Monday, September 30, 2013

Gov't Shutdown Threatens Skiing!

Major storms have been rumbling through the Pacific Northwest over the weekend and snow is beginning to pile up in the high country.  With snow levels near 4000-4500 feet, it is currently 32ºF with heavy snow at Paradise Ranger Station on Mt. Rainier (~5500 ft), one of more accessible higher elevation locations that you can access by car in the Washington Cascades.  As a result, Paradise is frequently a very good choice for early season turns.  
Paradise Ranger Station parking lot, Mt. Rainier National Park.  Source: NPS
Although they only have several inches of snow on the ground at present, satellite imagery shows a classic pattern for heavy snowfall today with a continuous fetch of unstable cold air extending from the Gulf of Alaska into the Washington Cascades.  

Thus, the forecast calls for more snow, potentially enough given the density of Cascade snow for turns tomorrow.

Source: NCEP
Conditions will be marginal, but it could be enough for diehard northwestern skiers to take a look.  Then again, perhaps they should consider other options.  While the road to Paradise frequently closes during inclement weather, closure tomorrow looks to be a virtual lock thanks to the pending government shutdown.  Curses to this Congress!
The fall back for northwesterners is Chinook Pass (or perhaps the Mt. Baker area), also a very snowy high elevation location (which is closed to traffic during the winter).  My son and I toured there in July 2011.  

In contrast to that lovely day, a hard shell and a hearty appetite for gloppy snow with minimal cover will be required this week.  Nevertheless, we're talking turns in September or very early October.  

Sunday, September 29, 2013

You Decide

Editors note: This post was updated on 24 January 2017 to include the new, updated link for the Summary for Policy Makers (the old one had expired). 

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (referred to by scientists as the AR5) consists of four reports:
  1. Working Group I report on the physical science basis of climate change
  2. Working Group II report on impacts, adaptation, and vulnerability
  3. Working Group III report on mitigation of climate change
  4. The Synthesis Report
The final draft of the Working Group I report, which attempts to provide a snapshot of current understanding of climate change, will be released and available at tomorrow.  The Summary for Policy Makers was approved and released on Friday.  

Below is a Cliff Notes version of the key figures, provided with as little commentary as possible.  Perhaps we will take a closer look at key findings and areas requiring further research in future posts.  

Observed Global Temperature Change during Historical Record
Source: IPCC

Other Changes to the Climate System
Source: IPCC

Carbon Dioxide Concentrations
Note: Trends in other greenhouse gasses not included in the Summary for Policy Makers but should be included in the full report issued tomorrow.
Source: IPCC

Estimated Changes in Radiative Forcing since 1750
Note: Radiative forcing can be thought of as a shift in the Earth's energy balance.  A basic description is available here.  
Source: IPCC

Simulations of Recent Climate Change with and without Anthropogenic Forcings
Source: IPCC

Climate Model Projections
Note: RCP2.6, RCP4.5, RCP6.0, and RCP8.5 are scenarios for future concentrations of greenhouse gasses and other climate forcing agents with RCP2.6 representing a highly aggressive mitigation scenario designed to limit the increase in global mean temperature to 2ºC and RCP8.5 representing the most extreme scenario. 
Source: IPCC
Source: IPCC

Friday, September 27, 2013

Ski Kennecott!

Observations from Alta-Collins suggest that 3-5 inches of snow fell overnight, but the big winner in last night's lake-effect lottery looks to be Oquirrh Mountains and the "hallowed backcountry" owned by Kennecott.

The Dry Fork Snotel (DRFU1) located at 7160 feet just to the north of the Kennecott's open-pit copper mine picked up a fast 7 inches from 4–10 am this morning with about 0.70 inches of snow-water equivalent.

Mountain snow showers and valley rain showers of the lake- and non-lake variety will continue for a few more hours before this event winds down later today.

The best thing that could happen now is that we get a good warmup to melt all this snow.  The last thing we need is a persistent snow cover rotting on north facing aspects and providing all sorts of avalanche booby traps when real winter starts in a few weeks.

Addendum @ 11:30 AM: 

Reports of mixed snow and rain this morning along I-80 near Tooele and accumulating snow in the Oquirrhs down to about 4800 feet.

Thursday, September 26, 2013

Things Are About to Get Interesting

I like our chances of picking up several inches of mountain snow in the Cottonwoods later today, tonight, and early tomorrow as a cold, "juicy" northwesterly flow develops over northern Utah.  Perhaps we'll do better if the lake effect can kick in with the right orientation.  If we can get high precipitation rates, you might even see some of the white stuff on the benches.

