difference is like taking an additional vitamin C pill in the morning when what you really need is some B-12 to go with it.  Making this even more difficult is that the models still struggle forecasting those ingredients - especially moisture.  I haven't verified this carefully, but a quick look at forecast and observed soundings suggests that we came in drier than yesterday's NAM suggested.

Tuesday, November 23, 2010

That was easy, now it gets hard

Hats off to the weather enterprise, including NCEP, NWS/Salt Lake Weather Forecast Office, TV broadcasters, etc., for a really great forecast.  This is a watershed event for the weather prediction in Utah.  Our own mini-version of the 1993 Superstorm.  It is often said that life in Utah is about 20 years behind the rest of America (don't believe me, spend a weekend in Provo...) and metaphorically that is where we are with forecast skill in part because many of our storm systems are very small in scale and not usually driven by strong synoptic forcing.  As a result, model skill here lags the rest of the country.  I was shocked at how difficult it was to forecast here when I moved from Seattle in 1995.

Since then, however, the observing system has improved substantially (e.g., MesoWest), the models have improved substantially, and forecasters have learned a lot about Intermountain weather.  We still lag, but now we can seize opportunities such as we had today when the system is strongly forced, well captured by our much improved observing system, and well simulated by the models.  The forecasters really went after it, spread the word (the web and social media revolution have helped here too), and spared the Wasatch Front a very difficult rush hour.

Unfortunately, it gets much harder from here.  Tonight and tomorrow we are once again dealing with the Dreaded Lake Effect, which is highly nonlinear, poorly resolved by NWP, very sensitive to small changes in the upstream flow (especially given the small lake size), etc.  It appears that lake-effect will happen, but where, when, and how much?  We don't have tools to reliably answer these questions yet (even probabalistic ones), which makes this a great research challenge for you students out there.

Like a good meteorologist, I'm hoping we get pounded.  It will be unbelievably cold in the mountains tomorrow (probably below -10F above 9000 ft) so I'd like to go skiing in the foothills behind my house where it will be a more tolerable 10F.


~5PM MST.  Now let the snow begin.

Looking toward KSLC from the Salt Lake Avenues

Thar she blows!

The cold front is just upstream of Hat Island and we can now get a pretty good look at its strength in terms of the airmass change and post-frontal winds.  The MesoWest analysis shows a 16F temperature contrast between Hat and Gunnison Islands.

The temperature at Gunnison Island dropped from about 43F to 28F with frontal passage, which was accompanied by a 55 mph wind gust.  Post frontal winds, however, have subsided to about 25 mph sustained with gusts to just over 35.  

It's a bit nastier along I-80 in the West Desert.  DPG17 warmed to 48F and then observed an abrupt temperature drop to 28F with fropa.  Impressive!  Post-frontal winds are sustained 40 mph with gusts to over 45.  This is blizzard strength for sure and it must be a mess and really lonely out there on I-80.  I suspect the surge to 48F may have been fueled a bit by a clear slot that was evident in the satellite earlier this afternoon (mentioned at weather discussion).

It should be a spectacular cloud display as the front pushes into the Salt Lake Valley.  

Noon update

The frontal precipitation band has advanced into far NW Utah and both pre- and post-frontal winds are quite impressive.

An interesting aspect of the front is the thermal structure.  There is a nice frontal nose (see below), but the coldest air lags well behind the front.  

What this means is that we'll have an exciting frontal passage, but that it will take several hours for the really cold air to move in.   

And so it begins...

The most anticipated winter storm in many years is on our doorstep and let's hope it doesn't disappoint.  Right now we have the "calm before the storm" so to speak, with a south wind and temperatures in the mid 30's over the Salt Lake Valley.  However, the 1430 UTC MesoWest analysis shows surface temperatures in the teens and single digits across east-central Oregon - a harbinger of things to come.

Where is the front?  Well it just passed Jerome County Airport (KJER) in central Snake River Plain.

At present, the front has not yet strongly contracted as the initial temperature fall at KJER was only about 7F.  However, the 700-mb analysis shows a 14C temperature contrast from southeast Idaho to the Columbia basin with a temperature minimum below -24C, so there is broad post-frontal baroclinity in place.

The surface front appears to undergo scale contraction in the 12Z NAM and passes through KSLC between 5 and 8 PM tonight.  Weather conditions should be miserable during and following frontal passage when wind and turbulence should blow snow into every crevice and orifice.  The National Weather Service has issued a rare blizzard warning covering all of northern and southwest Utah.  Go home early.

