Saturday, November 22, 2014

Forecast and Secrets Update

Not much change from yesterday in the forecast for today and tonight (compare time-height section below with that in the previous post).  Looks like a pretty good dump for the Wasatch.  I suspect most of you will be happy in the morning.   I'm still perplexed about what to expect thereafter. It's the weekend, so I'll let you surf around and look a the divergent forecasts out there and pick what you want.

A quick update on my book Secrets of the Greatest Snow on Earth.  Amazon is finally reporting that it will be in stock on November 25.  I've heard delivery dates ranging from November 28th to 30th.  Thanks for your patience!  In addition, it is now available in Nook and paperback (shipping with 24 hours) from Barnes and Noble.  I'm a huge fan of e-books, but in this case, I recommend the paperback.  For those of you on or near the University of Utah campus, I will be doing a lecture and book signing (Thanks to King's English who should have the book in stock, but call ahead to be sure) at the Wallace Stegner Center at 12:15 PM on December 2nd.  Click here for more information.

Friday, November 21, 2014

Peering into Forecasts of the Coming Storm

Time-height sections are remarkably powerful tools for weather forecasting (see Forecast Tools: The Time-Height Section for a basic introduction).  The forecast NAM time-height section for Salt Lake City for the next couple of days is an absolute classic, showing beautifully the likely storm environment changes over northern Utah.

With time-height sections, we typically reverse the x-axis so that time increases to the left, as is the case above.  So, scan from right to left to see the passage of various weather systems between now and Monday afternoon.  Instead of temperature, we also use a thermodynamic variable known as equivalent potential temperature, also known as theta-e or θe, which considers temperature and moisture and can be useful for examining atmospheric stability.

As can be seen above, we begin today with a mainly dry (and polluted environment).  The leading edge of the precipitation system is marked by a pronounced warm front that descends overnight and passes at mountaintop level prior to noon (Sat 18Z) tomorrow.  As the warm front approaches, southwesterly low-level flow will increase and potentially mix out our the inversion overnight.  

Moisture moves in with the approach of the warm front at low levels, and precipitation will develop over the mountains tomorrow morning.  Snow levels will max out around the time of warm-frontal passage, reaching 7500 feet (note that the blue line is the freezing level and that the snow level is typically 1000 feet below this).  Later in the day, a surge of low-θe aloft will precede the surface cold front and destabilize the lower atmosphere.  Thus, I would expect the precipitation to become more convective tomorrow afternoon, with even the potential for a bolt or two of lightning and rumble of thunder.  During this period, there might be graupel at times, and snow levels will lower, but remain above the valley floor.

Cold frontal passage occurs late tomorrow or early tomorrow evening, at which point snow levels will crash to the valley floor.  This will be the first full-bore period of winter weather at all northern Utah elevations this year (hooray!).  Get your old bald tires replaced today!

Following frontal passage, we see a prolonged period of unstable northwesterly flow.  This is what I call the post-frontal crap shoot since much of what happens during this period depends on factors that are difficult to forecast, including lake-effect.  

Of course the hard part is figuring out how much snow is going to fall.  The graphs below show what the NAM is producing for Alta.  Precipitation develops during the day tomorrow, and becomes heavy during the frontal passage, and then tapers off overnight.    

Based on this, I'd probably go for a total accumulation of 12-20 inches tomorrow and tomorrow night (i.e., through  5 am Sunday) in upper Little Cottonwood (typically the NAM underestimates total snowfall, hence the increase, although there are unfortunately exceptions). After that, based on a survey of the various models, I think it's likely we will see periods of snow through Monday afternoon and that we'll get more snow than indicated by the NAM above.  Nevertheless, I consider the post-frontal environment to be more of a crap shoot, so I'm not sticking my neck out for specific accumulations in a 48-84 hour forecast, especially given the fickle nature of the lake effect, which could play a role beginning on Sunday morning.  I think you should be optimistic, but keep your fingers crossed.  

Thursday, November 20, 2014

Snow Is Coming, But How Much?

I've been distracted by Buffalo's snowmaggedon the past couple of days.  Incredibly, they are getting more again today.  Never has so much powder gone to waste.  If only Mother Nature could have arranged for the Adirondacks, especially the high peaks, to be located more strategically relative to Lakes Erie and Ontario.

Of course, there is a place where Mother Nature has fortuitously aligned the weather, topography, and geography for great skiing and that's the Cottonwood Canyons.  And, the good news is that we are going to be getting some beginning this weekend.  The storm will be warm on Saturday with a passing warm front.  Following the cold frontal passage late Saturday, temperatures will fall and we are looking at what will likely be an active northwesterly flow period through Monday Night and possibly Tuesday.