Here are some thoughts on lake effect that are relevant for tonight's forecast.  First, lake effect is relatively rare in September.  During the latter half of the month, you can expect an event on average every 4-5 years.  Events are more common later in the fall when there is a higher frequency of trough passages and cold air intrusions.

Source: Alcott et al. (2012)
Although events are scarce this time of year they do happen.  In addition, if we look at the environmental conditions in which lake effect occurs throughout the cool season, we find that during the fall (and spring), one generally needs a much larger temperature difference between the lake  and the overlying airmass [we usually use the 700-mb (about 10,000 ft) temperature as indicative of the overlying airmass] to generate lake effect.  The graph below is fairly busy, but the red box-and-whiskers show the range of lake-to-700-mb temperature differences during lake-effect periods and illustrate fall and spring events generally require larger differences than events in the spring and fall winter.

Source: Alcott et al. (2012)
The dashed red line reflects our best guess of the minimum lake-to-700-mb temperature difference needed to generate lake-effect in any given month.  In September, it's about 21ºC.  The good news is that we'll be beyond that tonight.  Lake temperatures as of yesterday were around 20ºC, whereas the 700-mb temperature is forecast to be -5 to -7ºC (depending on what model you look at) late tonight and early tomorrow morning.  Even though the lake will cool some over the next several hours, we should be above the necessary (but not sufficient) threshold.

The problem is that that threshold has been met before in September without producing lake-effect (the black box and whiskers show differentials without lake-effect, confirming this).  This is because there are a number of additional factors at play, one of which is the relative humidity.  It's simply harder (or impossible) to get lake effect from a dry low-level environment.  If we look at all lake-effect periods, we find that the likelihood of lake-effect increases with the size of the lake-to-700-mb temperature difference (relative to the dashed red line above, something we call ΔT excess, or how far you are above the magic threshold) and the low-level (850-700 mb) relative humidity.
Source: Alcott et al. (2012)
Of course, there are still other variables at play, such the time of day.  There is a strong diurnal modulation of lake-effect with events more common at night and especially just before and around sunrise and less common in the afternoon.  This effect is more pronounced in the Fall and Spring when solar heating is stronger than it is in the winter.
Source: Alcott et al. (2012)
So if you put all this scentific mumble jumble together, what do you have.  Well, we have several key ingredients coming together at that optimal time (late night and early morning): Big lake-to-700-mb temperature difference, high relative humidities, and flow from the west-northwest to northwest.  We can extract information about the environment forecast by the computer models and estimate the likelihood of lake-effect and you get the numbers below from the GFS and NAM.

So, the NAM is positively giddy, the GFS a bit more subdued.  In the case of the GFS, it's simply a little warmer and drier, so the odds are reduced a bit.  Note that those are instantaneous probabilities.  If you were to consider the odds of lake effect at any time during the late night or early morning, it would be higher than the maximum values provided in the tables above.

Put all this together, and you have a good chance of lake effect.  Whether or not it goes and goes big is something we really can't forecast reliably.  In addition, even if it goes, there's the issue of where and how much.   This is why I don't bet the house on the Dreaded Lake Effect.   

Wednesday, September 25, 2013

Donald Rumsfield Forecasts Powder

Cold air, northwesterly flow, lake stink.  Ah, fall is here, and so is the post-frontal crapshoot.

Let's talk about the easy stuff, or what Donald Rumsfield would call the known knowns.  A deep upper-level trough with the coldest air we've seen thusfar this fall will be rumbling through the Intermountain West over the next couple of days.  The coldest and juiciest air is expected to be over northern Utah on Friday morning.

At that time, the 700-mb temperature is forecast to be about -5 to -6ºC, which is why the National Weather Service is calling for snow levels down to about 6000 feet.  If we got a strong convective or lake-effect storm, I wouldn't be surprised if we say snow down to the benches.

The hard parts of this forecast are what Donald Rumsfield would call the the known unknowns and the unknown unknowns.  The known unknowns are things that we know we don't know.  In this case, we know we don't know how to predict with precision the distribution and intensity of precipitation that falls a post-frontal environment like the one that is picture above.  The problem is this.  Small changes in the position of the upper-level trough, the wind direction, the temperature of the airmass, and the humidity of the airmass can greatly alter precipitation structures and patterns.  Further, this is a lake-effect situation, and meteorologists don't call it the Dreaded Lake Effect for nothing (we actually call it something saltier than that, but I try to keep this blog clean).