NAM 700-mb temperatures plunge to -25C at KSLC by tomorrow morning.  Since 1956, this is uncharted territory for November.  We have a very good chance of breaking the all-time lowest maximum temperature at KSLC tomorrow.  Lake effect?  Likely, but we'll deal with that in a future post.

Monday, November 22, 2010

Historic cold surge

We are still on track for what should prove to be an historic surge of cold air into northern Utah as both the NAM and GFS are bringing in an airmass for Wednesday that would produce the coldest 700-mb temperatures observed in Utah since at least 1956.  The NWS is now calling for a maximum temperature at KSLC on Wednesday of 16F.

I did some quick Hysplit trajectories this morning.  These can be flaky (not sure what is going on with that one from SoCal), but show we're dealing with air originating from the Arctic and interior British Columbia (these are 84 hour backwards trajectories from 12Z Wed).

Sunday, November 21, 2010

Hell is about to freeze over

Photo: good-times.webshots.com
In an earlier post, I mentioned that there aren't alot of days when the 700 mb temperature is below -20C and that the GFS was going for something like -21C this week.  I also wondered if that would verify, thinking that it would probably not end up being that cold.

Well, the latest GFS runs are instead calling for it to be even colder.  The latest GFS 700-mb temperature forecast for 15Z Wednesday is -25.3C.  Trevor has examined the KSLC RAOBS for 1956-2010 and has found that the coldest November 700-mb temperature observed in that period is -22.6C.  Further, there have only been 14 soundings (out of 40,246) with a 700-mb temperature below -25C during the entire record.  The last day we were below -25C was on 30 December 1990 when the 700-mb temperature was -27.2C.

Let's convert to American units to put this into perspective.  -25.3C = -13.5F.  That is a seriously cold day of skiing in the mountains.  Well mixed, that's 17F in the valley.  Cold.  Bloody cold.

Saturday, November 20, 2010

Kyle Tietze 1984-2010

All of us in the Department of Atmospheric Sciences are deeply saddened at the loss of Kyle Tietze, who passed away last Friday night.

We will miss you Kyle
Kyle was an exceptional young man, scientist, and friend.  He brought out the best in everyone.  It was an honor and privilege to attend his memorial service this evening, which I found deeply inspiring.  Below is a remarkable photo of the frontal passage that occurred just prior to the service.  

Frontal passage just prior to Kyle's memorial service (photo T. Cruickshank)
I can't help but think the frontal passage was more than merely coincidence.  Our best to Kyle and all his family and friends.  Our thoughts and prayers are with you.  

Graupelfest continues

We've gone directly from Octoberfest to Graupelfest this year and the first part of this storm cycle is certainly bringing plenty of styrofoam balls to ski on (and I like it!).

We've spoken a little about Graupel formation in an earlier post, but I'll add a bit more here.  Graupel forms as supercooled cloud-liquid water collides and freezes with on an ice crystal, with the crystal so fully encased in frozen cloud droplets that it is no longer distinguishable.  

This is not your brain, it is graupel (USDA/Wikipedia Commons)
Graupel lies at the outer end of rimed particles according to the Magono and Lee (1966) snow crystal classification, which actually discusses four types of graupel: (1) graupel-like snow, (2) hexagonal graupel, (3) lump graupel, and (4) cone-like graupel.   They all hurt your face, but enable fun skiing.  I used to love to ski graupel storms in the Cascades not only because it was so smooth and silky, but also because they didn't stick to my shell and get me wet!

Graupel formation is favored in storms with high cloud-liquid water content and strong vertical velocities.  The former supplies the super-cooled water, and the later enables the hydrometeor to remain suspended as it is rimed.  Dry snow might have a fall speed of 1 m/s or less, but the fall speed of snow increases as it is rimed, with graupel attaining fall speeds of up to 3 m/s.  Thus, if you want big graupel, you need strong vertical motion.

For graupel to form, we typically want warm storms or at least warm cloud bases so that there is abundant super-cooled water to play with.  Further, accretion is more efficient if the cloud liquid water is spread out over a smaller number of cloud droplets (so the droplets are bigger), so it is best if the storm has a smaller number of cloud-condensation nuclei (CCN) on which cloud droplets form.  The combination of a high frequency of warm storms and maritime airmasses that are typically cleaner and have fewer CCN is one reason why the Cascades and Sierra see so much graupel.   Finally, we want strong rising motion, either produced by convection or direct orographic ascent.

Part of the reason that we've seen alot of graupel so far this year is that our storms have been warm and they have been convective.  Once things get colder later in this cycle, we'll see snow that forms primarily from vapor deposition again.

