Big Snows in the Western Alps

By popular demand, here's a quick post on the big snows in the western Alps.

You may have seen news stories about this.  Huge snows in portions of the French, Italian, and Swiss Alps.  Thirteen thousand stranded in Zermatt, etc.

I couldn't find a good map of recent Alpine snowfall, so I snagged just a few notable numbers from ski reports online.  On Monday, 8 January, Tignes (France) reported 31" of snow, Sestriere (Italy) 31", Cervina (Italy) 10", and Zermatt (Switzerland) 20".  The next day, Tignes got another 31", Sestriere 39", Cervina 20", and Zermatt 0.  I'm not sure exactly where those obs are taken, and if the 0 at Zermatt might indicate no report, but there were significant snows in the western Alps.  There can be huge contrasts in snowfall across and along the Alps, but I haven't bothered to detail these small scale variations here.  I will say that snow-depth forecasts from for the period ending late on 9 January showed a pronounced max along the Alpine chain at the upper reaches of the western Po Valley catchment.  This would include Sestriere and Cervina.  Tignes and Zermatt sit just across the Alpine crest.

The setup for the event is shown below and features a digging upper-level trough that develops over western Europe and closes off over Spain and the western Mediterranean Basin before moving downstream.

In the interest of time, I've thrown together a map of the event, valid 1200 UTC 8 January, that includes sea level pressure (black contours), 925 mb (~750 m above sea level) wind vectors, and 925 mb temperatures.  At this time, the surface cyclone was centered roughly over the idland of Menorca in the western Mediterranean.  The cold front was ahead of the low center, passing over the island of Sardegna.  As a result, low level southeasterly flow over the Tyrrhenian Sea and west of the Italian boot impinged on the western hook of the Alps.  At the same time, there was strong flow blocking on the southern side of the Alps, resulting in a "u-shaped" sea level pressure ridge and easterly flow over the Po Valley.  As a result, southeasterly flow over the Adriatic Sea curved cyclonically and became easterly over the Po Valley, where it ran into the formidable western Alps.

This pattern persisted for a significant period of time and I suspect that the convergence of these two airstreams and strong orographic ascent of the easterly flow over the Po Valley were important contributors to the heavy snowfall.  In pink shading, I've added the are of strongest 700-mb (3000 m above sea level) ascent and not surprisingly there is a big bull's eye over the east side of the western Alps.

Below is a sounding from Milan, in the Po Valley, 12 hours later at 0000 UTC 9 January.  You can see the low-level southeasterly flow, which is remarkably strong, featuring a 25 m/s (50 knot) lo-level jet at about 950 mb.  The temperature and dewpoint traces show saturated or near-saturated conditions through the depth of the atmosphere, with conditions that meteorologists would describe as "moist neutral."  This is a recipe for strong low-level moisture transport to the western Alps, strong orographic ascent on their eastern slopes, and in all likelihood some embedded convection to juice things up and crank up precipitation rates. 

Source: University of Wyoming
A case like this deserves a more detailed investigation than I can provide here.  I'm sure the event undergoes important evolution over the period that I haven't had time to look into, and that the terrain effects are probably very interesting and impressive if one has access to higher-resolution precipitation/snowfall data.  If you dig in, please feel free to comment and share your insights.

The Warm of the Storm

Let's start with the good news.

If my math is right, storm total water equivalent at Alta-Collins through 7 am (9662 ft) is 1.20", producing 9" of base-building Cascade concrete.  Snow, possibly heavy at times, will continue through much of the morning.  The HRRR has things winding down late morning.  The NAM keeps a few snow showers around in the afternoon.  I'll go with another 2-5", mainly from 7 to 11 AM this morning.  We'll do a bit better than that if we can a good burst going or snow showers in the afternoon are more productive than expected.  These numbers are lower than the NWS Cottonwood Canyons forecast, but looking at the radar and the HRRR, I don't see us doing much better than that.  Even the "5" in 2-5 was a stretch.  Hope I'm wrong and they are right.

Now, let's shift to the bad news.

The prefrontal southwesterly flow was even warmer than I anticipated and temperatures overnight have cooled more slowly than expected.  A look at the temperature and precipitation graphs for Alta Collins shows temperatures hovering from 31-32ºF until yesterday evening and then only cooling very gradually to the current (7 am) value of 29ºF.

Thus, while we've added base builder in the upper elevations, the mid and lower elevation snowpack has taken a serious hit.  Further, the snow level remains high.  Even now, it is 34ºF at 8500 feet and the highest traffic camera in Big Cottonwood Canyon, which is near 7500 feet, shows only wet roads. 

I hate to say it, but get out and enjoy the high elevation snow today.   We may get a bit of windy "dirty ridge" snowshowers Thursday and Friday, but it won't add up to much.  Then the ridge returns.

Beggars can't be choosers.

Addendum @ 10 am

5 inches and .48" of water at Alta Collins since 7am.  Storm just exploded.  Wonderful! 

Norwegian Cruise Ship Heads Right Into a Well-Forecast Storm

On January 4th, its cruise ship Breakaway (I did not make this name up!) sailed right into the most dangerous portion of an explosively deepening, intense winter cyclone off the east coast, causing damage to the ship and greatly discomforting the passengers. And it did so by ignoring emphatic warnings by the meteorological community.

Some of the videos taken by passengers were terrifying, with large waves, water cascading down staircases, and items falling off the walls.  Here are a few sample videos:

The ship has a store called the "Tides Boutique"---and during this storm it has real tides.

So what happened?  The cruise ship sailed directly into the most dangerous sector of the storm.  This plot show the NWS surface analysis (sea level pressures and fronts) at 4 AM January 4th and the position of the ship at that time.  The ship was just south of the low center in a region of very strong winds---a region that is often called the "sting jet" in the weather business.

Here is a sustained wind analysis from the NOAA/NWS HRRR model with speed shown with the color fill, wind vectors with the symbols, and the black dot my estimated position of ship at the same time (4 AM Thursday PST).  At this point, they were in sustained winds of around 45 knots (52 mph), with higher gusts.  The ship was heading northward towards NY at this time. 

You will notice even stronger winds in front (north) of them, with a long fetch---allowing the development of big waves.   That morning the WindSat scatterometer satellite, which can measure surface winds from space (using the relationship between wind and small ocean waves,) went over the storm (and ship).  Here are the winds it found (wind vectors, color coded shown).  Sustained winds from the NW of 40-50 knots at the ship position off of North Carolina.

The most dangerous location in the Pacific cyclone is to the south and southwest of the low center, in the region of strongest pressure gradient.  This figure from a NOAA document indicates the region of very strong winds (red shading).  Guess what ship was very close to the worst part of the storm?

The forecasts for this storm were excellent, days ahead of time.  The forecasts made 24 hr and 48hr before the time shown above, had good forecasts of the storm (see below). The cruise line and the Captain had no excuses for being there.
                                          24h                                                                          48h
The NOAA Ocean Prediction Center (OPC) had an excellent forecast the day before for 80 kt wind gusts and 33 ft seas in the storm area.   Waves around the Breakaway were estimated at 30 ft.

And the NOAA 4-day forecast was highly accurate.
The storm was moving rapidly to the Northeast, so a delay of 24hr would have made a world of difference.

Something is really wrong when a ship with 4000 passengers and over 1000 crew heads into an historic storm of great intensity.   Thousands of lives were endangered and everyone on board had a very unpleasant experience.   It appears that the Captain ignored highly reliable meteorological guidance and needlessly put the ship and its passengers at risk.