The unknown unknowns are those things that we don't know we don't know.  It's hard to speculate about something you don't know you don't know, but it is pretty clear that we haven't yet unlocked some of the key ingredients of the Great Salt Lake Effect.  Some situations with very similar atmospheric structures and lake temperatures produce lake effect, while others don't.  We haven't figured out why.  We also haven't figured out why lake-effect is sometimes banded while other times it is wide spread.  We have some ideas, but they don't really help us discriminate events in advance.

Different types of lake-effect: (a) Widespread, (b) hybrid, (c) banded. Source: Alcott et al. (2012).
These known unknowns and unknown unknowns are why I call the postfrontal environment a crapshoot.  There are few sure things in the post-frontal environment.  Sometimes we get it, sometimes we don't.  This is an instance where we're going to get something over the next couple of days, but we'll have to see if it proves to be a lot.  Personally, I'd be happy if it held off until early November, as discussed in the now classic Wasatch Weather Weenies post from 28 September 2011: Patience Young Jedi Knight

Tuesday, September 24, 2013

Powder Envy

Source: Silverton Mountain
This is an entirely unsubstantiated observation, but it seems that in most years Colorado gets an early season dump before the Wasatch.  If this is true, I suspect is is a function of altitude and the great diversity of ranges that they have, which greatly increases the likelihood that somewhere in the state they will get something when a trough rumbles through.  

Have faith.  We always win in the end...

Monday, September 23, 2013

Downstream Development and Utah Snow

Chances are you've heard rumors of flakes in the Uintas yesterday and snow coming to the Wasatch this week.  The setup for the potential snow later this week is something that frequently happens over the north Pacific, especially in the fall.  On Saturday morning [1200 UTC (0600 MDT) 21 Sep] and upper-level short wave trough (red line) was moving eastward over the Sea of Japan, while a plume of moisture (green line) extended northward from the subtropics near the dateline.  

IR satellite image and GFS 500-mb geopotential height (black lines) and integrated water vapor (color fill)
analysis valid 1200 UTC 21 Sep 2013
 Over the past two days, the upper-level trough amplified (western most red line below) and phased with that plume of moisture (green arrows).  This led to an amplification of the pattern downstream with a ridge building over the Aleutian islands (blue line) and a trough digging over the eastern Pacific (easternmost red line).

IR satellite image and GFS 500-mb geopotential height (black lines) and integrated water vapor (color fill)
analysis valid 1200 UTC 23 Sep 2013
This process is known as downstream development.  Here's a loop of the process.  Note in particular how the large scale pattern (the flow roughly parallels the black 500-mb height contours) over the entire  north Pacific transitions from something that is relatively "straight" (what meteorologists call zonal) to one that is "wavy" (what meteorologists call amplified).

The amplification of this pattern is expected to continue the next couple of days with the trough over the eastern Pacific strengthening and moving into the western U.S.

Forecasting pattern transitions like this couldn't be done reliably before the development of computer forecast models.  Nevertheless, although those forecast models are helpful, the forecast challenge for later this week is figuring out the details, such as how deep with the trough get, how quickly will it move downstream, and where will it generate precipitation.  That's why we can be fairly confident that we're going to see cooler weather and some mountain snow later in the week, but when, where, and how much remains a bit of a mystery.  The most likely scenario is a few inches at high elevations, but there's a wide range of possibilities depending on how everything comes (or doesn't come) together.

Saturday, September 21, 2013

Beat That Team Down South!

It looks windy and warm (by evening standards) for tonight's game, which will surely run to midnight (pictured below, note the very strong flow).

Games in recent years have frequently been decided by field goals.  Planning to have that wind at your back in the fourth quarter could prove prescient.  Hopefully Coach Whittingham is either aware or has a good plan to ensure we have a comfortable lead by then.  Given the porous nature of the Ute defense so far this year, I'm not thinking the latter is likely.

Finally, regardless of who wins, it is always worth remembering the top-10 reasons why Utah is better than BYU:

10. We have the Great Salt Lake effect, they have the Utah Lake effect
9. We chase tornadoes, 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 more than 60 individual national champions,  BYU races division II.
5. We can ski on Sundays
4. We have, they have, well, nothing anywhere near as cool as
3. We are the best university for skiers and snowboarders, they are the most stone-cold sober school
2. Our air quality sucks, but sometimes theirs is worse
1. We have an atmospheric sciences program, they don't

Friday, September 20, 2013

Recipe for a Large Temperature Rise

You will be shedding layers today.  Here's the recipe:

1. Preceding surge of cold air into the region

2. A dry, crystal clear night

3. Development of strong southwesterly flow and the transport of warm air aloft over northern Utah overnight

4. A clear, sunny day with continuing southwesterly flow

Thus we'll be rebounding from an overnight minimum of about 48ºF at the airport to something like 83ºF this afternoon.