Friday, November 19, 2010

Sierra Nevada and airmass transformation

During southwesterly flow, a pronounced gradient in precipitable water (i.e., the PW wall) often develops over the High Sierra crest with a cloud and precipitation shadow extending downstream over the Intermountain West.  During the day, sensible heating within the cloud and precipitation shadow can enhance frontogenesis directly through differential heating (assuming post-frontal cloud cover) and indirectly by driving a thermally forced circulation that reinforces the cross-front ageostrophic circulation.  Koch et al. (1995) describe this process nicely.

 A marvelous example of the Sierra PW wall and downstream cloud and precipitation shadow has developed.  Note the local minimum in PW that extends downstream of the High Sierra across nearly all of Nevada, the lack of cloud-cover within this minimum.

2045 UTC 19 Nov 2010 Visible Sat & RUC PW and 925 mb Wind
One can literally see the influence of the varying crest height of the Sierra Nevada in this image.  Higher PW air is able to penetrate across the relatively low Sierra crest north of Lake Tahoe and around the southern flanks of the High Sierra and through the Mohave Desert.  The PW minimum is found directly in the lee of the High Sierra, which is the most formidable part of the barrier.

At issue are the processes responsible for the PW minimum.  One possibility is airmass transformation in which water vapor is lost as orographic precipitation on the windward side of a mountain barrier, leading to a drier airmass in the lee (Smith et al. 2003).  In the present case, radar imagery suggests there is little windward precipitation over the High Sierra, except near Lake Tahoe.

2045 UTC 19 Nov 2010 Radar Composite
Perhaps radar coverage over the High Sierra is poor (anyone?), but even if it isn't, there are other mechanisms besides water vapor loss to orographic precipitation that might contribute to the Sierra PW wall including:
  1. Differences in elevation upwind and downwind of the Sierra.  Because the Intermountain West is at least 1500 m higher than the central valley, a portion of the PW decrease is simply the lack of the lower-troposphere.  On the otherhand, this doesn't account for the pronounced minimum in the lee of the High Sierra.
  2. Topographic blocking.  This is my personal favorite.  The Sierra frequently produce a strong barrier jet that advects large quantities of moisture poleward over their windward slopes (e.g., Marwitz 1987).   Essentially, the barrier jet acts like a robber baron.  Instead of the windward airmass being forced directly over the barrier and into the lee, blocking results in the low-level flow being diverted poleward until it encounters the lower portion of the barrier north of Lake Tahoe where it turns anticyclonically and penetrates into the Great Basin.
There is some evidence of the topographic blocking effects in the analysis above.  Note how the flow upstream of the high Sierra is SW, but the flow north of Lake Tahoe is more westerly.

Heather Reeves did a really nice job of explaining this process and its role in producing an isolated maximum in precipitation in the northern half of the Sierra Nevada (Reeves et al. 2008).  At issue is whether or not this process also contributes to the leeward PW minimum.  This would be an interesting topic for future work.  

Local and remote effects of Sierra Nevada

The next two days offer a great opportunity to examine the influence of the Sierra Nevada on the meteorology of the Intermountain West.  As of 1430Z this morning, a high-amplitude upper-level ridge and surface anticyclone were planted over the Gulf of Alaska with a positively tilted upper-level trough off the Pacific coast of the contiguous US.  As a result, large-scale southwesterly flow extends across California, the Sierra Nevada, and the Intermountain West.

IR Sat/GFS Forecast Valid ~1430 UTC 19 Nov 2010
On the regional scale, the leading edge of 700-mb baroclinity is draped across central Nevada and northern Utah and is collocated with a sea level pressure trough that extends from the Sierra Nevada across the Great Salt Lake Basin.

IR Sat/RUC Valid 1500 UTC 19 Nov 2010
The trough lies within the so-called Great Basin cyclone region identified by Jeglum et al. (2010) as having a high frequency of Intermountain cyclone occurrence and genesis downstream of the High Sierra.  Thus, although the existence of frontal troughing and baroclinity is consistent with the large scale pattern, I suspect the Sierra Nevada are enhancing the trough and influencing the position and intensity of the baroclinity over Nevada.  See also Shafer and Steenburgh (2008) and West et al. (2010).  

This pattern will persist over the next 24-36 hours as the large-scale trough amplifies and digs off the Pacific coast.  Keep an eye on things and consider the role of the Sierra Nevada.  In addition to how that mountain barrier affects mass and momentum, consider how it alters the thermodynamic and moisture characteristics of airmasses and how this in turn might affect the frontal and precipitation dynamics over the Intermountain West.  