The image below shows the 1200 UTC GFS forecast accumulated water-equivalent precipitation through 0000 UTC 25 Nov (6 PM Monday MST).  One can see the moisture corridor extending across the Pacific Northwest with precipitation maxima over the Wasatch–Teton ranges and the western Colorado Rockies.

Source: NCEP
If we extract a time series of accumulated precipitation for Salt Lake City from the GFS we see some healthy numbers, with a total of about 1.2 inches by late Monday.

There are, however, at least three critical factors to consider when interpreting such a forecast:

  1. The GFS does not adequately resolve the topography of the western U.S.  As a result, we typically have to adjust these precipitation amounts for topographic effects.  This is typically done in ad hoc fashion.  For example, the standard approach for estimating the total precipitation in upper Little Cottonwood is to multiply the GFS forecast for Salt Lake City by 2 or 2.5.  Using the latter would be a phat 3.6 inches of water, which would be marvelous for base building.
  2. For snowfall amount, we need to also forecast the snow-to-liquid ratio, or how much snow we'll get out of each inch of water.  There is no model that does this today, although we do have some algorithms that we can apply to model output to get a pretty good estimate for the upper Cottonwoods.
  3. Can I trust this forecast?  The atmosphere is quite chaotic.  A slight shift in the position of the jet stream and moisture plume in a situation like this could leave us either high and dry (north shift) or right in the thick of it (south shift).
Through the National Weather Service Collaborative Science, Technology and Applied Research (CSTAR) program, I've been working with Trevor Alcott to develop approaches for dealing with these issues.  More correctly, Trevor has been coming up with great ideas and implementing them, while I sit back, pat him on the back, and otherwise get the hell out of the way.  

The approach uses forecasts from the North American Ensemble Forecast System (NAEFS), which includes the Canadian ensemble system and the U.S. Global Ensemble Forecast System (GEFS).  These low-resolution forecasts are then downscaled using climatological precipitation –altitude relationships to generate high-resolution precipitation forecasts.  Then we apply a snow-to-liquid algorithm to estimate snowfall.  We do this to all members of the ensemble.  This allows us to:
  1. Calculate a mean of all the members, which over the long run is typically a better forecast than that produced by a single modeling system such as the GFS
  2. Evaluate the range of possibilities for a given period, including probabilities of exceeding specific thresholds.
Now, I'm going to show you these results, which are literally a day old.  I need to emphasize that this is an extremely experimental system, so don't plan your powder days or backcountry travel based on it.  The system has not been tested or calibrated, so right now, this is just great eye candy, but hopefully over time we can assess reliability and make refinements. I'm going to hold off on the snowfall estimates as I'm not sure we're even in the ballpark on those yet.  

The ensemble mean water equivalent forecast below is from last nights NAEFS and covers the period through 0000 UTC 25 November (5 PM Monday).  It shows a strip > 3 inches along most of the high Wasatch with a maximum of just over 5" on Ben Lomond Peak.  Structurally, this looks reasonable, but will it be skillful?  Time will tell.  I am skeptical of the Ben Lomond max.  Since we are using climatological precipitation-altitude relationships, they don't account for flow directions.  Typically Ben Lomond doesn't do well in northwesterly flow, so perhaps it will fall short.  Elsewhere perhaps we have a shot, but the storm will need to be productive.  If this does verify, we'll be in good shape for Thanksgiving skiing.

With an ensemble, however, we can take a look at many many forecasts to get a handle on the range of possibilities, as well as probabilities. Below is what we call a plume diagram and it shows the total accumulated precipitation at Alta-Collins from all the ensemble members (Canadian model light green, GEFS dark green) and the mean (thick green line). 

You can see that the mean gives almost 4 inches of water for Alta-Collins.  What a godsend that would be.  The range, however, is quite large.  The driest members produce around 1.5 inches of precipitation, and the wettest over 7 inches.  That's a lot of spread.  The Canadian model is in general wetter than the GEFS.  This appears to be due to the Canadian simply being more productive than the GFS since their large-scale forecasts are similar.  Canadians do like snow, eh? Of course, there is a member of the GEFS that goes for five inches.  

 Now, if this were a reliable and well calibrated system, one might draw an analogy of that plume to rolling two dice.  The most likely outcome is a 7 when you roll two dice.  In the case of the forecast above, the most likely outcome is probably something near the mean.  The extreme low and high precipitation amounts are more like rolling snake eyes or two sixes.  Less likely, but still possible outcomes.  Surely you see why it can be foolhardy to put specific numbers on winter storms many days in advance.  