Of course, there are better places to experience a full force temperature swing, such as the Rush Valley south of Tooele where overnight minimum temperatures were as low as 32ºF.

At the Faust observing site that reached 32ºF last night, they will likely see a temperature increase today of about 50ºF.  Not too shabby.

Thursday, September 19, 2013

Snow On the Ground and (Possibly) In the Future

Major changes are now underway that should warm the hearts of Utah skiers.  First, I'd like to whet your appetite with the web cam photo from Mt. Washburn in Yellowstone National Park (thanks to Jack Hales' Star Valley Weather Blog for pointing this out).

Source: National Park Service
Next, we have a couple of opportunities for snow at least in the Wasatch in the coming week.  The first is with the trough passage that will occur this weekend.  Overnight forecasts from the North American Ensemble Forecast System [NAEFS, a combination of the US Global Ensemble Forecast System (GEFS and the Canadian ensemble] include a few members that drop 700-mb temperatures down to 0ºC on Sunday, which would probably get snow levels down to about 8500 ft.

Source: NWS
On the other hand, the average of those members is a bit warmer than that and the moisture accompanying the trough is pretty limited.  So, this could disappoint, but I thought I'd note the potential for our first dusting at upper elevations.  

Then, an even stronger trough is forecast to move into the western United States next week by both the U.S. Global Ensemble Forecast System (GEFS) and the gold-standard ECMWF ensemble forecast system.

Source: NWS
Much will depend on the path and structure of the trough, but it looks like a major blast of colder air is coming to the west.  Perhaps the Winter Warlock can conjure up some magic and give us at least a dusting at upper elevations in the Wasatch.  Stay tuned.  

Wednesday, September 18, 2013

Climatological Perspectives

Here are some interesting perspectives on the summer of 2013 and this wonderful airmass that has pushed into northern Utah.

To begin, it looks like we can finally assume that all the 100ºF days are in the books for this year.  We hit 20, 1 shy of tying the all time record.  I guess you can't have everything.

Las night our overnight minimum was 56ºF, still higher than the average of 52ºF, but it is likely we will have a minimum either tonight before midnight or tomorrow that is below average, which would be our first day with a below average minimum since August 2nd.  August 2nd is the only day since June 23rd with a below average minimum, as illustrated nicely by the National Weather Service graphs below.  

So, over the last 86 days, we have had one with a below average minimum temperature. One!  That is in my view the most remarkable aspect of this summer.

Today's high will likely be near 70ºF, which will make it the coolest day since May 31, when we only eked out 69ºF.  Since then, the lowest maximum temperature was 73ºF on September 14, when it was cloudy and rainy, so enjoy the cool, clear air today as it's been a while since we saw an airmass like this.  

Tuesday, September 17, 2013

This Is the End

The last day of the 2013 monsoon is here.  A parting gift just rumbled through campus.  

Get your freak on and celebrate with The Doors.

Monday, September 16, 2013

Change Is Coming!

Sunrise over Salt Lake this morning
It hung on for as long as possible, but it is now apparent that our suffering is almost over.  The summer of 2013, characterized by extreme heat in Salt Lake City and an extremely active and prolonged monsoon across much of the state, will end this week.  

Change is coming in the form of a midlatitude upper-level trough and cold front that will sweep into the Pacific Northwest and penetrate into northern Utah early Wednesday.

By Wednesday afternoon, we will be enjoying the coldest airmass that we have seen around here since at least June, with 700-mb temperatures forecast to be near +2ºC.

It's tough to say for certain if this is a true stake-in-the-heart for the monsoon, but the forecasts for the next week or so keep us in large-scale southwesterly to westerly flow.  Temperatures will rebound by later in the week, but it appears the monsoon moisture will remain to the south, which hopefully means that we'll see some decent cooling at night as we would expect with the approach of fall.

Sunday, September 15, 2013

It Continues

Our hearts go out to everyone suffering in the flooding along Colorado's Front Range.  Mother Nature is showing no mercy, with heavy rain returning today.  Although accumulations aren't expected to be as outrageous as a couple of days ago, they will add further insult to injury.
Source: NCAR/RAL
Now added to the list of mind boggling observations from this storm is how the North Fork of the Big Thompson River at Drake has remained near or above record flood stage now for more than 60 hours.