Oh what a night

It was a remarkable night last night with "flat line" temperature conditions persisting at KSLC all night long.  Check out the meteogram, which shows temperatures nearly dropped only a few degrees in the late afternoon and evening and then remained remarkably steady all night long with a lowest 5-min observation of 54 F.

The combination of a thick cirrostratus deck combined with strong winds (sustained at > 15 mph for nearly the entire night) appears to have prevented the formation of any significant surface-based cold pool.  Even still, you don't see too many nights where there isn't a brief lull in the winds to enable a quick trop in temperature of a few degrees. 

Thursday, November 18, 2010

The 20/20 "rule"

This morning's GFS forecast drops 700-mb temperatures to about -21C by 0000 UTC Wednesday.

My 20/20 rule is not really a rule, but derives from my impression there are only a small number of days where the 700-mb temperature at KSLC is lower than -20C or greater than 20C.  Thus, anytime I see -20C or +20C air, it piques my interest and I know we're out there on the ends of the bell curve for temperature.  Plus, 20/20 is easy to remember.  On the otherhand, one should always verify such rules of thumb.  Perhaps someone can run some sounding stats and we'll see if my intuition is any good.

Also at issue is if this GFS forecast will verify.  Beyond just the usual issues related to the need to think probabalistically at this time scale, the source of air this cold is interior western Canada and in the past the GFS sometimes had a cold bias in these scenarios because the real-world topography produces more blocking and airmass transformation.  The GFS has been upgraded recently to an effect grid spacing of ~27km, so perhaps it will do better in these scenarios.

Wednesday, November 17, 2010

Potential warming

Tomorrow is going to be an interesting day.  The 18Z NAM is forecasting a midday 700-hPa temperature of 2C.  If we were to mix out to that level, that would give us a max temperature of about 19C (66F). 

We lack the solar energy to mix out to that level, but will the wind do the job for us?  There is a fairly strong along-barrier pressure gradient tomorrow, so I'm interested to see if we can get the southerlies going in the valley and really warm things up or if we keep a stable layer going as the NAM suggests.

PCAPS folks might want to keep an eye on this and run through some scenarios.  

Did it "blow like snot" yesterday?

CDC Public Health Image Library/Wikipedia Commons
As posted by Wendy yesterday, a peak gust of 96 mph was observed at Alta-Mount Baldy yesterday afternoon and appears to be the big winner in yesterday's storm.  A quick look at MesoWest reveals that 84 stations reported at least one gust in excess of 50 mph including:

  • Alta-Mt. Baldy - 96 mph
  • Logan Summit - 78 mph
  • Deer Valley Mt. Baldy - 75 mph
  • Powder Mountain - 75 mph
  • Alta-Collins - 73 mph
  • Ogden Peak - 71 mph
  • UT30 - 65 mph (appears to be highest gust at lowland station)
  • Curlew Junction - 63 mph
  • Utah Test and Training Range (TTU) - 61 mph
  • University of Utah - 57 mph

Thus, there is little doubt that it "blew like snot" yesterday.  However, in saying that, I'm speaking metaphorically and not scientifically.  In reality, there are wide-ranging estimates for the speed of snot, with Mythbusters producing a humorous 35-40 mph estimate. And, like the speed of snot, wind speed and gust reports are also subject to some uncertainty.  

Amongst the concerns are: (1) instrumentation accuracy/error, (2) averaging frequency and duration, and (3) representativeness error.  These issues become particularly important when cross-comparing across stations that have different sensors, processing algorithms, and exposures.  

For example, can we say with absolute certainty that Alta-Mt. Baldy had a stronger gust yesterday than Logan Summit?  The answer is no.  With some mental gymnastics, we could estimate how likely it was that Alta-Mt. Baldy had a stronger gust, but that is the best we can do.  

The bottom line is it is good to remember that "all observations are bad, but some are useful."

Tuesday, November 16, 2010

As winds nuke in the Wasatch

Check out Alta Mount Baldy (AMB) winds ...... gusting to 96!! The sensor was rimed of course until recently.....apparently you don't need a heater if the winds are around 100mph.......

Proposed blog changes

It's pretty clear that the restricted access of this site is a pain for a number of students.  Although I had some concerns that public access might restrict our ability to freelance, I'm not sure that really is the case.  Thus, I'm considering making it open access.  Please add a comment here and let me know your thoughts.