Of course, we don't know yet how reliable and calibrated this system is, which is why you should continue to listen to official forecasts and not mad scientists like me.  I thought, however, that it might be interesting to share and perhaps we'll get some of this running on in the near future.  Putting all this together, it looks like we're going to see a good storm cycle through Monday, but we'll have to see how it all comes together in the coming days.  

Wednesday, November 19, 2014

Buffalo to Get Even More

- Steve Casimiro, Editor, Powder Magazine, 1987-1998

The quote above is one of my favorites and one that I often think of during major snow events.  It came to mind this morning as I examined the snowfall totals for the Buffalo area so far, with maximum storm-total accumulations of more than 60 inches.  

At the moment, it's actually not snowing in Buffalo.  The camera image below shows the New York State Thruway near West Seneca, near the heart of the most intense snowfall.  This is a major thoroughfare and one of the best maintained highways in America.  They have finally carved a trough through the snow, but there's a lot of work still to do.  

A few miles to the north accumulations are much lower and the road looks fine.  

Although Buffalo can exhale this morning, more snow is expected later today, tonight, and tomorrow.   At first, the snow will be of the widespread "garden variety" type, but later tonight, the lake-effect will return with another raging lake band possible.  Below is the 4-km NAM forecast for 1200 UTC (0700 EST) tomorrow).

Bets on maximum total snowfall?  

Tuesday, November 18, 2014

Buffalo in the Lake-Effect Crosshairs

Note: This post has been updated to include a remarkable video of the lake-effect band at the end. 

You don't see this every day.  Even in metric, 40-60 (cm) is a pretty good storm.  But this is the forecast in inches.  Source:
Contrary to popular belief, Buffalo is not the snowiest city in the United States, it's Syracuse (96 vs. 126 inches).  However, when it comes to behemoth storms, Buffalo is king.  Why?  It sits on the eastern end of highly elongated Lake Erie and thus can be pounded by intense snowbands that form along the major lake axis.  Such snowbands are sometimes called Long-Lake Axis Parallel or LLAP bands and are responsible for some of the most intense snowstorms in the world.  Last night and today portions of the Buffalo Metro Area have been getting pounded by a persistent LLAP band, as illustrated by the 18-hour-long radar loop below.  The approximate position of the Buffalo metro area is outline by the red box.

There are a number of reasons why these bands are intense.  First, they form when the large-scale flow is moving over the longest axis of a lake, which means the airmass experiences the maximum heating and moistening possible.  Second, the local heating over the lake surface generates land breezes that converge near the center lake axis, triggering and organizing the snowfall in an intense, localized band. The schematic below from my recently released book shows how this process works over the Great Salt Lake, but the concept is the same over Lake Erie.

Source: Secrets of the Greatest Snow on Earth (Steenburgh 2014)
LLAP bands can be very narrow with sharp contrasts in snowfall rates and accumulations.  If you look carefully at the loop above you'll see that once the band sets up, it snows only briefly in the northern portion of the Buffalo metro area.  In contrast, the southern portion is getting absolutely pounded.  A great photo of what I think is the northern edge of the band was taken by the Buffalo News 4 staff.  On the right you are in the clear.  On the left, snowpocalypse.

The Weather Channel is currently reporting that up to 4 feet of snow has fallen already in the Buffalo area.  In the satellite imagery, you can see very clearly the elongated, corrugated nature of the cumulus clouds in the primary lake-effect band, with clear skies immediately to the north.

Source: College of DuPage
In northern Utah, the Great Salt Lake occasionally produces LLAP bands.  In fact, about 20% of lake-effect periods feature such bands.  However, they lack the intensity and duration of the LLAP beasties that form downstream of Lake Erie.  Of course, as good as the Lake Erie bands are, it is Lake Ontario and the Tug Hill Plateau that see the biggest and baddest LLAP bands in the world.

Addendum @ 1:40 PM 18 Nov: Great video below from YouTube user Alfonzo Cutaia showing remarkable updrafts and northern edge of the band.


Monday, November 17, 2014

Update on Secrets of the Greatest Snow on Earth

I've been told by some of you that you've received e-mails from Amazon stating that they are having trouble obtaining copies of Secrets of the Greatest Snow on Earth.  I'm not sure what the problem is with their cyborgs but I suspect that there's been some minor glitch in their receipt or processing that's pushed their ship date past November 14.  Sit tight as I suspect it won't be long now.

I do know that copies are available currently at Weller Book Works in Trolley Square.  You can find them on their signed copies table and in their winter sports section.  You should also be able to order directly from Utah State University Press or by calling their customer service center at 1-800-621-2736.  I just bought 20 copies and they will be here by the end of the week.  Alternatively, check with your favorite book store as the book is beginning to appear at other retailers.  

Thanks for the interest and patience!