Assuming the gauge is still functioning correctly, it appears there will be no quick retreat of the flood waters along the Front Range.  Videos of the flood damage are disheartening.  You can find them all over the web, but a large collection is available at The Lede.

A couple of perspectives on the flooding that might interest you include the following:

Both of these have a bit of a Boulder focus.  I suspect as bad as things are there, they are much worse to the north.   

Saturday, September 14, 2013

TDWR to the Rescue

The flooding in Colorado has disrupted telecommunications, halting the the flow and distribution of radar data from the Salt Lake City radar (KMTX).  What is a meteorologist or weather enthusiast to do?

One option is to use the Salt Lake City Terminal Doppler Weather Radar (TDWR).  The TDWR is a smaller, shorter-range radar deployed primarily to detect wind shear for aircraft safety.  Nevertheless, it provides high quality radar images just like KMTX.  Below is an example of the imagery of this afternoon's thunderstorms.

The TDWR is located near the shore of the Great Salt Lake west of Centerville.  It thus has the advantage of providing radar scans at much lower elevations over the Salt Lake Valley compared to KMTX (which is at 6800 ft on top of Promontory Point).  On the other hand, it is completely blocked by the Wasatch Mountains to the east, and thus provides no guidance on what is happening on the Wasatch Back and beyond.

If you need a radar fix, you can access the Salt Lake TDWR data at this Weather Underground site.

Friday, September 13, 2013

A Storm of Biblical Proportions

Widespread flooding in Boulder.  Courtesy Ed Szoke. 
It is very rare that I will use the same hyperbole as USA Today (See "Biblical" rains trigger flooding that kills 3 in Colo.), but the storm that hit Boulder and the Colorado Front Range the past two days is so mind boggling that I don't know how else to describe it.  Boulder is home to the National Center for Atmospheric Research, NOAA/EarthSystems Research Lab, and numerous other meteorological organizations and our thoughts go out to our colleagues and other residents of the area.

According to the Western Region Climate Center, Boulder has an average annual precipitation of 18.72 inches and an average monthly precipitation in September of 1.53 inches.  Until this event, records for the wettest month and day were 9.59 inches in May 1995 and 4.80 inches on July 31, 1919.  Both of these records have been obliterated.

Rainfall estimates from the Advanced Hydrologic Prediction Service show rainfall maxima in or near Boulder of more than 8 inches for the 24-hour period ending 1200 UTC (0600 MDT) 12 September and more than 6 inches for the 24-hour period ending 1200 UTC (0600 MDT) 13 September.

Reports to the Community Cooperative Rain, Hail, & Snow Network (CoCoRaHS) show accumulations of more than 8 inches at several locations in the Boulder area over the 24-hour period ending 7 AM 12 September and approaching 6 inches over the 24-hour period ending 7 AM 13 September (reports are still trickling in).

My colleagues in the Boulder area have reported as much as 14 inches over the 2-day period which jives with these reports.

To the northwest of Boulder, the gauge along the North Fork of the Big Thompson River at Drake reached what appears to be a record crest of 10.55 feet.

It appears that this equates to a flow of 59,600 cubic feet per second, exceeding that observed during the 1976 Big Thompson Flood.  This has led to widespread evacuations along the the Big Thompson and other rivers issuing from the Front Range. (Addendum @ 9:05 am: The 10.55 looks a bit spiky, so perhaps the true crest was closer to 10 ft, which would still be a record 42000 cfs).

Lying in the climatological lee of the Rockies, the Boulder area is relatively dry, but experiences episodes of heavy precipitation (often in the form of snow) during periods when the flow develops an easterly component and can tap into moisture from the Gulf of Mexico, as was the case, for instance, at 1200 UTC (0600 MDT) 12 September and for much of the period.

In addition, precipitable water (a.k.a. integrated water vapor) values in the sounding taken from Denver International Airport, were likely the highest ever observed in September (records go back to 1948).  Prior to this event, the highest value observed was just under 1.25 inches, whereas they reached as high as 1.33 inches at 1200 UTC (0600 MDT) 12 September.

Those are easily identified factors, but more work is needed to understand this event and why it was so productive and prolonged.  With over 1000 meteorologists in the Boulder area, the causes of such an extreme event will certainly be investigated in depth.