Monday, November 15, 2010

Welcome to the Cascades

Whatever happened to the Greatest Snow on Earth?  The UAC is reporting that 10 inches of heavy dense graupel fell last night in LCC, with a water content of 20%!  There were numerous reports of riming and drizzle in the Cottonwoods yesterday as well.  Green light conditions are over in the backcountry.

To understand the 24 hours, we need to recognize that liquid water needs a "partner", known as an ice nuclei, to turn into ice, even at temperatures below freezing.  Liquid water's preferred partner is ice, and if it comes in contact with ice, it will gladly freeze if the temperature is below 0C.  On the otherhand, liquid water is more selective with other potential ice nuclei.  If the water comes in contact with a particle that has a molecular structure similar to ice, it may freeze at relatively warm temperatures that are only a few degrees below freezing.  On the otherhand, if the water comes in contact with a particle that is dissimilar to ice, it may not freeze until temperatures are well below freezing, perhaps -15 C or colder.  There are not alot of ice nuclei floating around, so it can be hard to get a cloud to glaciate, especially if cloud top temperatures are not well below freezing.

During yesterdays riming event, we were dealing with a very stable cloud deck, with a base near 750 mb where the temperature was around -6C and a top near 675 mb where the temperature was -8C.

Under such a scenario, it is very hard to get water droplets to freeze because there are not alot of ice nuclei.  As a result, you get alot of riming as cloud droplets freeze on contact with the ground, trees, your goggles, etc., and you get drizzle as the droplets grow through collision and coalescence.  Sure there can be some snow, but you are dealing with a really messy mixed regime.

Major changes occurred, however, with the precipitation dynamics overnight as temperatures cooled aloft and the moisture deepened.  By 12Z this morning, cloud top extended to 500 mb, where temperatures were below -20C, and lapse rates were moist-neutral through a deep layer.

In this scenario, getting particles to freeze and snowflakes to form in the upper-portions of the cloud is not so challenging.  BUT, we have tons of supercooled water to play with near cloud base where temperatures are very near 0C.  Given the near-neutral conditions, graupel is forming as these supercooled droplets freeze on contact with falling ice particles in the cloud.  This is riming too, but it is leading to hydrometeor growth within the cloud.

These processes are very common in maritime climates such as the Cascades and Sierra Nevada, but less common here where are storms are typically colder, have more ice nuclei, and have less supercooled liquid water.  For now, however, we are dealing with an environment that is more maritime than continental, so we get to enjoy Cascadian snow for a while.  In my view, it's all good when you need base.

Sunday, November 14, 2010

Computers, guns, and tequila

I happened to notice this morning that something is askew with regards to one of the place names on the Google map image behind the MesoWest plot below (click on it to make it bigger).  Can you find it?

I'll post the answer in a couple of days if nobody figures it out, but backcountry skiers with a tendency to study topo maps and gape at big lines in Little Cottonwood Canyon should find it quickly.

This is a nice little reminder of the Mitch Ratcliffe quote that "a computer lets you make more mistakes faster than any invention in human history -- with the possible exception of handguns and tequila."

Saturday, November 13, 2010

Dirty ridge

With us teetering on the edge of really getting the ski season going, I've been looking at the guidance regularly hoping to see a glimmer of hope for the "big one."  Nothing would please me more than to see us lay down another 2-3 feet of the white stuff, especially at lower elevations, in order to open up the backcountry fully.

At issue is whether or not the GFS and NAM are teasing me.  I've been fairly surprised at just how wet they are for Monday.  As shown below, we're downstream of a long-wave ridge, in NNW flow, and yet the GFS is going berserk for precip.  The NAM is also fairly wet as well.  

Experience suggests that we'll get some periods of snow out of a pattern like this, but not the behemoth dump I'm looking for.  Sometimes in a pattern like this we get a strong riming event.  Typically that happens if we're on the anticyclonic side of the jet so that we get a layer of altostratus with cloud-top temperatures that are fairly mild, say -5C.  In that situation, it's hard to get ice nucleation going.  Snowfall in that situation is more limited.  

Keep your eye to the sky the next couple of days as this could be an interesting period to observe.  Don't despair if the snow doesn't fly big time.  The longer-range guidance suggests we'll have some opportunities further down the road. 

Tuesday, November 9, 2010

SOLPEX IOP2 photos

Storm chasing Utah style.  These photos are from Christy Wall and all but the first were taken at the east-side site near the Antelope Island Causeway.

Student-designed mobile mesonet unit
Jeff by the Perry mercury observing trailer
Balloon inflation
Wait, come back!  Just kidding
Cozy spot...internet and all