White Thanksgiving for Wasatch Front?

If you are dreaming of a white Thanksgiving you may be in luck.

Regional satellite, radar, and 700-mb temperatures over the past 18 hours show the landfall of a Pacific frontal system along the northern California coast followed by its redevelopment over north-central Nevada.

That frontal system has its sights set on the Wasatch Front for later today.  The latest HRRR shows the surface cold front pushing through the Salt Lake Valley around 2300 UTC (4 PM MST) this afternoon, with a band of frontal precipitation in its wake for much of the Wasatch Front.

Snow levels will drop quickly after frontal passage to bench and eventually valley levels.  This looks to be a quick hitting event, but 1-3 inches on cold surfaces along the east bench of the Salt Lake Valley seems like a good possibility.  Valley accumulations will be less, but perhaps enough to whiten cold surfaces.

The HRRR (above) generates lake-effect in the wake of the front, but one needs to be cautious interpreting that forecast.  First, lake geometry uses by the HRRR is based on 2001 data, leading to a Great Salt Lake that is much larger than it is at present.  U graduate student Brian Blaylock send me the image below showing the HRRR surface temperatures (degrees Kelvin) and you can clearly see the massive area of the HRRR Great Salt Lake compared to a more recent lake-shoreline outline (black line).  This means that the reliability of HRRR lake-effect forecasts will likely be low.  Of course, this doesn't mean we won't get lake effect tonight.  It is a modest possibility, although the odds of an intense, long-lived band are low (but not zero).  If you get some, consider yourself fortunate.

Temperatures during the frontal passage are favorable for relatively low density snow up in the mountains.  The 1200 UTC NAM generates 0.44" of water for Alta-Collins.  Overnight NCAR ensemble forecasts have 9 members between about 0.3 and .75 inches of water, and one less optimistic member going for just 0.1".

Based on that range of water equivalents, I would normally lean for 5-10" of snow, but the environment tonight looks favorable for lower density snow, perhaps with 5-6% water content.  That would mean something like 6-15" of snow if we ignore the less optimistic NCAR ensemble member (and I really want to do that).   Fifteen seems like a lot, however, unless the post-frontal and lake-effect precipitation really gets going.  I think something in the range of 6-12" is more likely and will stick with that and hope for more.

Two Ensemble Pitfalls to Avoid

If you are a user of some of our ensemble products on weather.utah.edu, I thought I would share a two pitfalls to avoid.

The first pitfall is assuming that ensemble members in the middle of the distribution are more likely to verify than those on the tails.  Although that could be the case, there are times when that's a bad assumption. 

Here's an example based on the NCAR ensemble forecast for Alta Collins for the 2-day period ending this afternoon and encompassing our most recent storm (focus on the top graph).  In the NCAR ensemble, every member is an equally likely outcome.  And, in this particular forecast, the outcomes are relatively evenly distributed.  The odds of 0.6 inches are about the same as 1.4 inches (the latter verified).  

Hedging your forecast to a subset of the plumes makes little sense when the distribution of forecasts is so even.  It might make sense when there is a strong clustering of forecasts, but one also needs to keep in mind that this is only a 10-member ensemble, so in some instances, that clustering might not be significant and you might want to be cautious about biting on it.  

The second is assuming that the full spread of the ensemble (from the low to high members) captures the full range of possible outcomes.  All of the ensemble forecast systems in operations today are underdispersive.  That means they don't fully capture the full range of possibilities in a given forecast period.  For example, at mountain sites in the western U.S., the spread of downscaled 5-day forecasts derived from the Global Ensemble Forecast System, which represent a portion of the forecasts used for the NAEFS-downscaled products on weather.utah.edu, encompasses the observed amount produced during major precipitation events only 56% of the time.  During the other 44%, the observed amount lies outside the downscaled GEFS spread.  

This Year, Ski Season Will Come with a Whimper, Not a Bang

Over the past few days I've been looking at the medium range ensembles and, although I don't see a storm in the next week to get excited about, there is a series of modest events that should slowly but surely add to our non-existent to meager snowpack.  We start with last night's precipitation (upper-elevation snow only), see a bit more today and tonight (with lower snow levels), and continue with a couple more storms over the next week.

Source: weather.utah.edu

Water and snowfall totals presently being advertised by the models for the next week are not game changers, except perhaps for the wettest, snowiest ensemble members.  Looking beyond the 1-week period presented above, there's hope of continued activity, but I don't like to talk about details in such long-range forecasts.  

Thus, it looks like this ski season will come in with a whimper, not a bang.  We'll get a little here and there.  Resorts may open a trail or two with snowmaking during the Thanksgiving weekend (no promises as we don't have snowguns in our models).  Backcountry skiers will be teased, not satiated.  It is what it is.

Beautiful Weather for Everything but Snowmaking

Everyone along the Wasatch Front and Back awakes this morning to a beautiful day with a think veil of high clouds blanketed over snowy mountains.

Everyone that is except for the snowmakers.

Toying with the idea of skinning and skiing the white ribbon of death down Collins Gulch tomorrow, I took a look at the web cams from Alta and saw nothing in the way of operating snow guns.

How can this be?  A building ridge and southwesterly flow conspired to raise our temperatures dramatically last night.  Check out the temperatures at Alta Base over the past 24 hours.  We peaked at 33ºF yesterday and cooled into the mid 20s shortly after sunset.  Then, after midnight, temperatures rose dramatically and currently hover near 40ºF.

There are, however, a few places where shallow pools of cold air did not scour out of valleys or basins.  For example, temperatures in the Snyderville Basin and near the base of Park City Mountain Resort remain below freezing this morning.  Yup, they can make snow at the base, but nowhere else.

It won't take long for temperatures there to rise above freezing, so if they are running a few guns down there, they won't be able to do it for long.

Kiss and Make Up with Mother Nature NOW!

Our first all elevation "winter storm" of the season is winding down.  Radar imagery from 1221–1512 UTC (0521–0812 MST) shows the last gasp of lake- and mountain enhanced showers decaying and moving downstream.

We had just enough to coat the grass around campus.  Up at Alta-Collins, there is a 5" snow total with 0.27" of water equivalent, clearly on the low end of the most likely range we discussed yesterday.  Maybe they will pick up a bit more, but these are pretty paltry numbers and nothing to get excited about.

I don't know what you people have done to irritate Mother Nature, but kiss and make up now.  This has been going on for 5 years now.  FIVE LONG YEARS.  I'm not getting any younger and I don't want this year to be a sixth.

Known Knowns, Known Unknowns, and Unknown Unknowns

In this post, I will be channeling Donald Rumsfeld, who once famously said.

"There are known knowns. There are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don't know. But there are also unknown unknowns. There are things we do not know we don't know."

Rumsfeld was lampooned for that quote, which many considered to be nonsense, but I understand exactly what he meant.

Known Knowns

Our computer models have gotten pretty good at predicting large-scale airmass changes.  That's why the massive drop in temperature from yesterday (record high of 73ºF and the latest 73ºF on record at the Salt Lake City airport) to tomorrow morning (low 30s) is a known known.  Think of this period as a time machine in which you are transported from early Fall to late Fall in two days.  We are in transit today as cooler air is already moving in aloft and the surface front, marking the leading edge of even cooler air, is scheduled to arrive this afternoon.

Known Unknowns

Difficulty predicting the timing and amount of precipitation, especially snowfall, are known unknowns.   Adding insult to injury is the fact that we are dealing with a front that does not have a well developed precipitation band this morning, as illustrated by the 1400 UTC (7 AM MST) analysis below.

Some models are, however, bullish on precipitation developing along the front.  Others are bullish on precipitation behind the front.  This leads to significant differences in the timing and amount of precipitation tonight and tomorrow morning.

From the operational models, we see water equivalent totals for the Salt Lake City airport ranging from only 0.1" (0000 UTC GFS) to .26" (0600 UTC NAM).  All models produce some frontal precip (step part of the curves, lags the actual surface front by a few hours), but of differing amounts.  Only couple produce a burst of post-frontal precipitation (increases around 1200 UTC give or take).  The National Weather Service forecast (green line) is the wettest of all (.32"), primarily because they call for post-frontal (and presumbaly lake-effect) precipitation to continue late tonight and tomorrow morning.  Most of the models below do not realistically simulate such precipitation.

For experimental models, we can look at the NCAR ensemble, which has the advantage of being much higher in resolution (3 km).  We'll also shift our attention to Alta as I know just a few of you have an interest in that.  Again, we see dramatic differences in precipitation amount produced during the frontal passage (from about 00-06 UTC) and then during the post-frontal period (after 0600 UTC).  Totals range from .3 to 1 inch of water equivalent.

The final known unknown is uncertainty in the snow-to-liquid ratio for the storm.  Drier snow yields larger accumulations per unit of water.  I expect the snow in the mountains from this event to be at or below average water content.  In the valley, accumulations will be limited by snow densification as it melts on the warm ground (accumulations will be greatest on cold surfaces).  However, there is uncertainty in predictions of snow-to-liquid ratio, and this is an additional source of error in forecasts.

Unknown Unknowns

I have spent much of my career studying lake effect so it pains me to say this, but we really can't predict the location and amount of lake effect worth a damn.  We have gotten better predicting the likelihood of lake effect (will it happen or not) and we can look at why lake-effect is likely tonight using the graphs below.  We have, for example, a forecast for a remarkably cold airmass moving over a remarkably warm lake (illustrated by top two left hand panels), a moist low-level airmass (lower left panel), and NW-N flow (top right).  Based on training to past events, this gives us a probability that lake-effect develops (note: this just means that we get some, not that we get a lot) that peaks at about 90% tomorrow morning.

That's all fine and dandy, but we still struggle with these forecasts for a number of reasons.  One is that the models aren't perfect.  They have errors in wind direction and humidity.  Another is that lake effect comes in many flavors.  It can be banded (intense and localized) or non-banded (weaker, but broader in coverage).  What causes those differences in flavor are basically unknown unknowns.  We don't know why some events feature banding and others don't.  Finally, despite the fact that all of the large-scale factors appear to add up to lake effect, we still have events where it doesn't happen (it's not as simple as cold northerly or northwesterly flow moving over the lake as that happens all the time with no lake effect).  Again, this is an additional unknown unknown.

Putting it all together

I think this remains a forecast with a broad range of possibilities.  I'm sticking with my forecast of 1-2 inches of snow for the benches (cold surface accumulation) and 5-10 inches of snow for Alta as I those ranges represent the most likely outcomes.  Could we get skunked and come in below those numbers.  Yes, if the front is less productive than advertised and the post-frontal doesn't come through.  Could we do better?  Yes, if the post-frontal showers late tonight and early tomorrow, possibly with lake effect, get going.

One thing to note about the lake effect is that the forecast flow for tomorrow morning is somewhat sheared, with the wind veering (turning clockwise) with height from northwesterly to near northerly.   As a result, in the NCAR ensemble, the probability of more than 1" of water equivalent is highest in the Oquirrh Mountains, maxing at 60%.  In the central Wasatch odds are generally around 20%.  

Clearly the best option is to keep expectations low and hope for the low probability, heavy precipitation outcome to verify in the central Wasatch.